OBJECTIVE LENS ELEMENT AND OPTICAL HEAD DEVICE INCLUDING THE SAME

Abstract

An objective lens element having improved diffraction efficiency at at least one of used wavelengths is provided. The objective lens element includes one surface including: a first region including an optical axis; and a second region surrounding the first region. A periodic first diffraction structure is formed on the first region, and a periodic second diffraction structure different from the first diffraction structure is formed on the second region. The objective lens element satisfies the following conditions. |A1−B1|<|A2−B2|  (1) |B1|≧|B2|  (2) Here, A1 and B1 are diffraction orders at the first region to converge light of a first wavelength and light of a second wavelength on a recording surface, respectively, and A2 and B2 are diffraction orders at the second region to converge the light of the first wavelength and the light of the second wavelength on a recording surface, respectively.

Description

CROSS REFERENCE TO RELATED APPLICATION

The disclosure of Japanese Patent Application Nos. 2011-029371, filed on Feb. 15, 2011, and 2012-29063, filed on Feb. 14, 2012, is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an objective lens element used for performing at least one of recording, reproducing, and erasing of information on an optical information medium such as an optical disc, and an optical head device including the objective lens element.

2. Description of the Background Art

In recent years, research and development has been actively carried out concerning high-density optical discs that have an increased recording density and thus have an increased storage capacity. A standard of such a high-density optical disc is Blu-Ray (R) Disc (hereinafter, referred to as “BD”) in which the image side numerical aperture (NA) of an objective lens element is set to about 0.85 and the thickness of a protective base plate formed on an information recording surface of an optical disc is set to about 0.1 mm.

In addition to the above BD standard, a standard (so-called DVD standard) in which a red laser beam with a wavelength of about 680 nm is used and the thickness of a protective base plate formed on an information recording surface of an optical disc is set to about 0.6 mm, and a standard (so-called CD standard) in which an infrared laser beam with a wavelength of about 780 nm is used and the thickness of a protective base plate is set to about 1.2 mm, have also been used. Various objective lens elements that are compatible with at least two of these three standards have been developed.

For example, an objective lens element that is compatible with two types of information storage media, BD and DVD, and an objective lens element that is compatible with three types of information storage media, BD, DVD, and CD, are known (for the latter, for example, see Japanese Laid-Open Patent Publication No. 2010-170694).

In the objective lens element that is compatible with two types of information storage media, BD and DVD, a first surface is divided into two regions which are concentric about a symmetry axis (optical axis), a compatible region which performs aberration compensation for two types of wavelengths for BD and DVD is formed in the region close to the optical axis, and an outer region optimized for BD is formed outside the compatible region. The compatible region has a diffraction structure and achieves spherical aberration compensation for two types of formats of BD and DVD by using a difference in diffraction angle caused by a difference in wavelength. It should be noted that the first surface refers to a surface located closer to a light source, among two optically functional surfaces of the objective lens element. In other words, the first surface is an incident surface of the objective lens element. In addition, a surface opposed to the first surface is referred to as a second surface. In other words, the second surface is an exit surface of the objective lens element.

In the objective lens element that is compatible with three types of information storage media, BD, DVD, and CD, a first surface is divided into three regions which are concentric about the optical axis, a compatible region which performs aberration compensation for three types of wavelengths for BD, DVD, and CD is formed in the region closest to the optical axis, another compatible region which performs aberration compensation for two types of wavelengths for BD and DVD is formed outside the compatible region, and an outer region optimized for BD is formed outside the other compatible region. The compatible region has a diffraction structure and achieves spherical aberration compensation for three types of formats of BD, DVD, and CD by using a difference in diffraction angle caused by a difference in wavelength.

However, when the objective lens element having the above configuration is used, it is necessary to increase the diffraction power of the compatible region in order to ensure a sufficient working distance (in particular, when DVD or CD is used) while aberration compensation is performed for a plurality of formats.

When the diffraction power is increased, the interval of the periodic diffraction structure in the compatible region narrows as distance from the optical axis of the objective lens element increases.

When the interval of the diffraction structure narrows, the diffraction efficiency decreases. As a result, an amount of light passing through the compatible region decreases. Thus, in the conventional objective lens element, performance sufficient to perform recording, reproducing, and erasing of information on an optical information medium such as an optical disc is not obtained.

Further, the interval of the diffraction structure narrows as distance from the optical axis increases. When the interval of the diffraction shape narrows, an amount of light at a peripheral portion of the compatible region greatly decreases. When DVD or CD is used, it is necessary to form a predetermined convergence spot only with light passing through the compatible region. Thus, when an amount of light which determines a numerical aperture for DVD or CD decreases, the numerical aperture effectively decreases. As a result, reproducing/recording performance deteriorates, since a convergence spot on the optical disc effectively enlarges.

SUMMARY OF THE INVENTION

Therefore, the present invention provides an objective lens element having improved diffraction efficiency at least one of a plurality of used wavelengths.

The present invention is directed to an objective lens element capable of converging each of fight of a first wavelength and light of a second wavelength longer than the first wavelength, on an information recording surface of an optical disc. The objective lens element includes one surface including: a first region including an optical axis; and a second region surrounding the first region. Aperiodic first diffraction structure is formed on the first region. A periodic second diffraction structure that is different from the first diffraction structure is formed on the second region. The objective lens element satisfies the following condition formulas.

|A1−B1|<|A2−B2|  (1)

|B1|≧|B2|  (2)

Here

A1 is a diffraction order at the first region to converge the light of the first wavelength on the information recording surface,

B1 is a diffraction order at the first region to converge the light of the second wavelength on the information recording surface,

A2 is a diffraction order at the second region to converge the light of the first wavelength on the information recording surface, and

B2 is a diffraction order at the second region to converge the light of the second wavelength on the information recording surface.

Further, the present invention is directed to an objective lens element capable of converging each of light of a first wavelength, light of a second wavelength longer than the first wavelength, and light of a third wavelength longer than the second wavelength, on an information recording surface of an optical disc. The objective lens element satisfies the following conditions.

L1<0  (3)

L2>0  (4)

Here

L1 is the distance from an incident surface of the objective lens element to an object point of a light source of the second wavelength, and

L2 is the distance from the incident surface of the objective lens element to an object point of a light source of the third wavelength.

According to the present invention, diffraction efficiency improves at at least one of a plurality of used wavelengths.

These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram of an optical head device including an objective lens element according to a first embodiment;

FIG. 2 is a schematic cross-sectional view of the objective lens element according to the first embodiment;

FIG. 3 is a partially enlarged view showing a diffraction shape of the objective lens element according to the first embodiment;

FIG. 4 is a schematic configuration diagram of an optical head device including an objective lens element according to a second embodiment;

FIG. 5 is a schematic cross-sectional view of the objective lens element according to the second embodiment;

FIG. 6 is a partially enlarged view showing a diffraction shape of the objective lens element according to the second embodiment;

FIG. 7A is an optical path diagram of an objective lens element according to a third embodiment (when DVD is used);

FIG. 7B is an optical path diagram of the objective lens element according to the third embodiment (when CD is used);

FIG. 8 is a partially enlarged view showing a diffraction shape of an objective lens element according to Example 1;

FIG. 9 is a partially enlarged view showing a diffraction shape of an objective lens element according to Example 2;

FIG. 10 is a partially enlarged view showing a diffraction shape of an objective lens element according to Example 3;

FIG. 11 is a partially enlarged view showing a diffraction shape of an objective lens element according to Example 4;

FIG. 12 is a partially enlarged view showing a diffraction shape of an objective lens element according to Example 5;

FIG. 13 is a partially enlarged view showing a diffraction shape of an objective lens element according to Example 6; and

FIG. 14 is a partially enlarged view showing a diffraction shape of an objective lens element according to Example 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

1. Configuration of Optical Head Device

FIG. 1 is a schematic configuration diagram of an optical head device including an objective lens element according to a first embodiment. A blue light beam 61 described below corresponds to a light of the wavelength λ1. The wavelength λ1 is an example of first wavelength. Further, a red light beam described below corresponds to a light of the wavelength λ2. The wavelength λ2 is an example of second wavelength.

The optical head device according to the first embodiment is configured to be compatible with the BD standard and the DVD standard.

A blue light beam 61 emitted from a laser beam source 1 passes through a relay lens 2 and a three-beam grating 3, is reflected by a beam splitter 4, and then is converted into a substantially parallel light beam by a collimating lens 8. The collimating lens 8 is movable in an optical axis direction. By moving in the optical axis direction, the collimating lens 8 compensates for a spherical aberration caused by an error of a base material thickness of an optical disc and a spherical aberration caused by a difference in base material thickness between information recording surfaces in a medium having multiple information recording surfaces. The blue light beam 61 having passed through the collimating lens 8 passes through a quarter wavelength plate 5, is reflected by an upward reflection mirror 12, is incident on the objective lens element 143, and is converged on an information recording surface of an optical disc 9 to form a desired spot thereon. The blue light beam 61 reflected by an information recording surface of an optical disc 9 passes through the objective lens element 143 again, is reflected by the upward reflection mirror 12, and passes through the quarter wavelength plate 5, the collimating lens 8, and the beam splitter 4 in order. The blue light beam 61 outputted from the beam splitter 4 is reflected by a beam splitter 16, is converged on a photodetector 33 by a detection lens 32, and is detected as an optical signal by the photodetector 33.

A red light beam 62 emitted from a laser beam source 20 passes through a three-beam grating 22, the beam splitter 16 and the beam splitter 4, is incident on the collimating lens 8, and is converted into diverging light. The collimating lens 8 can adjust the parallelism of the red light beam 62 by moving in the optical axis direction. In addition, similarly to the case where the optical disc 9 is used, by moving in the optical axis direction, the collimating lens 8 compensates for a spherical aberration caused by a difference in base material thickness of an optical disc, a temperature change, a wavelength change, and the like. The red light beam 62 having passed through the collimating lens 8 passes through the quarter wavelength plate 5, is reflected as diverging light by the upward reflection mirror 12, is incident on the objective lens element 143, and is converged on an information recording surface of an optical disc 10 to form a desired spot thereon. The red light beam 62 reflected by the information recording surface of the optical disc 10 passes through the objective lens element 143 again, is reflected by the upward reflection mirror 12, and passes through the quarter wavelength plate 5, the collimating lens 8, and the beam splitter 4 in order. The red light beam 62 outputted from the beam splitter 4 is reflected by the beam splitter 16, is converged on the photodetector 33 by the detection lens 32, and is detected as an optical signal by the photodetector 33.

2. Description of Objective Lens Element

Next, the objective lens element 143 according to the present embodiment will be described. FIG. 2 is a schematic cross-sectional view of the objective lens element 143 according to the first embodiment.

The objective lens element 143 according to the first embodiment is compatible with the BD standard and the DVD standard, converges blue light of a wavelength λ1 (about 400 nm) on an information recording surface through a base plate having a thickness of 0.1 mm to form a spot thereon, and converges red light of a wavelength λ2 (about 680 nm) on an information recording surface through a base plate having a thickness of 0.6 mm to form a spot thereon.

An incident side optically functional surface of the objective lens element 143 is divided into three regions each having a center on the optical axis, namely, a first region 131A including the optical axis, a ring-shaped second region 131B surrounding the first region 131A, and a ring-shaped outer region 131F surrounding the second region 131B. A stair-like diffraction structure is provided on the first region 131A. A stair-like diffraction structure that is different from that provided on the first region 131A is provided on the second region 131B. A sawtooth-like diffraction structure is provided on the outer region 131F.

Each of the first region 131A and the second region 131B is a region which contributes to formation of spots of light with two wavelengths for BD and DVD. Meanwhile, the outer region 131F is a region dedicated for BD, which contributes to formation of a spot of only light for BD.

3. Description of Diffraction Structure

Next, the diffraction structure of the objective lens element 143 according to the present embodiment will be described. FIG. 3 is a partially enlarged view for illustrating the diffraction structure of the objective lens element 143. In FIG. 3, a broken line represents the surface shape of the diffraction structure, a portion below the broken line is a lens material such as glass, and a portion above the broken line is air. It should be noted that in later-described partially enlarged views of diffraction structures, similarly, a portion below a broken line is a lens material, and a portion above a broken line is air.

The objective lens element 143 according to the present embodiment mainly includes the first region 131A, the second region 131B, and the outer region 131F.

In the objective lens element 143 according to the present embodiment, the diffraction structure formed on the first region 131A, the diffraction structure formed on the second region 131B, and the diffraction structure formed on the outer region 131F have different shapes, respectively. The diffraction structure shown in FIG. 3 is an example and may be a diffraction structure of another shape. In addition, the shapes of connection portions between the diffraction structures which are shown in FIG. 3 are examples, and the shapes of the connection portions between the diffraction structures can be set as appropriate.

Hereinafter, each region will be described.

The stair-like diffraction structure provided on the first region 131A is a periodic structure in which one cycle is composed of 4-level steps whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases. Here, the level number indicates the number of portions substantially parallel to the base surface of the objective lens element, in one cycle of the periodic structure.

The step height of the stair-like diffraction structure of the first region 131A is set such that the diffraction efficiency of +1st order diffracted light is at its maximum when the blue light of the wavelength λ1 is used and the diffraction efficiency of −1st order diffracted light is at its maximum when the red light of the wavelength λ2 is used. Here, positive and negative of diffraction order will be described. First, a direction in which light incident on the first surface is refracted is set as a reference direction. When light incident on the first surface travels in a direction in which the light is converged toward the inner side of the reference direction (the optical axis side) by diffraction at the second surface, the diffraction order is positive. In addition, when light incident on the first surface travels in a direction in which the light is converged toward the outer side of the reference direction (the outer periphery side) by diffraction, the diffraction order is negative.

One cycle of the stair-like diffraction structure provided on the first region 131A does not necessarily have to be composed of 4-level steps and may be composed of steps other than 4-level steps.

The stair-like diffraction structure provided on the second region 131B is a periodic structure in which one cycle is composed of 4-level steps whose height monotonically increases step by step as distance from the optical axis of the objective lens element increases.

The step height of the stair-like diffraction structure of the second region 131B is set such that the diffraction efficiency of +2nd order diffracted light is at its maximum when the blue light of the wavelength λ1 is used and the diffraction efficiency of −1st order diffracted light is at its maximum when the red light of the wavelength λ2 is used. One cycle of the stair-like diffraction structure provided on the second region 131B does not necessarily have to be composed of 4-level steps and may be composed of steps other than 4-level steps. In addition, in the present embodiment, a value with which the diffraction efficiency at each wavelength is at its maximum is selected as the diffraction order, but a value with which the diffraction efficiency is not at its maximum may be used as the diffraction order.

The height of the sawtooth-like diffraction structure provided on the outer region 131F is set such that the diffraction efficiency of +3rd order diffracted light is at its maximum when the light of the wavelength λ1 for BD is used. The diffraction order having the maximum diffraction efficiency may be a diffraction order other than +3rd order. However, the outer region 131F is a region dedicated for BD, and thus preferably has an aperture limiting function for adjusting an effective NA, with respect to light of a wavelength other than light for BD. In other words, it is desired that light of a wavelength other than the wavelength which is incident on the outer region 131F, does not contribute to a spot and does not return as stray light onto the photodetector 33. The stray light refers to light which is reflected by a surface of an optical disc, a recording surface of the optical disc, an optical component on an optical path, a lens surface, or the like and which influences the intensity of signal light on the photodetector.

4. Regarding Characteristic Portions

The objective lens element 143 according to the present embodiment satisfies the following condition formulas (1) and (2).

|A1−B1|<|A2−B2|  (1)

|B1|≧|B2|  (2)

Here,

A1 is the diffraction order at the first region to converge the light of the wavelength λ1 on the information recording surface,

B1 is the diffraction order at the first region to converge to the light of the wavelength λ2 on the information recording surface,

A2 is the diffraction order at the second region to converge the light of the wavelength λ1 on the information recording surface, and

B2 is the diffraction order at the second region to converge the light of the wavelength λ2 on the information recording surface.

By selecting the diffraction order at the first region 131A and the diffraction order at the second region 131B such that the condition formulas (1) and (2) are satisfied, the width of each ring zone constituting the periodic structure of the second region 131B can be made larger than the width of each ring zone constituting the periodic structure of the first region 131A. As a result, the diffraction efficiency improves. The reason why the diffraction efficiency improves will be described below.

First, the working distances for BD and DVD will be described. The working distance is the distance between the objective lens element 143 and a surface of an information storage medium.

When the working distance is shortened, there is the possibility that the objective lens element 143 and the information storage medium will be brought into contact with each other. However, in the case of BD, when the working distance is lengthened, the focal distance increases, and thus the diameter of the objective lens element has to be increased in order to obtain a desired numerical aperture. In addition, when the working distance is long, deterioration of performance caused by a manufacturing error is great. Thus, it is difficult to lengthen the working distance when BD is used.

Meanwhile, regarding DVD, sufficient performance can be ensured even when the working distance is lengthened to some extent as compared to BD. Thus, the working distance when DVD is used can be set so as to be long to some extent, in order to prevent the objective lens element 143 and the information storage medium (here, DVD) from being brought into contact with each other.

In order to lengthen the working distance for DVD, it is necessary to increase diffraction power.

In order to increase the diffraction power, it is necessary to shorten the cycle of the diffraction structure. However, when the cycle of the diffraction structure is shortened, the diffraction efficiency decreases. In particular, the diffraction efficiency of light for DVD decreases. As a result, performance required for DVD cannot be realized.

Thus, in the present embodiment, in order to prevent the diffraction efficiency of light of at least one wavelength (here, the diffraction efficiency of light for DVD) from decreasing, the diffraction order for BD at the first region 131A is set to +1st order, the diffraction order for DVD at the first region 131A is set to −1st order, the diffraction order for BD at the second region 131B is set to +2nd order, and the diffraction order for DVD at the second region 131B is set to −1st order.

As described above, in the present embodiment, the diffraction order for the light for BD at the second region 131B is made higher than the diffraction order for the light for BD at the first region 131A, and the diffraction order for the light for DVD at the second region 131B is made the same as the diffraction order for the light for DVD at the first region 131A. In other words, the difference between the diffraction order for BD and the diffraction order for DVD at the second region 131B is made greater than the difference between the diffraction order for BD and the diffraction order for DVD at the first region 131A.

When a difference in diffraction order is great, the angular difference between diffracted light for BD and diffracted light for DVD also increases. In order to converge diffracted light for BD or diffracted light for DVD, which has passed through the first region 131A and the second region 131B, to form a desired spot in a state where a difference in diffraction order is great, it is necessary to decrease the angular difference between the diffracted light for BD and the diffracted light for DVD in the second region 131B. Thus, in order to decrease the angular difference between the diffracted light for BD and the diffracted light for DVD, the diffraction power of the second region 131B is decreased.

Since the diffraction power is decreased, the cycle of the diffraction structure can be widened. By widening the cycle of the diffraction structure, decrease of the diffraction efficiency can be prevented. In the case of the present embodiment, the diffraction efficiency of the light for DVD greatly improves.

Second Embodiment

1. Description of Optical Head Device

FIG. 4 is a schematic configuration diagram of an optical head device including an objective lens element according to a second embodiment.

The optical head device according to the second embodiment is configured to be compatible with the BD standard, the DVD standard, and the CD standard. A blue light beam 61 described below corresponds to a light of the wavelength λ1. The wavelength λ1 is an example of first wavelength. Further, a red light beam described below corresponds to a light of the wavelength λ2. The wavelength λ2 is an example of second wavelength. Further, an infrared light beam 63 corresponds to the wavelength λ3. The wavelength λ3 is an example of third wavelength.

A blue light beam 61 emitted from a laser beam source 1 passes through a relay lens 2 and a three-beam grating 3, is reflected by a beam splitter 4, and then is converted into a substantially parallel light beam by a collimating lens 8. The collimating lens 8 is movable in an optical axis direction. By moving in the optical axis direction, the collimating lens 8 compensates for a spherical aberration caused by an error of a base material thickness of an optical disc and a spherical aberration caused by a difference in base material thickness between information recording surfaces in a medium having multiple information recording surfaces. The blue light beam 61 having passed through the collimating lens 8 passes through a quarter wavelength plate 5, is reflected by an upward reflection mirror 12, is incident on the objective lens element 163, and is converged on an information recording surface of an optical disc 9 to form a desired spot thereon. The blue light beam 61 reflected by the information recording surface of the optical disc 9 passes through the objective lens element 163 again, is reflected by the upward reflection mirror 12, and passes through the quarter wavelength plate 5, the collimating lens 8, and the beam splitter 4 in order. The blue light beam 61 outputted from the beam splitter 4 is reflected by a beam splitter 16, is converged on a photodetector 33 by a detection lens 32, and is detected as an optical signal by the photodetector 33.

A laser beam source 20 according to the present embodiment is a two-wavelength laser beam source which selectively emits red light and infrared light. A red light beam 62 emitted from a laser beam source 20 passes through a three-beam grating 22, the beam splitter 16, and the beam splitter 4, is incident on the collimating lens 8, and is converted into diverging light. The collimating lens 8 can adjust the parallelism of the red light beam 62 by moving in the optical axis direction. In addition, similarly to the case where the optical disc 9 is used, by moving in the optical axis direction, the collimating lens 8 compensates for a spherical aberration caused by a difference in base material thickness of an optical disc, a temperature change, a wavelength change, and the like. The red light beam 62 having passed through the collimating lens 8 passes through the quarter wavelength plate 5, is reflected as diverging light by the upward reflection mirror 12, is incident on the objective lens element 163, and is converged on an information recording surface of an optical disc 10 to form a desired spot thereon. The red light beam 62 reflected by the information recording surface of the optical disc 10 passes through the objective lens element 163 again, is reflected by the upward reflection mirror 12, and passes through the quarter wavelength plate 5, the collimating lens 8, and the beam splitter 4 in order. The red light beam 62 outputted from the beam splitter 4 is reflected by the beam splitter 16, is converged on the photodetector 33 by the detection lens 32, and is detected as an optical signal by the photodetector 33.

An infrared light beam 63 emitted from the laser beam source 20 passes through the three-beam grating 22, the beam splitter 16, and the beam splitter 4, is incident on the collimating lens 8, and is converted into diverging light. The infrared light beam 63 outputted from the collimating lens 8 passes through the quarter wavelength plate 5, is reflected by the upward reflection mirror 12, is incident on the objective lens element 163, and is converged on an information recording surface of an optical disc 11 to form a desired spot thereon. The infrared light beam 63 reflected by the information recording surface of the optical disc 11 passes through the objective lens element 163 again, is reflected by the upward reflection mirror 12, passes through the quarter wavelength plate 5, the collimating lens 8, and the beam splitter 4 in order, and is reflected by the beam splitter 16. Then, the infrared light beam 63 is converged by the detection lens 32 and detected as an optical signal by the photodetector 33.

2. Description of Objective Lens Element

Next, the objective lens element 163 according to the present embodiment will be described. FIG. 5 is a schematic cross-sectional view of the objective lens element 163 according to the present embodiment.

The objective lens element 163 according to the second embodiment is compatible with the BD standard, the DVD standard, and the CD standard, converges blue light of a wavelength λ1 (about 400 nm) on an information recording surface through a base plate having a thickness of 0.1 mm to form a spot thereon, converges red light of a wavelength λ2 (about 680 nm) on an information recording surface through a base plate having a thickness of 0.6 mm to form a spot thereon, and converges infrared light of a wavelength λ3 (about 780 nm) on an information recording surface through a base plate having a thickness of 1.2 mm to form a spot thereon.

An incident side optically functional surface of the objective lens element 163 is divided into four regions each having a center on the optical axis, namely, a first region 151A including the optical axis, a ring-shaped second region 151B surrounding the first region 151A, a ring-shaped third region 151C surrounding the second region 151B, and a ring-shaped outer region 151F surrounding the third region 151C. Stair-like diffraction structures that are different from each other are provided on the first region 151A, the second region 151B, and the third region 151C, respectively. A sawtooth-like diffraction structure is provided on the outer region 151F.

Each of the first region 151A and the second region 151B is a region which contributes to formation of spots of light with three wavelengths for BD, DVD, and CD. The third region 151C is a region which contributes to formation of spots of light with two wavelengths for BD and DVD. The outer region 151F is a region dedicated for BD, which contributes to formation of a spot of only light for BD.

3. Description of Diffraction Structure

Next, the diffraction structure of the objective lens element 163 according to the present embodiment will be described. FIG. 6 is a partially enlarged view for illustrating the diffraction structure of the objective lens element 163.

The objective lens element 163 according to the present embodiment mainly includes the first region 151A, the second region 151B, the third region 151C, and the outer region 151F.

In the objective lens element 163 according to the present embodiment, the diffraction structure formed on the first region 151A, the diffraction structure formed on the second region 151B, the diffraction structure formed on the third region 151C, and the diffraction structure formed on the outer region 151F have different shapes, respectively. The diffraction structure shown in FIG. 6 is an example and may be a diffraction structure of another shape. In addition, the shapes of connection portions between the diffraction structures which are shown in FIG. 6 are examples, and the shapes of the connection portions between the diffraction structures can be set as appropriate.

Hereinafter, each region will be described.

The stair-like diffraction structure provided on the first region 151A is a periodic structure in which one cycle is composed of 6-level steps whose height monotonically decreases step by step as distance from the optical axis of the objective lens element 163 increases. Here, the level number indicates the number of portions substantially parallel to the base surface of the objective lens element 163, in one cycle of the periodic structure.

The step height of the stair-like diffraction structure of the first region 151A is set such that the diffraction efficiency of +2nd order diffracted light is at its maximum when the blue light of the wavelength λ1 is used, the diffraction efficiency of −1st order diffracted light is at its maximum when the red light of the wavelength λ2 is used, and the diffraction efficiency of −2nd order diffracted light is at its maximum when the infrared light of the wavelength λ3 is used. Here, positive and negative of diffraction order will be described. First, a direction in which light incident on the first surface is refracted is set as a reference direction. When light incident on the first surface travels in a direction in which the light is converged toward the inner side of the reference direction (the optical axis side) by diffraction at the second surface, the diffraction order is positive.

One cycle of the stair-like diffraction structure provided on the first region 151A does not necessarily have to be composed of 6-level steps and may be composed of steps other than 6-level steps.

The stair-like diffraction structure provided on the second region 151B is a periodic structure in which one cycle is composed of 8-level steps whose height monotonically decreases step by step as distance from the optical axis of the objective lens element 163 increases. The step height of the stair-like diffraction structure of the second region 151B is set such that the diffraction efficiency of +2nd order diffracted light is at its maximum when the blue light of the wavelength λ1 is used, the diffraction efficiency of −2nd order diffracted light is at its maximum when the red light of the wavelength λ2 is used, and the diffraction efficiency of −3rd order diffracted light is at its maximum when the infrared light of the wavelength λ3 is used. One cycle of the stair-like diffraction structure provided on the second region 151B does not necessarily have to be composed of 8-level steps and may be composed of steps other than 8-level steps. In addition, in the present embodiment, a value with which the diffraction efficiency at each wavelength is at its maximum is selected as the diffraction order, but a value with which the diffraction efficiency is not at its maximum may be used as the diffraction order.

The stair-like diffraction structure provided on the third region 151C is a periodic structure in which one cycle is composed of 4-level steps whose height monotonically decreases step by step as distance from the optical axis of the objective lens element 163 increases. The step height of the stair-like diffraction structure of the third region 151C is set such that the diffraction efficiency of +1st order diffracted light is at its maximum when the blue light of the wavelength λ1 is used and the diffraction efficiency of −1st order diffracted light is at its maximum when the red light of the wavelength λ2 is used. One cycle of the stair-like diffraction structure provided on the third region 151C does not necessarily have to be composed of 4-level steps and may be composed of steps other than 4-level steps. In addition, in the present embodiment, a value with which the diffraction efficiency at each wavelength is at its maximum is selected as the diffraction order, but a value with which the diffraction efficiency is not at its maximum may be used as the diffraction order.

The step height of the sawtooth-like diffraction structure provided on the outer region 151F is set such that the diffraction efficiency of +3rd order diffracted light is at its maximum when the blue light of the wavelength λ1 is used. The diffraction order having the maximum diffraction efficiency may be a diffraction order other than +3rd order. However, the outer region 151F is a region dedicated for BD, and thus preferably has an aperture limiting function for adjusting an effective NA, with respect to light of a wavelength other than the light for BD. In other words, it is desired that light of a wavelength other than the wavelength λ1, which is incident on the outer region 151F, does not contribute to a spot and does not return as stray light onto the photodetector 33. The stray light refers to light which is reflected by a surface of an optical disc, a recording surface of the optical disc, an optical component on an optical path, a lens surface, or the like and which influences the intensity of signal light on the photodetector.

4. Regarding Characteristic Portions

The objective lens element 163 according to the present embodiment satisfies the following condition formulas (1) and (2).

|A1−B1|<|A2−B2|  (1)

|B1|≧|B2|  (2)

Here,

A1 is the diffraction order at the first region to converge the light of the wavelength λ3 on the information recording surface,

B1 is the diffraction order at the first region to converge the light of the wavelength λ3 on the information recording surface,

A2 is the diffraction order at the second region to converge the light of the wavelength λ3 on the information recording surface, and

B2 is the diffraction order at the second region to converge the light of the wavelength λ1 on the information recording surface.

By selecting the diffraction order at the first region 131A and the diffraction order at the second region 131B such that the condition formulas (1) and (2) are satisfied, the width of each ring zone of the periodic structure of the second region 151B can be made larger than the width of each ring zone of the periodic structure of the first region 151A. As a result, the diffraction efficiency improves. The reason why the diffraction efficiency improves will be described below.

First, the working distances for BD and CD will be described. The working distance is the distance between the objective lens element 163 and a surface of an information storage medium.

When the working distance is shortened, there is the possibility that the objective lens element 163 and the information storage medium will be brought into contact with each other. However, in the case of BD, when the working distance is lengthened, the focal distance increases, and thus the diameter of the objective lens element has to be increased in order to obtain a desired numerical aperture. In addition, when the working distance is long, deterioration of performance caused by a manufacturing error is great. Thus, it is difficult to lengthen the working distance when BD is used.

Meanwhile, regarding CD, sufficient performance can be ensured even when the working distance is lengthened to some extent as compared to BD. Thus, the working distance when CD is used can be set so as to be long to some extent, in order to prevent the objective lens element 163 and the information storage medium (here, CD) from being brought into contact with each other.

In order to lengthen the working distance for CD, it is necessary to increase diffraction power.

In order to increase the diffraction power, it is necessary to shorten the cycle of the diffraction structure. However, when the cycle of the diffraction structure is shortened, the diffraction efficiency decreases. In particular, the diffraction efficiency of light for CD decreases. As a result, performance required for CD cannot be realized.

Thus, in the present embodiment, in order to prevent the diffraction efficiency of light of at least one wavelength (here, the diffraction efficiency of light for CD) from decreasing, the diffraction order for BD at the first region 151A is set to +2nd order, the diffraction order for CD at the first region 151A is set to −2nd order, the diffraction order for BD at the second region 151B is set to +2nd order, and the diffraction order for CD at the second region 151B is set to −3rd order.

As described above, in the present embodiment, the diffraction order for the light for BD at the second region 151B is made the same as the diffraction order for the light for BD at the first region 151A, and the absolute value of the diffraction order for the light for CD at the second region 151B is made higher than the absolute value of the diffraction order for the light for CD at the first region 151A. In other words, the difference between the diffraction order for BD and the diffraction order for CD at the second region 151B is made greater than the difference between the diffraction order for BD and the diffraction order for CD at the first region 151A.

When a difference in diffraction order is great, the angular difference between diffracted light for BD and diffracted light for CD also increases. In order to converge diffracted light for BD or diffracted light for CD, which has passed through the first region 151A and the second region 151B, to form a desired spot in a state where a difference in diffraction order is great, it is necessary to decrease the angular difference between the diffracted light for BD and the diffracted light for CD in the second region 151B. Thus, in order to decrease the angular difference between the diffracted light for BD and the diffracted light for DVD, the diffraction power of the second region 151B is decreased.

Since the diffraction power is decreased, the cycle of the diffraction structure can be widened. By widening the cycle of the diffraction structure, decrease of the diffraction efficiency can be prevented. This is because increase of the diffraction power can be suppressed by using diffracted light of a high order.

While the diffraction order of the light (the wavelength λ1) for BD at the first region 151A is +2nd and the diffraction order of the light (the wavelength λ3) for CD at the first region 151A is −2nd, the diffraction order of the light (the wavelength λ1) for BD at the second region 151B is +2nd and the diffraction order of the light (the wavelength λ3) for CD at the second region 151B is −3rd. The absolute value of the diffraction order of the light for CD is higher at the second region 151B than at the first region 151A, and thus the diffraction angle of the light for CD is higher at the second region 151B than at the first region 151A. However, in the case of BD, the diffraction orders at the first region 151A and the second region 151B are the same.

Since the difference between the diffraction orders of the light for BD and the light for CD is increased at the second region 151B compared to the first region 151A as described above, the diffraction power for ensuring the same working distance can be small. In other words, the interval of the periodic diffraction shape of the second region 151B is wider than the interval of the diffraction shape of the first region 151A. In the present embodiment, by providing such a configuration, the diffraction efficiency of the light for BD greatly improves.

In the second embodiment, the regions which contribute to formation of spots of light with the three wavelengths for BD, DVD, and CD have been described as an example. However, the second embodiment may be applied to a region which contributes to formation of spots of light with the two wavelengths for BD and DVD. In other words, where B1 is the diffraction order at the first region with respect to the light of the wavelength λ2 and B2 is the diffraction order at the second region with respect to the light of the wavelength λ2, it suffices that a diffraction order is selected such that the condition formulas (1) and (2) are satisfied.

In other words, the first region may be formed as a region which contributes to formation of spots of light with the three wavelengths for BD, DVD, and CD, each of the second region and the third region may be formed as a region which contributes to formation of spots of light with the two wavelengths for BD and DVD, and the outer region may be formed as a region dedicated for BD, which contributes to formation of a spot of only the light for BD. In this case, it suffices that each of the diffraction structures of the second region and the third region satisfies the condition formulas (1) and (2) in the first embodiment. By so doing, it is possible to improve the diffraction efficiency for either BD or DVD.

Third Embodiment

FIGS. 7A and 7B are optical path diagrams of an objective lens according to a third embodiment of the present invention. Specifically, FIG. 7A is an optical path diagram when DVD is used, and FIG. 7B is an optical path diagram when CD is used.

The objective lens element 183 according to the present embodiment is a BD/DVD/CD compatible lens. The objective lens element 183 can converge light of a wavelength λ1 for BD on an information recording surface through a base plate having a thickness t1 to form a spot thereon, can converge light of a wavelength λ2 for DVD on an information recording surface through a base plate having a thickness t2 to form a spot thereon, and can converge light of a wavelength λ3 for CD on an information recording surface through a base plate having a thickness t3 to form a spot thereon (λ1<λ2<λ3 and t1<t2<t3). Here, the objective lens element 183 is designed such that when the light for BD is used, a spherical aberration which occurs when a parallel light beam is incident on an incident surface thereof is small. In addition, the objective lens element 183 is designed such that: when the light for DVD is used, a spherical aberration which occurs when a converging light beam is incident on the incident surface is smaller than a spherical aberration which occurs when a parallel light beam of the same wavelength is incident on the incident surface; and when the light for CD is used, a spherical aberration which occurs when a diverging light beam is incident on the incident surface is smaller than a spherical aberration which occurs when a parallel light beam of the same wavelength is incident on the incident surface.

The objective lens element 183 according to the present embodiment satisfies the following conditions.

L1<0  (3)

L2>0  (4)

Here,

L1 is the distance from the incident surface of the objective lens element to an object point of a light source of the wavelength λ2, and

L2 is the distance from the incident surface of the objective lens element to an object point of a light source of the wavelength λ3.

It should be noted that the object point distance is set so as to be positive when the object point is on the light source side (the left side in FIGS. 7A and 7B) of the incident surface of the objective lens element 183, and is set so as to be negative when the object point is on the optical disc side (the right side in FIGS. 7A and 7B) of the incident surface of the objective lens element 183.

According to the present embodiment, while the optical performance when the light for BD is used is optimized, optical performance for DVD and CD can also be ensured. Thus, the three-wavelength compatible objective lens element 183 having excellent optical performance can be realized.

EXAMPLES

Hereinafter, Examples of the present invention will be described with construction data and specific values of diffraction efficiencies. It should be noted that in each Example, a surface to which an aspheric coefficient is provided indicates a refractive optical surface having an aspherical shape or a surface (e.g., a diffractive surface) having a refraction function equal to that of an aspheric surface. The surface shape of an aspheric surface is defined by the following formula.

$X=\frac{C_jh^2}{1+\sqrt{1-\left(1+k_j\right)C_j^2h^2}}+\sum A_{j,n}h^n$

Here,

X is the distance from an on-the-aspheric-surface point at a height h relative to the optical axis to a tangential plane at the top of the aspheric surface,

h is the height relative to the optical axis,

C_j is the radius of curvature at the top of an aspheric surface of a j^th surface of a lens (C_j=1/R_j),

k_j is the conic constant of the j^th surface of the lens, and

A_j,n is the n^th-order aspheric constant of the j^th surface of the lens.

Further, a phase difference caused by a diffraction structure added to an optical surface is provided by the following formula.

φ(h)=MΣP_j,mh^2m

Here,

φ(h) is a phase function,

h is the height relative to the optical axis,

P_j,m is the 2m^th-order phase function coefficient of the j^th surface of the lens, and

M is a diffraction order.

FIGS. 8 to 12 are partially enlarged views of diffraction structures of objective lens elements according to Examples 1 to 5, respectively. Specifically, FIGS. 8 to 12 each are an enlarged view of a compatible region composed of a first region and a second region. In FIGS. 8 to 12, a portion below a diffraction shape represented by a broken line is a lens material, and a portion above the diffraction shape is air.

Example 1

Example 1 corresponds to the first embodiment. The first surface of an objective lens element according to Example 1 is divided into a first region including a symmetry axis, a second region surrounding the first region, and an outer region surrounding the second region. A 4-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases is provided on the first region of the first surface. A 4-level stair-like diffraction structure whose height monotonically increases step by step as distance from the optical axis of the objective lens element increases is provided on the second region. A sawtooth-like diffraction structure is provided on the outer region. The second surface is also divided into a first region and an outer region, and different aspheric surfaces are provided on these regions, respectively. The objective lens element according to Example 1 is a BD/DVD compatible lens. With regard to designed values for BD, the wavelength is 408 nm; the focal length is 1.30 mm; the numerical aperture (NA) is 0.86; and the protective layer thickness of an information storage medium is 0.1 mm. With regard to designed values for DVD, the wavelength is 660 nm; the focal length is 1.45 mm; the NA is 0.6; and the protective layer thickness of an information storage medium is 0.6 mm.

Tables 1 and 2 show construction data of the objective lens element according to Example 1.

	TABLE 1
		BD	DVD
	Wavelength	0.408	0.660
	Effective diameter	2.24	1.79
	NA	0.86	0.60
	Working distance (WD)	0.38	0.30
	Disc thickness (DT)	0.10	0.60
	Focal length	1.30	1.45
	First surface,_ Inner region	1	−1
	Diffraction order
	First surface, Middle region	2	−1
	Diffraction order
	First surface, Outer region	3	—
	Diffraction order
	Object point (OP)	∞	150
	Radius of
Surface	curvature at
No.	the top	Thickness	Material	Remarks
0		OP
1	0.865865	1.561992	n1	Inner region
				(Diffractive surface),
				Middle region
				(Diffractive surface),
				Outer region
				(Diffractive surface)
2	−1.383884	WD		Inner region
				(Aspheric surface),
				Outer region
				(Aspheric surface)
3	∞	DT	disc	Planar
4	∞			Planar
	Wavelength	0.408	0.660
	n1	1.52196	1.50413
	disc	1.61642	1.57815

TABLE 2
               Inner region
First surface  Diffractive surface
Region         0 mm-0.86 mm
               Aspherical constant
RD             0.86586496
k              −0.65493979
A0             0
A2             0
A4             0.043157648
A6             0.022669249
A8             −0.011628554
A10            0.04337541
A12            −0.020193608
               Phase function
P2             −286.87093
P4             34.691426
P6             −35.053222
               Middle region
First surface  Diffractive surface
Region         0.86 mm-0.90 mm
               Aspherical constant
RD             0.6889239
k              −1.123762
A0             6.93E−02
A2             0
A4             −0.72506052
A6             −0.36650133
A8             1.2488416
A10            5.7418087
A12            1.2738183
A14            −39.026355
A16            45.308213
A18            8.1660937
A20            −37.137781
A22            15.526237
               Phase function
P2             −428.05758
P4             306.21925
P6             −144.05443
               Outer region
First surface  Diffractive surface
Region         0.90 mm-1.12 mm
               Aspherical constant
RD             0.8865722
k              −0.650895
A0             7.76E−03
A2             0
A4             0.035157518
A6             0.060338707
A8             0.021728949
A10            −0.04642263
A12            −0.006640622
A14            0.002299187
A16            0.01567928
A18            0.041621567
A20            −0.038273541
               Phase function
P2             −195.47933
P4             −107.2491
               Inner region
Second surface Aspheric surface
Region         0 mm-0.51 mm
               Aspherical constant
RD             −1.383884
k              −25.670907
A0             0
A2             0
A4             0.43509192
A6             −0.82598537
A8             −0.64638874
A10            5.2735825
A12            −6.8737179
               Outer region
Second surface Aspheric surface
Region         0.51 mm-0.88 mm
               Aspherical constant
RD             −1.383884
k              −33.488315
A0             −0.000255988
A2             0
A4             0.28103759
A6             −0.40400028
A8             −0.14767444
A10            0.68091898
A12            −0.039113439
A14            −1.4783633
A16            1.8259917
A18            −0.5551804
A20            −0.41580555
A22            0.26520865

Tables 3A-3E show ring zone cycles of the stair-like step structure provided on the first region of the first surface, and cycles of steps arranged in each ring zone.

	TABLE 3A
	Cycle [μm]		Cycle [μm]
First ring zone	147.67	First step	74.02
		Second step	30.70
		Third step	23.58
		Fourth step	19.89
Second ring zone	61.64	First step	17.54
		Second step	15.88
		Third step	14.61
		Fourth step	13.61
Third ring zone	47.47	First step	12.80
		Second step	12.11
		Third step	11.53
		Fourth step	11.03
Fourth ring zone	40.14	First step	10.58
		Second step	10.19
		Third step	9.84
		Fourth step	9.53
Fifth ring zone	35.47	First step	9.24
		Second step	8.98
		Third step	8.74
		Fourth step	8.52
Sixth ring zone	32.15	First step	8.31
		Second step	8.12
		Third step	7.94
		Fourth step	7.78
Seventh ring zone	29.64	First step	7.62
		Second step	7.47
		Third step	7.34
		Fourth step	7.21
Eighth ring zone	27.64	First step	7.08
		Second step	6.96
		Third step	6.85
		Fourth step	6.74

	TABLE 3B
	Cycle [μm]		Cycle [μm]
Ninth ring zone	26.00	First step	6.64
		Second step	6.55
		Third step	6.45
		Fourth step	6.36
Tenth ring zone	24.63	First step	6.28
		Second step	6.20
		Third step	6.12
		Fourth step	6.04
Eleventh ring zone	23.45	First step	5.97
		Second step	5.90
		Third step	5.83
		Fourth step	5.76
Twelfth ring zone	22.42	First step	5.70
		Second step	5.64
		Third step	5.58
		Fourth step	5.52
Thirteenth ring zone	21.52	First step	5.46
		Second step	5.41
		Third step	5.35
		Fourth step	5.30
Fourteenth ring zone	20.71	First step	5.25
		Second step	5.20
		Third step	5.15
		Fourth step	5.10
Fifteenth ring zone	19.97	First step	5.06
		Second step	5.01
		Third step	4.97
		Fourth step	4.93
Sixteenth ring zone	19.31	First step	4.89
		Second step	4.85
		Third step	4.81
		Fourth step	4.77

	TABLE 3C
	Cycle [μm]		Cycle [μm]
Seventeenth ring zone	18.70	First step	4.73
		Second step	4.69
		Third step	4.66
		Fourth step	4.62
Eighteenth ring zone	18.13	First step	4.58
		Second step	4.55
		Third step	4.52
		Fourth step	4.48
Nineteenth ring zone	17.61	First step	4.45
		Second step	4.42
		Third step	4.39
		Fourth step	4.35
Twentieth ring zone	17.12	First step	4.32
		Second step	4.29
		Third step	4.26
		Fourth step	4.23
Twenty-first ring zone	16.66	First step	4.21
		Second step	4.18
		Third step	4.15
		Fourth step	4.12
Twenty-second ring zone	16.22	First step	4.10
		Second step	4.07
		Third step	4.04
		Fourth step	4.02
Twenty-third ring zone	15.81	First step	3.99
		Second step	3.97
		Third step	3.94
		Fourth step	3.92
Twenty-fourth ring zone	15.42	First step	3.89
		Second step	3.87
		Third step	3.84
		Fourth step	3.82

	TABLE 3D
	Cycle [μm]		Cycle [μm]
Twenty-fifth ring zone	15.05	First step	3.80
		Second step	3.77
		Third step	3.75
		Fourth step	3.73
Twenty-sixth ring zone	14.70	First step	3.71
		Second step	3.69
		Third step	3.66
		Fourth step	3.64
Twenty-seventh ring zone	14.36	First step	3.62
		Second step	3.60
		Third step	3.58
		Fourth step	3.56
Twenty-eighth ring zone	14.04	First step	3.54
		Second step	3.52
		Third step	3.50
		Fourth step	3.48
Twenty-ninth ring zone	13.73	First step	3.46
		Second step	3.44
		Third step	3.42
		Fourth step	3.40
Thirtieth ring zone	13.43	First step	3.39
		Second step	3.37
		Third step	3.35
		Fourth step	3.33
Thirty-first ring zone	13.14	First step	3.31
		Second step	3.29
		Third step	3.28
		Fourth step	3.26
Thirty-second ring zone	12.86	First step	3.24
		Second step	3.22
		Third step	3.21
		Fourth step	3.19

	TABLE 3E
	Cycle [μm]		Cycle [μm]
Thirty-third ring zone	12.60	First step	3.17
		Second step	3.16
		Third step	3.14
		Fourth step	3.12

On the first region of Example 1, one ring zone cycle is composed of consecutive 4-level stair-like steps whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases. Each ring zone cycle in Tables 3A-3E indicate the width of a ring zone in a radial direction (in a direction perpendicular to the optical axis) as indicated by an arrow in FIG. 8. On the first region, a first ring zone, a second ring zone, a third ring zone, . . . , and a thirty-third ring zone are provided in order from the optical axis side toward the outer periphery side of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow in FIG. 8. In each ring zone, the steps are referred to as a first step, a second step, a third step, and a fourth step in order from the optical axis side toward the outer periphery side.

Table 4 shows ring zone cycles of the stair-like step structure provided on the second region of the first surface, and cycles of steps arranged in each ring zone.

	TABLE 4
	Cycle [μm]		Cycle [μm]
First ring zone	17.06	First step	4.30
		Second step	4.28
		Third step	4.25
		Fourth step	4.22
Second ring zone	16.62	First step	4.20
		Second step	4.17
		Third step	4.14
		Fourth step	4.11

On the second region of Example 1, one ring zone cycle is composed of consecutive 4-level stair-like steps whose height monotonically increases step by step as distance from the optical axis of the objective lens element increases. Each ring zone cycle in Table 4 indicates the width of a ring zone in the radial direction (in the direction perpendicular to the optical axis) as indicated by an arrow in FIG. 8. On the second region, a first ring zone and a second ring zone are provided in order from the optical axis side toward the outer periphery side of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow in FIG. 8. In each ring zone, the steps are referred to as a first step, a second step, a third step, and a fourth step in order from the optical axis side toward the outer periphery side.

Table 5 shows ring zone cycles of the sawtooth-like diffraction structure provided on the outer region of the first surface.

TABLE 5
Cycle [μm]
First ring zone   40.40
Second ring zone  24.83
Third ring zone   23.62
Fourth ring zone  22.53
Fifth ring zone   21.55
Sixth ring zone   20.65
Seventh ring zone 19.84
Eighth ring zone  19.10
Ninth ring zone   18.41

On the outer region, a first ring zone, a second ring zone, a third ring zone, a fourth ring zone, . . . , a ninth ring zone are provided in order from the optical axis side toward the outer periphery side of the objective lens element.

Tables 6A-6E show step heights of the stair-like diffraction structure provided on the first region of the first surface. In one cycle of the stair-like diffraction structure, the height of each of the first to third steps is set such that a phase difference of 1.25 wavelengths is provided to light of the designed wavelength for BD, and the height of the fourth step is set such that a phase difference of 3.75 wavelengths is provided in the opposite direction.

	TABLE 6A
	Depth [μm]
	First ring zone	First step	0.97848
		Second step	0.97988
		Third step	0.98128
		Fourth step	2.94806
	Second ring zone	First step	0.98411
		Second step	0.98554
		Third step	0.98699
		Fourth step	2.96531
	Third ring zone	First step	0.98990
		Second step	0.99138
		Third step	0.99286
		Fourth step	2.98305
	Fourth ring zone	First step	0.99585
		Second step	0.99738
		Third step	0.99890
		Fourth step	3.00131
	Fifth ring zone	First step	1.00198
		Second step	1.00354
		Third step	1.00511
		Fourth step	3.02006
	Sixth ring zone	First step	1.00828
		Second step	1.00988
		Third step	1.01150
		Fourth step	3.03938
	Seventh ring zone	First step	1.01476
		Second step	1.01640
		Third step	1.01806
		Fourth step	3.05921
	Eighth ring zone	First step	1.02143
		Second step	1.02311
		Third step	1.02483
		Fourth step	3.07965

	TABLE 6B
	Depth [μm]
	Ninth ring zone	First step	1.02828
		Second step	1.03003
		Third step	1.03178
		Fourth step	3.10065
	Tenth ring zone	First step	1.03534
		Second step	1.03713
		Third step	1.03894
		Fourth step	3.12229
	Eleventh ring zone	First step	1.04260
		Second step	1.04444
		Third step	1.04630
		Fourth step	3.14453
	Twelfth ring zone	First step	1.05006
		Second step	1.05198
		Third step	1.05389
		Fourth step	3.16744
	Thirteenth ring zone	First step	1.05776
		Second step	1.05971
		Third step	1.06169
		Fourth step	3.19103
	Fourteenth ring zone	First step	1.06568
		Second step	1.06770
		Third step	1.06973
		Fourth step	3.21533
	Fifteenth ring zone	First step	1.07384
		Second step	1.07591
		Third step	1.07800
		Fourth step	3.24034
	Sixteenth ring zone	First step	1.08224
		Second step	1.08438
		Third step	1.08653
		Fourth step	3.26610

	TABLE 6C
	Depth [μm]
	Seventeenth ring zone	First step	1.09089
		Second step	1.09309
		Third step	1.09530
		Fourth step	3.29261
	Eighteenth ring zone	First step	1.09980
		Second step	1.10206
		Third step	1.10435
		Fourth step	3.31999
	Nineteenth ring zone	First step	1.10898
		Second step	1.11131
		Third step	1.11368
		Fourth step	3.34815
	Twentieth ring zone	First step	1.11844
		Second step	1.12085
		Third step	1.12329
		Fourth step	3.37718
	Twenty-first ring zone	First step	1.12820
		Second step	1.13068
		Third step	1.13319
		Fourth step	3.40710
	Twenty-second ring zone	First step	1.13825
		Second step	1.14081
		Third step	1.14339
		Fourth step	3.43796
	Twenty-third ring zone	First step	1.14861
		Second step	1.15125
		Third step	1.15390
		Fourth step	3.46976
	Twenty-fourth ring zone	First step	1.15929
		Second step	1.16200
		Third step	1.16474
		Fourth step	3.50250

	TABLE 6D
	Depth [μm]
	Twenty-fifth ring zone	First step	1.17029
		Second step	1.17309
		Third step	1.17591
		Fourth step	3.53629
	Twenty-sixth ring zone	First step	1.18163
		Second step	1.18451
		Third step	1.18741
		Fourth step	3.57105
	Twenty-seventh ring zone	First step	1.19330
		Second step	1.19628
		Third step	1.19928
		Fourth step	3.60686
	Twenty-eighth ring zone	First step	1.20533
		Second step	1.20839
		Third step	1.21148
		Fourth step	3.64373
	Twenty-ninth ring zone	First step	1.21771
		Second step	1.22086
		Third step	1.22404
		Fourth step	3.68171
	Thirtieth ring zone	First step	1.23045
		Second step	1.23369
		Third step	1.23696
		Fourth step	3.72075
	Thirty-first ring zone	First step	1.24356
		Second step	1.24689
		Third step	1.25025
		Fourth step	3.76088
	Thirty-second ring zone	First step	1.25704
		Second step	1.26046
		Third step	1.26391
		Fourth step	3.80216

	TABLE 6E
	Depth [μm]
	Thirty-third ring zone	First step	1.27088
		Second step	1.27440
		Third step	1.27794

Table 7 shows step heights of the stair-like diffraction structure provided on the second region of the first surface. In one cycle of the stair-like diffraction structure, the height of each of the first to third steps is set such that a phase difference of 0.25 wavelength is provided to light of the designed wavelength for DVD, and the height of the fourth step is set such that a phase difference of 0.75 wavelength is provided in the opposite direction.

	TABLE 7
	Depth [μm]
	First ring zone	First step	0.43228
		Second step	0.43394
		Third step	0.43562
		Fourth step	1.31194
	Second ring zone	First step	0.43903
		Second step	0.44076
		Third step	0.44252
		Fourth step	1.33292

Table 8 shows step heights of the sawtooth-like diffraction structure provided on the outer region of the first surface. The step heights of the sawtooth-like diffraction structure are set such that a phase difference of 3 wavelengths is provided to the light of the designed wavelength for BD, and +3rd order diffracted light is used.

TABLE 8
Depth [μm]
First ring zone   3.36561
Second ring zone  3.45768
Third ring zone   3.54477
Fourth ring zone  3.61974
Fifth ring zone   3.67212
Sixth ring zone   3.68745
Seventh ring zone 3.64692
Eighth ring zone  3.52776
Ninth ring zone   3.30591

Table 9 shows diffraction efficiencies at the thirty-third ring zone of the first region and at the second ring zone of the second region. The second ring zone of the second region is an outermost region which contributes to formation of a spot of the light for DVD in the present example.

	TABLE 9
	Diffraction
	efficiency(%)
BD	Inner region	First ring zone	80
		Thirty-third ring zone	61
	Middle region	Second ring zone	29
DVD	Inner region	First ring zone	78
		Thirty-third ring zone	58
	Middle region	Second ring zone	72

The ring zone cycle of the thirty-third ring zone of the first region is about 13 μm, and the diffraction efficiency of the light for DVD is about 58%. Meanwhile, the ring zone cycle of the first ring zone of the first region is about 148 μm, and the diffraction efficiency of the light for DVD is about 78%. Thus, the diffraction efficiency of the light for DVD at the thirty-third ring zone of the first region is much lower than the diffraction efficiency at the first ring zone. When the present invention is not applied, the diameter of a spot on a recording surface is increased with this diffraction efficiency, and recording/reproducing performance of DVD deteriorates.

In contrast, the ring zone cycle of the second ring zone of the second region is about 17 μm. The diffraction efficiency of the light for DVD at the second ring zone of the second region is about 72% and is greatly improved as compared to that at the thirty-third ring zone of the first region. Thus, enlargement of a beam spot formed when the light for DVD is incident is suppressed. As a result, the recording/reproducing performance improves.

Example 2

Example 2 corresponds to the second embodiment. The first surface of an objective lens element according to Example 2 is divided into a first region including a symmetry axis, a second region surrounding the first region, a third region surrounding the second region, and an outer region surrounding the third region. A 6-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases is provided on the first region of the first surface. An 8-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases is provided on the second region. A 4-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases is provided on the third region. A sawtooth-like diffraction structure is provided on the outer region. The second surface is an aspheric surface. The objective lens element according to Example 2 is a BD/DVD/CD compatible lens. With regard to designed values for BD, the wavelength is 405 nm; the focal length is 1.20 mm; and the protective layer thickness of an information storage medium is 0.085 mm. With regard to designed values for DVD, the wavelength is 650 nm; the focal length is 1.45 mm; and the protective layer thickness of an information storage medium is 0.6 mm. With regard to designed values for CD, the wavelength is 780 nm; the focal length is 1.64 mm; and the protective layer thickness of an information storage medium is 1.2 mm.

Tables 10 and 11 show construction data of the objective lens element according to Example 2.

	TABLE 10
		BD	DVD	CD
	Wavelength	0.405	0.650	0.780
	Effective diameter	1.66	1.64	1.66
	Working distance (WD)	0.47	0.44	0.30
	Disc thickness (DT)	0.085	0.60	1.20
	Focal length	1.20	1.45	1.64
	First surface, First region	2	−1	−2
	Diffraction order
	First surface, Second region	2	−2	−3
	Diffraction order
	First surface, Third region	1	−1	—
	Diffraction order
	First surface, Outer region	3	—	—
	Diffraction order
	Object point (OP)	∞	∞	400
	Radius of
Surface	curvature at
No.	the top	Thickness	Material	Remarks
0		OP
1	0.80094682	1.16723	n1	First region
				(Diffractive surface),
				Second region
				(Diffractive surface),
				Third region
				(Diffractive surface),
				Outer region
				(Diffractive surface)
2	−2.36979	WD		Aspheric surface
3	∞	DT	disc	Planar
4	∞			Planar
	Wavelength	0.405	0.650	0.780
	n1	1.52550	1.50746	1.50385
	disc	1.61913	1.57881	1.57180

TABLE 11
               First region
First surface  Diffractive surface
Region         0 mm-0.76 mm
               Aspherical constant
RD             0.80094682
k              −0.99630763
A0             0
A2             0
A4             0.15911155
A6             0.10260604
A8             0.012767725
A10            −1.9922639
A12            9.054756
A14            −14.670648
A16            8.6186121
               Phase function
P2             −307.57297
P4             −18.255432
P6             13.625204
               Second region
First surface  Diffractive surface
Region         0.76 mm-0.82 mm
               Aspherical constant
RD             0.38100041
k              −1.8508273
A0             −1.21E−01
A2             0
A4             −0.035003385
A6             0.99082251
A8             −1.8892425
A10            −2.4967727
A12            17.673196
A14            −25.890832
A16            12.211077
A18            0.7497486
               Phase function
P2             −291.36683
P4             −215.20791
P6             306.73161
               Aspherical
Second surface constant
RD             −2.36979
k              16.74851
A0             0
A2             0
A4             1.5488076
A6             −10.726282
A8             36.424214
A10            553.29212
A12            −7238.9707
A14            37431.492
A16            −93807.005
A18            94316.616

Tables 12A-12F show ring zone cycles of the stair-like step structure provided on the first region of the first surface, and cycles of steps arranged in each ring zone.

	TABLE 12A
	Cycle [μm]		Cycle [μm]
First ring zone	142.39	First step	58.34
		Second step	24.16
		Third step	18.53
		Fourth step	15.62
		Fifth step	13.76
		Sixth step	12.43
Second ring zone	59.05	First step	11.43
		Second step	10.64
		Third step	9.99
		Fourth step	9.44
		Fifth step	8.98
		Sixth step	8.58
Third ring zone	45.24	First step	8.22
		Second step	7.91
		Third step	7.63
		Fourth step	7.38
		Fifth step	7.15
		Sixth step	6.94
Fourth ring zone	38.08	First step	6.75
		Second step	6.57
		Third step	6.41
		Fourth step	6.26
		Fifth step	6.11
		Sixth step	5.98
Fifth ring zone	33.50	First step	5.86
		Second step	5.74
		Third step	5.63
		Fourth step	5.52
		Fifth step	5.42
		Sixth step	5.33

	TABLE 12B
	Cycle [μm]		Cycle [μm]
Sixth ring zone	30.25	First step	5.24
		Second step	5.16
		Third step	5.07
		Fourth step	5.00
		Fifth step	4.92
		Sixth step	4.85
Seventh ring zone	27.78	First step	4.78
		Second step	4.72
		Third step	4.66
		Fourth step	4.60
		Fifth step	4.54
		Sixth step	4.48
Eighth ring zone	25.83	First step	4.43
		Second step	4.38
		Third step	4.33
		Fourth step	4.28
		Fifth step	4.23
		Sixth step	4.19
Ninth ring zone	24.23	First step	4.14
		Second step	4.10
		Third step	4.06
		Fourth step	4.02
		Fifth step	3.98
		Sixth step	3.94
Tenth ring zone	22.90	First step	3.90
		Second step	3.87
		Third step	3.83
		Fourth step	3.80
		Fifth step	3.76
		Sixth step	3.73

	TABLE 12C
	Cycle [μm]		Cycle [μm]
Eleventh ring zone	21.76	First step	3.70
		Second step	3.67
		Third step	3.64
		Fourth step	3.61
		Fifth step	3.58
		Sixth step	3.55
Twelfth ring zone	20.77	First step	3.53
		Second step	3.50
		Third step	3.47
		Fourth step	3.45
		Fifth step	3.42
		Sixth step	3.40
Thirteenth ring zone	19.91	First step	3.38
		Second step	3.35
		Third step	3.33
		Fourth step	3.31
		Fifth step	3.28
		Sixth step	3.26
Fourteenth ring zone	19.15	First step	3.24
		Second step	3.22
		Third step	3.20
		Fourth step	3.18
		Fifth step	3.16
		Sixth step	3.14
Fifteenth ring zone	18.47	First step	3.12
		Second step	3.10
		Third step	3.09
		Fourth step	3.07
		Fifth step	3.05
		Sixth step	3.03

	TABLE 12D
	Cycle [μm]		Cycle [μm]
Sixteenth ring zone	17.85	First step	3.02
		Second step	3.00
		Third step	2.98
		Fourth step	2.97
		Fifth step	2.95
		Sixth step	2.94
Seventeenth ring zone	17.30	First step	2.92
		Second step	2.90
		Third step	2.89
		Fourth step	2.88
		Fifth step	2.86
		Sixth step	2.85
Eighteenth ring zone	16.79	First step	2.83
		Second step	2.82
		Third step	2.80
		Fourth step	2.79
		Fifth step	2.78
		Sixth step	2.77
Nineteenth ring zone	16.33	First step	2.75
		Second step	2.74
		Third step	2.73
		Fourth step	2.71
		Fifth step	2.70
		Sixth step	2.69
Twentieth ring zone	15.90	First step	2.68
		Second step	2.67
		Third step	2.66
		Fourth step	2.64
		Fifth step	2.63
		Sixth step	2.62

	TABLE 12E
	Cycle [μm]		Cycle [μm]
Twenty-first ring zone	15.51	First step	2.61
		Second step	2.60
		Third step	2.59
		Fourth step	2.58
		Fifth step	2.57
		Sixth step	2.56
Twenty-second ring zone	15.15	First step	2.55
		Second step	2.54
		Third step	2.53
		Fourth step	2.52
		Fifth step	2.51
		Sixth step	2.50
Twenty-third ring zone	14.81	First step	2.49
		Second step	2.48
		Third step	2.47
		Fourth step	2.46
		Fifth step	2.45
		Sixth step	2.45
Twenty-fourth ring zone	14.49	First step	2.44
		Second step	2.43
		Third step	2.42
		Fourth step	2.41
		Fifth step	2.40
		Sixth step	2.39
Twenty-fifth ring zone	14.20	First step	2.39
		Second step	2.38
		Third step	2.37
		Fourth step	2.36
		Fifth step	2.35
		Sixth step	2.35

	TABLE 12F
	Cycle [μm]		Cycle [μm]
Twenty-sixth ring zone	13.92	First step	2.34
		Second step	2.33
		Third step	2.32
		Fourth step	2.32
		Fifth step	2.31
		Sixth step	2.30
Twenty-seventh ring zone	13.67	First step	2.30
		Second step	2.29
		Third step	2.28
		Fourth step	2.27
		Fifth step	2.27
		Sixth step	2.26
Twenty-eighth ring zone	13.42	First step	2.25
		Second step	2.25
		Third step	2.24
		Fourth step	2.23
		Fifth step	2.23

On the first region of Example 2, one ring zone cycle is composed of consecutive 6-level stair-like steps whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases. Each ring zone cycle in Tables 12A-12F indicate the width of a ring zone in a radial direction (in a direction perpendicular to the optical axis) as indicated by an arrow in FIG. 9. On the first region, a first ring zone, a second ring zone, a third ring zone, . . . , and a twenty-eighth ring zone are provided in order from the optical axis side toward the outer periphery side of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow in FIG. 9. In each ring zone, the steps are referred to as a first step, a second step, a third step, . . . , and a sixth step in order from the optical axis side toward the outer periphery side.

Table 13 shows ring zone cycles of the stair-like step structure provided on the second region of the first surface, and cycles of steps arranged in each ring zone.

	TABLE 13
	Cycle [μm]		Cycle [μm]
First ring zone	19.67	First step	2.25
		Second step	2.26
		Third step	2.28
		Fourth step	2.30
		Fifth step	2.31
		Sixth step	2.33
		Seventh step	2.35
		Eighth step	2.37
Second ring zone	19.91	First step	2.40
		Second step	2.42
		Third step	2.44
		Fourth step	2.47
		Fifth step	2.50
		Sixth step	2.53
		Seventh step	2.56
		Eighth step	2.59
Third ring zone	22.25	First step	2.63
		Second step	2.67
		Third step	2.71
		Fourth step	2.75
		Fifth step	2.80
		Sixth step	2.85
		Seventh step	2.90
		Eighth step	2.96

On the second region of Example 2, one ring zone cycle is composed of consecutive 8-level stair-like steps whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases. Each ring zone cycle in Table 13 indicates the width of a ring zone in the radial direction (in the direction perpendicular to the optical axis) as indicated by an arrow in FIG. 9. On the second region, a first ring zone, a second ring zone, and a third ring zone are provided in order from the optical axis side toward the outer periphery side of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow in FIG. 9. In each ring zone, the steps are referred to as a first step, a second step, a third step, . . . , and an eighth step in order from the optical axis side toward the outer periphery side.

Tables 14A-14F show step heights of the stair-like diffraction structure provided on the first region of Example 2. In one cycle of the stair-like diffraction structure, the height of each of the first to fifth steps is set such that a phase difference of 1.33 wavelengths is provided to light of the designed wavelength for BD, and the height of the sixth step is set such that a phase difference of 6.65 wavelengths is provided in the opposite direction.

	TABLE 14A
	Depth [μm]
	First ring zone	First step	1.02853
		Second step	1.02972
		Third step	1.03090
		Fourth step	1.03210
		Fifth step	1.03331
		Sixth step	5.17257
	Second ring zone	First step	1.03574
		Second step	1.03695
		Third step	1.03818
		Fourth step	1.03942
		Fifth step	1.04066
		Sixth step	5.20950
	Third ring zone	First step	1.04315
		Second step	1.04442
		Third step	1.04569
		Fourth step	1.04695
		Fifth step	1.04823
		Sixth step	5.24755
	Fourth ring zone	First step	1.05079
		Second step	1.05210
		Third step	1.05339
		Fourth step	1.05470
		Fifth step	1.05602
		Sixth step	5.28668
	Fifth ring zone	First step	1.05866
		Second step	1.05999
		Third step	1.06133
		Fourth step	1.06268
		Fifth step	1.06403
		Sixth step	5.32693

	TABLE 14B
	Depth [μm]
	Sixth ring zone	First step	1.06675
		Second step	1.06812
		Third step	1.06949
		Fourth step	1.07088
		Fifth step	1.07227
		Sixth step	5.36832
	Seventh ring zone	First step	1.07506
		Second step	1.07648
		Third step	1.07789
		Fourth step	1.07932
		Fifth step	1.08074
		Sixth step	5.41085
	Eighth ring zone	First step	1.08362
		Second step	1.08506
		Third step	1.08651
		Fourth step	1.08798
		Fifth step	1.08945
		Sixth step	5.45464
	Ninth ring zone	First step	1.09241
		Second step	1.09389
		Third step	1.09538
		Fourth step	1.09689
		Fifth step	1.09839
		Sixth step	5.49956
	Tenth ring zone	First step	1.10143
		Second step	1.10296
		Third step	1.10451
		Fourth step	1.10604
		Fifth step	1.10760
		Sixth step	5.54581

	TABLE 14C
	Depth [μm]
	Eleventh ring zone	First step	1.11074
		Second step	1.11231
		Third step	1.11389
		Fourth step	1.11548
		Fifth step	1.11708
		Sixth step	5.59347
	Twelfth ring zone	First step	1.12031
		Second step	1.12193
		Third step	1.12357
		Fourth step	1.12521
		Fifth step	1.12686
		Sixth step	5.64266
	Thirteenth ring zone	First step	1.13020
		Second step	1.13188
		Third step	1.13357
		Fourth step	1.13528
		Fifth step	1.13698
		Sixth step	5.69351
	Fourteenth ring zone	First step	1.14043
		Second step	1.14218
		Third step	1.14393
		Fourth step	1.14569
		Fifth step	1.14746
		Sixth step	5.74623
	Fifteenth ring zone	First step	1.15105
		Second step	1.15286
		Third step	1.15467
		Fourth step	1.15651
		Fifth step	1.15835
		Sixth step	5.80108

	TABLE 14D
	Depth [μm]
	Sixteenth ring zone	First step	1.16208
		Second step	1.16396
		Third step	1.16586
		Fourth step	1.16777
		Fifth step	1.16969
		Sixth step	5.85814
	Seventeenth ring zone	First step	1.17359
		Second step	1.17555
		Third step	1.17752
		Fourth step	1.17952
		Fifth step	1.18153
		Sixth step	5.91772
	Eighteenth ring zone	First step	1.18558
		Second step	1.18764
		Third step	1.18972
		Fourth step	1.19180
		Fifth step	1.19390
		Sixth step	5.98010
	Nineteenth ring zone	First step	1.19815
		Second step	1.20030
		Third step	1.20247
		Fourth step	1.20466
		Fifth step	1.20686
		Sixth step	6.04542
	Twentieth ring zone	First step	1.21131
		Second step	1.21358
		Third step	1.21584
		Fourth step	1.21814
		Fifth step	1.22045
		Sixth step	6.11394

	TABLE 14E
	Depth [μm]
	Twenty-first ring zone	First step	1.22513
		Second step	1.22751
		Third step	1.22991
		Fourth step	1.23232
		Fifth step	1.23474
		Sixth step	6.18599
	Twenty-second ring zone	First step	1.23968
		Second step	1.24218
		Third step	1.24470
		Fourth step	1.24725
		Fifth step	1.24982
		Sixth step	6.26203
	Twenty-third ring zone	First step	1.25502
		Second step	1.25767
		Third step	1.26034
		Fourth step	1.26303
		Fifth step	1.26575
		Sixth step	6.34248
	Twenty-fourth ring zone	First step	1.27128
		Second step	1.27408
		Third step	1.27692
		Fourth step	1.27980
		Fifth step	1.28269
		Sixth step	6.42813
	Twenty-fifth ring zone	First step	1.28860
		Second step	1.29161
		Third step	1.29465
		Fourth step	1.29773
		Fifth step	1.30085
		Sixth step	6.52004

	TABLE 14F
	Depth [μm]
	Twenty-sixth ring zone	First step	1.30721
		Second step	1.31046
		Third step	1.31375
		Fourth step	1.31708
		Fifth step	1.32048
		Sixth step	6.61961
	Twenty-seventh ring zone	First step	1.32741
		Second step	1.33097
		Third step	1.33459
		Fourth step	1.33825
		Fifth step	1.34200
		Sixth step	6.72898
	Twenty-eighth ring zone	First step	1.34968
		Second step	1.35362
		Third step	1.35765
		Fourth step	1.36175
		Fifth step	1.36594

Table 15 shows step heights of the stair-like diffraction structure provided on the second region of Example 2. In one cycle of the stair-like diffraction structure, the height of each of the first to seventh steps is set such that a phase difference of 1.25 wavelengths is provided to the light of the designed wavelength for BD, and the height of the eighth step is set such that a phase difference of 8.75 wavelengths is provided in the opposite direction.

	TABLE 15
	Depth [μm]
	First ring zone	First step	1.31640
		Second step	1.32066
		Third step	1.32509
		Fourth step	1.32966
		Fifth step	1.33443
		Sixth step	1.33936
		Seventh step	1.34451
		Eighth step	9.44913
	Second ring zone	First step	1.35549
		Second step	1.36134
		Third step	1.36748
		Fourth step	1.37393
		Fifth step	1.38069
		Sixth step	1.38783
		Seventh step	1.39535
		Eighth step	9.82310
	Third ring zone	First step	1.41174
		Second step	1.42069
		Third step	1.43023
		Fourth step	1.44041
		Fifth step	1.45131
		Sixth step	1.46303
		Seventh step	1.47563
		Eighth step	10.42484

It should be noted that although not shown, the 4-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases is provided on the third region of Example 2. Further, the sawtooth-like diffraction structure is provided on the outer region of Example 2.

Table 16 shows diffraction efficiencies at the twenty-eighth ring zone of the first region and at the third ring zone of the second region. The third ring zone of the second region is an outermost region which contributes to formation of a spot of light for CD in the present example.

	TABLE 16
	Diffraction
	efficiency (%)
BD	Inner region	First ring zone	67
		Twenty-ninth ring zone	44
	Middle region	Third ring zone	60
DVD	Inner region	First ring zone	71
		Twenty-ninth ring zone	56
	Middle region	Third ring zone	34
CD	Inner region	First ring zone	65
		Twenty-ninth ring zone	30
	Middle region	Third ring zone	19

The ring zone cycle of the twenty-eighth ring zone of the first region is about 13 μm, and the diffraction efficiency of the light for BD is about 44%. Meanwhile, the ring zone cycle of the first ring zone of the first region is about 142 μm, and the diffraction efficiency of the light for BD is about 67%. Thus, the diffraction efficiency of the light for BD at the twenty-eighth ring zone of the first region is much lower than the diffraction efficiency at the first ring zone. When the present invention is not applied, an amount of light in reproducing/recording on BD is insufficient with this diffraction efficiency.

In contrast, the ring zone cycle of the third ring zone of the second region is about 22 μm. The diffraction efficiency of the light for BD at the third ring zone of the second region is about 60% and is greatly improved as compared to the diffraction efficiency at the twenty-eighth ring zone of the first region. Thus, insufficiency of the light amount in reproducing/recording on BD is suppressed.

Example 3

The first surface of an objective lens element according to Example 3 is divided into a first region including a symmetry axis, a second region surrounding the first region, a third region surrounding the second region, and an outer region surrounding the third region. A 7-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases is provided on the first region of the first surface. A 9-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases is provided on the second region. A 4-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases is provided on the third region. A sawtooth-like diffraction structure is provided on the outer region. The second surface is an aspheric surface. The objective lens element according to Example 3 is a BD/DVD/CD compatible lens. With regard to designed values for BD, the wavelength is 408 nm; the focal length is 1.30 mm; and the protective layer thickness of an information storage medium is 0.087 mm. With regard to designed values for DVD, the wavelength is 660 nm; the focal length is 1.57 mm; and the protective layer thickness of an information storage medium is 0.6 mm. With regard to designed values for CD, the wavelength is 785 nm; the focal length is 1.75 mm; and the protective layer thickness of an information storage medium is 1.2 mm.

Tables 17 and 18 show construction data of the objective lens element according to Example 3.

	TABLE 17
		BD	DVD	CD
	Wavelength	0.408	0.660	0.785
	Effective diameter	1.66	1.66	1.66
	Working distance (WD)	0.49	0.46	0.30
	Disc thickness (DT)	0.087	0.60	1.20
	Focal length	1.30	1.57	1.75
	First surface, First region	1	−2	−3
	Diffraction order
	First surface, Second region	1	−3	−4
	Diffraction order
	First surface, Third region	1	−1	—
	Diffraction order
	First surface, _Outer region	3	—	—
	Diffraction order
	Object point (OP)	∞	∞	∞
	Radius of
Surface	curvature at
No.	the top	Thickness	Material	Remarks
0		OP
1	0.80042572	1.284183	n1	First region
				(Diffractive surface),
				Second region
				(Diffractive surface),
				Third region
				(Diffractive surface),
				Outer region
				(Diffractive surface)
2	−2.757568	WD		Aspheric surface
3	∞	DT	disc	Planar
4	∞			Planar
	Wavelength	0.408	0.660	0.785
	n1	1.52505	1.50711	1.50374
	disc	1.61805	1.57812	1.57160

TABLE 18
               First region
First surface  Diffractive surface
Region         0 mm-0.71 mm
               Aspherical constant
RD             0.80042572
k              −0.97591321
A0             0
A2             0
A4             0.20000627
A6             −1.0643006
A8             8.1610405
A10            −35.521826
A12            88.358799
A14            −115.97361
A16            62.150855
               Phase function
P2             −224.64389
P4             −50.652842
P6             −2.6251858
               Second region
First surface  Diffractive surface
Region         0.71 mm-0.83 mm
               Aspherical constant
RD             0.84517215
k              −0.68523888
A0             −8.41E−03
A2             0
A4             0.28575967
A6             −0.42240361
A8             1.5410588
A10            −4.7704736
A12            6.8216812
A14            −4.5550413
A16            1.3664987
               Phase function
P2             −226.34345
P4             −171.83969
P6             245.90644
Second surface Aspherical constant
RD             −2.757568
k              4.138653
A0             0
A2             0
A4             0.68975206
A6             −8.8967286
A8             82.669118
A10            −419.18394
A12            1083.2938
A14            −1248.8704
A16            480.15496

Tables 19A-19E show ring zone cycles of the stair-like step structure provided on the first region of the first surface, and cycles of steps arranged in each ring zone.

	TABLE 19A
	Cycle [μm]		Cycle [μm]
First ring zone	166.13	First step	63.18
		Second step	26.13
		Third step	20.02
		Fourth step	16.86
		Fifth step	14.83
		Sixth step	13.39
		Seventh step	12.30
Second ring zone	68.33	First step	11.43
		Second step	10.72
		Third step	10.13
		Fourth step	9.62
		Fifth step	9.18
		Sixth step	8.79
		Seventh step	8.45
Third ring zone	51.95	First step	8.14
		Second step	7.87
		Third step	7.61
		Fourth step	7.38
		Fifth step	7.17
		Sixth step	6.98
		Seventh step	6.80
Fourth ring zone	43.41	First step	6.63
		Second step	6.47
		Third step	6.32
		Fourth step	6.19
		Fifth step	6.06
		Sixth step	5.93
		Seventh step	5.82

	TABLE 19B
	Cycle [μm]		Cycle [μm]
Fifth ring zone	37.91	First step	5.71
		Second step	5.60
		Third step	5.50
		Fourth step	5.41
		Fifth step	5.32
		Sixth step	5.23
		Seventh step	5.15
Sixth ring zone	33.99	First step	5.07
		Second step	4.99
		Third step	4.92
		Fourth step	4.85
		Fifth step	4.78
		Sixth step	4.72
		Seventh step	4.65
Seventh ring zone	30.99	First step	4.59
		Second step	4.54
		Third step	4.48
		Fourth step	4.42
		Fifth step	4.37
		Sixth step	4.32
		Seventh step	4.27
Eighth ring zone	28.61	First step	4.22
		Second step	4.18
		Third step	4.13
		Fourth step	4.09
		Fifth step	4.04
		Sixth step	4.00
		Seventh step	3.96

	TABLE 19C
	Cycle [μm]		Cycle [μm]
Ninth ring zone	26.66	First step	3.92
		Second step	3.88
		Third step	3.84
		Fourth step	3.81
		Fifth step	3.77
		Sixth step	3.74
		Seventh step	3.70
Tenth ring zone	25.02	First step	3.67
		Second step	3.64
		Third step	3.60
		Fourth step	3.57
		Fifth step	3.54
		Sixth step	3.51
		Seventh step	3.48
Eleventh ring zone	23.61	First step	3.45
		Second step	3.43
		Third step	3.40
		Fourth step	3.37
		Fifth step	3.35
		Sixth step	3.32
		Seventh step	3.29
Twelfth ring zone	22.39	First step	3.27
		Second step	3.25
		Third step	3.22
		Fourth step	3.20
		Fifth step	3.17
		Sixth step	3.15
		Seventh step	3.13

	TABLE 19D
	Cycle [μm]		Cycle [μm]
Thirteenth ring zone	21.32	First step	3.11
		Second step	3.09
		Third step	3.07
		Fourth step	3.04
		Fifth step	3.02
		Sixth step	3.00
		Seventh step	2.98
Fourteenth ring zone	20.36	First step	2.96
		Second step	2.95
		Third step	2.93
		Fourth step	2.91
		Fifth step	2.89
		Sixth step	2.87
		Seventh step	2.85
Fifteenth ring zone	19.51	First step	2.84
		Second step	2.82
		Third step	2.80
		Fourth step	2.79
		Fifth step	2.77
		Sixth step	2.75
		Seventh step	2.74
Sixteenth ring zone	18.73	First step	2.72
		Second step	2.71
		Third step	2.69
		Fourth step	2.68
		Fifth step	2.66
		Sixth step	2.65
		Seventh step	2.63

	TABLE 19E
	Cycle [μm]		Cycle [μm]
Seventeenth ring zone	18.03	First step	2.62
		Second step	2.60
		Third step	2.59
		Fourth step	2.58
		Fifth step	2.56
		Sixth step	2.55
		Seventh step	2.54
Eighteenth ring zone	17.39	First step	2.52
		Second step	2.51
		Third step	2.50
		Fourth step	2.48
		Fifth step	2.47
		Sixth step	2.46
		Seventh step	2.45
Nineteenth ring zone	16.80	First step	2.43
		Second step	2.42
		Third step	2.41
		Fourth step	2.40
		Fifth step	2.39
		Sixth step	2.38
		Seventh step	2.37
Twentieth ring zone	16.25	First step	2.35
		Second step	2.34
		Third step	2.33
		Fourth step	2.32
		Fifth step	2.31
		Sixth step	2.30
		Seventh step	2.29

On the first region of Example 3, one ring zone cycle is composed of consecutive 7-level stair-like steps whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases. Each ring zone cycle in Tables 19A-19E indicate the width of a ring zone in a radial direction (in a direction perpendicular to the optical axis) as indicated by an arrow in FIG. 10. On the second region, a first ring zone, a second ring zone, a third ring zone, . . . , and a twentieth ring zone are provided in order from the optical axis side toward the outer periphery side of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow in FIG. 10. In each ring zone, the steps are referred to as a first step, a second step, a third step, . . . , and a seventh step in order from the optical axis side toward the outer periphery side.

Tables 20A and 20B show ring zone cycles of the stair-like step structure provided on the second region of the first surface, and cycles of steps arranged in each ring zone.

	TABLE 20A
	Cycle [μm]		Cycle [μm]
First ring zone	21.12	First step	2.32
		Second step	2.32
		Third step	2.33
		Fourth step	2.34
		Fifth step	2.35
		Sixth step	2.35
		Seventh step	2.36
		Eighth step	2.37
		Ninth step	2.38
Second ring zone	21.98	First step	2.39
		Second step	2.40
		Third step	2.41
		Fourth step	2.43
		Fifth step	2.44
		Sixth step	2.45
		Seventh step	2.47
		Eighth step	2.48
		Ninth step	2.50
Third ring zone	23.35	First step	2.51
		Second step	2.53
		Third step	2.55
		Fourth step	2.57
		Fifth step	2.59
		Sixth step	2.61
		Seventh step	2.63
		Eighth step	2.66
		Ninth step	2.68

	TABLE 20B
	Cycle [μm]		Cycle [μm]
Fourth ring zone	25.62	First step	2.71
		Second step	2.74
		Third step	2.77
		Fourth step	2.80
		Fifth step	2.84
		Sixth step	2.88
		Seventh step	2.92
		Eighth step	2.96
		Ninth step	3.00
Fifth ring zone	29.92	First step	3.05
		Second step	3.11
		Third step	3.17
		Fourth step	3.23
		Fifth step	3.30
		Sixth step	3.38
		Seventh step	3.46
		Eighth step	3.56
		Ninth step	3.66

On the second region of Example 3, one ring zone cycle is composed of consecutive 9-level stair-like steps whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases. Each ring zone cycle in Tables 20A and 20B indicate the width of a ring zone in the radial direction (in the direction perpendicular to the optical axis) as indicated by an arrow in FIG. 10. On the second region, a first ring zone, a second ring zone, a third ring zone, . . . , and a fifth ring zone are provided in order from the optical axis side toward the outer periphery side of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow in FIG. 10. In each ring zone, the steps are referred to as a first step, a second step, a third step, . . . , and a ninth step in order from the optical axis side toward the outer periphery side.

Tables 21A-21E show step heights of the stair-like diffraction structure provided on the first region of Example 3. In one cycle of the stair-like diffraction structure, the height of each of the first to sixth steps is set such that a phase difference of 1.14 wavelengths is provided to light of the designed wavelength for BD, and the height of the seventh step is set such that a phase difference of 6.84 wavelengths is provided in the opposite direction.

	TABLE 21A
	Depth [μm]
	First ring zone	First step	0.88924
		Second step	0.89029
		Third step	0.89135
		Fourth step	0.89241
		Fifth step	0.89347
		Sixth step	0.89453
		Seventh step	5.37365
	Second ring zone	First step	0.89668
		Second step	0.89776
		Third step	0.89884
		Fourth step	0.89993
		Fifth step	0.90100
		Sixth step	0.90210
		Seventh step	5.41912
	Third ring zone	First step	0.90428
		Second step	0.90538
		Third step	0.90648
		Fourth step	0.90758
		Fifth step	0.90869
		Sixth step	0.90978
		Seventh step	5.46535
	Fourth ring zone	First step	0.91201
		Second step	0.91312
		Third step	0.91424
		Fourth step	0.91536
		Fifth step	0.91648
		Sixth step	0.91761
		Seventh step	5.51246

	TABLE 21B
	Depth [μm]
	Fifth ring zone	First step	0.91987
		Second step	0.92101
		Third step	0.92215
		Fourth step	0.92329
		Fifth step	0.92444
		Sixth step	0.92558
		Seventh step	5.56040
	Sixth ring zone	First step	0.92789
		Second step	0.92904
		Third step	0.93020
		Fourth step	0.93136
		Fifth step	0.93253
		Sixth step	0.93369
		Seventh step	5.60916
	Seventh ring zone	First step	0.93603
		Second step	0.93720
		Third step	0.93838
		Fourth step	0.93956
		Fifth step	0.94075
		Sixth step	0.94192
		Seventh step	5.65867
	Eighth ring zone	First step	0.94430
		Second step	0.94550
		Third step	0.94669
		Fourth step	0.94789
		Fifth step	0.94909
		Sixth step	0.95029
		Seventh step	5.70894

	TABLE 21C
	Depth [μm]
	Ninth ring zone	First step	0.95270
		Second step	0.95391
		Third step	0.95513
		Fourth step	0.95634
		Fifth step	0.95755
		Sixth step	0.95877
		Seventh step	5.75997
	Tenth ring zone	First step	0.96123
		Second step	0.96245
		Third step	0.96369
		Fourth step	0.96492
		Fifth step	0.96616
		Sixth step	0.96740
		Seventh step	5.81188
	Eleventh ring zone	First step	0.96989
		Second step	0.97114
		Third step	0.97240
		Fourth step	0.97365
		Fifth step	0.97491
		Sixth step	0.97618
		Seventh step	5.86462
	Twelfth ring zone	First step	0.97872
		Second step	0.97999
		Third step	0.98127
		Fourth step	0.98255
		Fifth step	0.98383
		Sixth step	0.98511
		Seventh step	5.91839

	TABLE 21D
	Depth [μm]
	Thirteenth ring zone	First step	0.98769
		Second step	0.98899
		Third step	0.99028
		Fourth step	0.99159
		Fifth step	0.99289
		Sixth step	0.99419
		Seventh step	5.97304
	Fourteenth ring zone	First step	0.99681
		Second step	0.99812
		Third step	0.99944
		Fourth step	1.00075
		Fifth step	1.00208
		Sixth step	1.00339
		Seventh step	6.02832
	Fifteenth ring zone	First step	1.00603
		Second step	1.00736
		Third step	1.00869
		Fourth step	1.01001
		Fifth step	1.01134
		Sixth step	1.01268
		Seventh step	6.08401
	Sixteenth ring zone	First step	1.01533
		Second step	1.01666
		Third step	1.01799
		Fourth step	1.01933
		Fifth step	1.02066
		Sixth step	1.02200
		Seventh step	6.14004

	TABLE 21E
	Depth [μm]
	Seventeenth ring zone	First step	1.02468
		Second step	1.02603
		Third step	1.02736
		Fourth step	1.02871
		Fifth step	1.03007
		Sixth step	1.03143
		Seventh step	6.19675
	Eighteenth ring zone	First step	1.03416
		Second step	1.03555
		Third step	1.03693
		Fourth step	1.03832
		Fifth step	1.03974
		Sixth step	1.04116
		Seventh step	6.25559
	Nineteenth ring zone	First step	1.04405
		Second step	1.04552
		Third step	1.04702
		Fourth step	1.04854
		Fifth step	1.05009
		Sixth step	1.05165
		Seventh step	6.31958
	Twentieth ring zone	First step	1.05490
		Second step	1.05659
		Third step	1.05830
		Fourth step	1.06008
		Fifth step	1.06190
		Sixth step	1.06379

Tables 22A and 22B show step heights of the stair-like diffraction structure provided on the second region of Example 3. In one cycle of the stair-like diffraction structure, the height of each of the first to eighth steps is set such that a phase difference of 1.11 wavelengths is provided to the light of the designed wavelength for BD, and the height of the ninth step is set such that a phase difference of 8.88 wavelengths is provided in the opposite direction.

	TABLE 22A
	Depth [μm]
	First ring zone	First step	1.03439
		Second step	1.03557
		Third step	1.03676
		Fourth step	1.03797
		Fifth step	1.03919
		Sixth step	1.04044
		Seventh step	1.04170
		Eighth step	1.04298
		Ninth step	8.35422
	Second ring zone	First step	1.04560
		Second step	1.04695
		Third step	1.04832
		Fourth step	1.04972
		Fifth step	1.05114
		Sixth step	1.05259
		Seventh step	1.05408
		Eighth step	1.05560
		Ninth step	8.45731
	Third ring zone	First step	1.05875
		Second step	1.06039
		Third step	1.06208
		Fourth step	1.06380
		Fifth step	1.06559
		Sixth step	1.06742
		Seventh step	1.06932
		Eighth step	1.07127
		Ninth step	8.58643

	TABLE 22B
	Depth [μm]
	Fourth ring zone	First step	1.07540
		Second step	1.07758
		Third step	1.07984
		Fourth step	1.08219
		Fifth step	1.08465
		Sixth step	1.08722
		Seventh step	1.08990
		Eighth step	1.09272
		Ninth step	8.76536
	Fifth ring zone	First step	1.09878
		Second step	1.10206
		Third step	1.10554
		Fourth step	1.10922
		Fifth step	1.11314
		Sixth step	1.11733
		Seventh step	1.12181
		Eighth step	1.12663
		Ninth step	9.05467

It should be noted that although not shown, the 4-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases is provided on the third region of Example 3. The sawtooth-like diffraction structure is provided on the outer region of Example 3.

Table 23 shows diffraction efficiencies at the twentieth ring zone of the first region and at the fifth ring zone of the second region. The fifth ring zone of the second region is an outermost region which contributes to formation of a spot of light for CD in the present example.

	TABLE 23
	Diffraction
	efficiency(%)
BD	Inner region	First ring zone	89
		Twentieth ring zone	43
	Middle region	Fifth ring zone	58
DVD	Inner region	First ring zone	62
		Twentieth ring zone	26
	Middle region	Fifth ring zone	12
CD	Inner region	First ring zone	50
		Twentieth ring zone	21
	Middle region	Fifth ring zone	6

The ring zone cycle of the twentieth ring zone of the first region is about 16 μm, and the diffraction efficiency of the light for BD is about 43%. Meanwhile, the ring zone cycle of the first ring zone of the first region is about 166 μm, and the diffraction efficiency of the light for BD at the first ring zone is about 89%. Thus, the diffraction efficiency of the light for BD at the twentieth ring zone of the first region is much lower than the diffraction efficiency at the first ring zone. When the present invention is not applied, an amount of light in reproducing/recording on BD is insufficient with this diffraction efficiency. In contrast, the ring zone cycle of the fifth ring zone of the second region is about 13 μm. The diffraction efficiency of the light for BD at the fifth ring zone of the second region is about 58% and is greatly improved as compared to the diffraction efficiency at the twentieth ring zone of the first region. Thus, insufficiency of the light amount in reproducing/recording on BD is suppressed.

Example 4

The first surface of an objective lens element according to Example 4 is divided into a first region including a symmetry axis, a second region surrounding the first region, and an outer region surrounding the second region. A 6-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases is provided on the first region of the first surface. A 6-level stair-like diffraction structure whose height monotonically increases step by step as distance from the optical axis of the objective lens element increases is provided on the second region. A sawtooth-like diffraction structure is provided on the outer region. The second surface is provided with an aspheric surface. The objective lens element according to Example 4 is a BD/DVD compatible lens. With regard to designed values for BD, the wavelength is 408 nm; the focal length is 1.22 mm; and the protective layer thickness of an information storage medium is 0.1 mm. With regard to designed values for DVD, the wavelength is 660 nm; the focal length is 1.40 mm; and the protective layer thickness of an information storage medium is 0.6 mm.

Tables 24 and 25 show construction data of the objective lens element according to Example 4.

	TABLE 24
		BD	DVD
	Wavelength	0.408	0.660
	Effective diameter	1.68	1.68
	Working distance (WD)	0.40	0.40
	Disc thickness (DT)	0.10	0.60
	Focal length	1.22	1.40
	First surface, First region	2	−1
	Diffraction order
	First surface, Second region	1	−3
	Diffraction order
	First surface, _Outer region	3	—
	Diffraction order
	Object point (OP)	∞	∞
	Radius of
Surface	curvature at
No.	the top	Thickness	Material	Remarks
0		OP
1	0.81915814	1.296403	n1	First region
				(Diffractive surface),
				Second region
				(Diffractive surface),
				Outer region
				(Diffractive surface)
2	−2.029534	WD		Aspheric surface
3	∞	DT	disc	Planar
4	∞			Planar
	Wavelength	0.408	0.660
	n1	1.52505	1.50711
	disc	1.61642	1.57815

TABLE 25
               First region
First surface  Diffractive surface
Region         0 mm-0.69 mm
               Aspherical constant
RD             0.81915814
k              −1.0331387
A0             0
A2             0
A4             0.15889713
A6             −0.026375804
A8             0.54253097
A10            −1.7713649
A12            3.6778599
A14            −3.8852706
A16            1.7582532
               Phase function
P2             −334.63934
P4             −2.2724443
P6             −12.706397
               Second region
First surface  Diffractive surface
Region         0.69 mm-0.84 mm
               Aspherical constant
RD             0.30439226
k              −1.5749807
A0             −1.81E−01
A2             0
A4             −0.22148822
A6             0.81109812
A8             −1.9054705
A10            −1.9178654
A12            18.412031
A14            −27.313249
A16            12.802057
               Phase function
P2             −720.17443
P4             1117.5096
P6             −677.1287
Second surface Aspherical constant
RD             −2.029534
k              13.40262
A0             0
A2             0
A4             1.7105509
A6             −14.232857
A8             145.08836
A10            −740.00426
A12            2202.1175
A14            −16617.021
A16            143605.41
A18            −530438.03
A20            685600.44

Tables 26A-26F show ring zone cycles of the stair-like step structure provided on the first region of the first surface, and cycles of steps arranged in each ring zone.

	TABLE 26A
	Cycle [μm]		Cycle [μm]
First ring zone	136.29	First step	55.94
		Second step	23.17
		Third step	17.78
		Fourth step	14.99
		Fifth step	13.20
		Sixth step	11.94
Second ring zone	56.73	First step	10.98
		Second step	10.22
		Third step	9.59
		Fourth step	9.07
		Fifth step	8.63
		Sixth step	8.25
Third ring zone	43.52	First step	7.91
		Second step	7.61
		Third step	7.34
		Fourth step	7.10
		Fifth step	6.88
		Sixth step	6.68
Fourth ring zone	36.68	First step	6.50
		Second step	6.33
		Third step	6.17
		Fourth step	6.03
		Fifth step	5.89
		Sixth step	5.76
Fifth ring zone	32.30	First step	5.64
		Second step	5.53
		Third step	5.43
		Fourth step	5.33
		Fifth step	5.23
		Sixth step	5.14

	TABLE 26B
	Cycle [μm]		Cycle [μm]
Sixth ring zone	29.18	First step	5.06
		Second step	4.97
		Third step	4.90
		Fourth step	4.82
		Fifth step	4.75
		Sixth step	4.68
Seventh ring zone	26.82	First step	4.62
		Second step	4.56
		Third step	4.50
		Fourth step	4.44
		Fifth step	4.38
		Sixth step	4.33
Eighth ring zone	24.95	First step	4.28
		Second step	4.23
		Third step	4.18
		Fourth step	4.13
		Fifth step	4.09
		Sixth step	4.04
Ninth ring zone	23.42	First step	4.00
		Second step	3.96
		Third step	3.92
		Fourth step	3.88
		Fifth step	3.84
		Sixth step	3.81
Tenth ring zone	22.13	First step	3.77
		Second step	3.74
		Third step	3.70
		Fourth step	3.67
		Fifth step	3.64
		Sixth step	3.61

	TABLE 26C
	Cycle [μm]		Cycle [μm]
Eleventh ring zone	21.03	First step	3.58
		Second step	3.55
		Third step	3.52
		Fourth step	3.49
		Fifth step	3.46
		Sixth step	3.44
Twelfth ring zone	20.08	First step	3.41
		Second step	3.38
		Third step	3.36
		Fourth step	3.33
		Fifth step	3.31
		Sixth step	3.28
Thirteenth ring zone	19.24	First step	3.26
		Second step	3.24
		Third step	3.22
		Fourth step	3.19
		Fifth step	3.17
		Sixth step	3.15
Fourteenth ring zone	18.49	First step	3.13
		Second step	3.11
		Third step	3.09
		Fourth step	3.07
		Fifth step	3.05
		Sixth step	3.03
Fifteenth ring zone	17.82	First step	3.01
		Second step	3.00
		Third step	2.98
		Fourth step	2.96
		Fifth step	2.94
		Sixth step	2.93

	TABLE 26D
	Cycle [μm]		Cycle [μm]
Sixteenth ring zone	17.22	First step	2.91
		Second step	2.89
		Third step	2.88
		Fourth step	2.86
		Fifth step	2.85
		Sixth step	2.83
Seventeenth ring zone	16.67	First step	2.81
		Second step	2.80
		Third step	2.78
		Fourth step	2.77
		Fifth step	2.76
		Sixth step	2.74
Eighteenth ring zone	16.16	First step	2.73
		Second step	2.71
		Third step	2.70
		Fourth step	2.69
		Fifth step	2.67
		Sixth step	2.66
Nineteenth ring zone	15.70	First step	2.65
		Second step	2.63
		Third step	2.62
		Fourth step	2.61
		Fifth step	2.60
		Sixth step	2.59
Twentieth ring zone	15.27	First step	2.57
		Second step	2.56
		Third step	2.55
		Fourth step	2.54
		Fifth step	2.53
		Sixth step	2.52

	TABLE 26E
	Cycle [μm]		Cycle [μm]
Twenty-first ring zone	14.87	First step	2.51
		Second step	2.49
		Third step	2.48
		Fourth step	2.47
		Fifth step	2.46
		Sixth step	2.45
Twenty-second ring zone	14.50	First step	2.44
		Second step	2.43
		Third step	2.42
		Fourth step	2.41
		Fifth step	2.40
		Sixth step	2.39
Twenty-third ring zone	14.15	First step	2.38
		Second step	2.37
		Third step	2.36
		Fourth step	2.35
		Fifth step	2.34
		Sixth step	2.34
Twenty-fourth ring zone	13.82	First step	2.33
		Second step	2.32
		Third step	2.31
		Fourth step	2.30
		Fifth step	2.29
		Sixth step	2.28
Twenty-fifth ring zone	13.52	First step	2.27
		Second step	2.27
		Third step	2.26
		Fourth step	2.25
		Fifth step	2.24
		Sixth step	2.23

	TABLE 26F
	Cycle [μm]		Cycle [μm]
Twenty-sixth ring zone	13.23	First step	2.22
		Second step	2.22
		Third step	2.21
		Fourth step	2.20
		Fifth step	2.19
		Sixth step	2.18

On the first region of Example 4, one ring zone cycle is composed of consecutive 6-level stair-like steps whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases. Each ring zone cycle in Tables 26A-26F indicate the width of a ring zone in a radial direction (in a direction perpendicular to the optical axis) as indicated by an arrow in FIG. 11. On the first region, a first ring zone, a second ring zone, a third ring zone, . . . , and a twenty-sixth ring zone are provided in order from the optical axis side toward the outer periphery side of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow in FIG. 11. In each ring zone, the steps are referred to as a first step, a second step, a third step, . . . , and a sixth step in order from the optical axis side toward the outer periphery side.

Tables 27A and 27B show ring zone cycles of the stair-like step structure provided on the second region of the first surface, and cycles of steps arranged in each ring zone.

	TABLE 27A
	Cycle [μm]		Cycle [μm]
First ring zone	20.01	First step	3.29
		Second step	3.31
		Third step	3.33
		Fourth step	3.35
		Fifth step	3.36
		Sixth step	3.37
Second ring zone	20.29	First step	3.38
		Second step	3.38
		Third step	3.39
		Fourth step	3.39
		Fifth step	3.38
		Sixth step	3.38
Third ring zone	19.97	First step	3.37
		Second step	3.36
		Third step	3.34
		Fourth step	3.32
		Fifth step	3.30
		Sixth step	3.28
Fourth ring zone	19.04	First step	3.25
		Second step	3.22
		Third step	3.19
		Fourth step	3.16
		Fifth step	3.13
		Sixth step	3.09
Fifth ring zone	17.69	First step	3.05
		Second step	3.01
		Third step	2.97
		Fourth step	2.93
		Fifth step	2.89
		Sixth step	2.84

	TABLE 27B
	Cycle [μm]		Cycle [μm]
Sixth ring zone	16.14	First step	2.80
		Second step	2.76
		Third step	2.71
		Fourth step	2.67
		Fifth step	2.62
		Sixth step	2.58
Seventh ring zone	14.61	First step	2.54
		Second step	2.50
		Third step	2.45
		Fourth step	2.41
		Fifth step	2.37
		Sixth step	2.33

On the second region of Example 4, one ring zone cycle is composed of consecutive 6-level stair-like steps whose height monotonically increases step by step as distance from the optical axis of the objective lens element increases. Each ring zone cycle in Tables 27A and 27B indicate the width of a ring zone in the radial direction (in the direction perpendicular to the optical axis) as indicated by an arrow in FIG. 11. On the second region, a first ring zone, a second ring zone, a third ring zone, . . . , and a seventh ring zone are provided in order from the optical axis side toward the outer periphery side of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow in FIG. 11. In each ring zone, the steps are referred to as a first step, a second step, a third step, . . . , and a sixth step in order from the optical axis side toward the outer periphery side.

Tables 28A-28F show step heights of the stair-like diffraction structure provided on the first region of Example 4. In one cycle of the stair-like diffraction structure, the height of each of the first to fifth steps is set such that a phase difference of 1.33 wavelengths is provided to light of the designed wavelength for BD, and the height of the sixth step is set such that a phase difference of 6.65 wavelengths is provided in the opposite direction.

	TABLE 28A
	Depth [μm]
	First ring zone	First step	1.03458
		Second step	1.03566
		Third step	1.03674
		Fourth step	1.03783
		Fifth step	1.03892
		Sixth step	5.20003
	Second ring zone	First step	1.04111
		Second step	1.04221
		Third step	1.04332
		Fourth step	1.04444
		Fifth step	1.04555
		Sixth step	5.23335
	Third ring zone	First step	1.04780
		Second step	1.04893
		Third step	1.05006
		Fourth step	1.05121
		Fifth step	1.05235
		Sixth step	5.26747
	Fourth ring zone	First step	1.05465
		Second step	1.05581
		Third step	1.05698
		Fourth step	1.05813
		Fifth step	1.05932
		Sixth step	5.30244
	Fifth ring zone	First step	1.06167
		Second step	1.06286
		Third step	1.06405
		Fourth step	1.06525
		Fifth step	1.06645
		Sixth step	5.33829

	TABLE 28B
	Depth [μm]
	Sixth ring zone	First step	1.06887
		Second step	1.07009
		Third step	1.07132
		Fourth step	1.07254
		Fifth step	1.07378
		Sixth step	5.37506
	Seventh ring zone	First step	1.07625
		Second step	1.07750
		Third step	1.07876
		Fourth step	1.08001
		Fifth step	1.08129
		Sixth step	5.41277
	Eighth ring zone	First step	1.08383
		Second step	1.08511
		Third step	1.08640
		Fourth step	1.08769
		Fifth step	1.08899
		Sixth step	5.45147
	Ninth ring zone	First step	1.09161
		Second step	1.09293
		Third step	1.09424
		Fourth step	1.09557
		Fifth step	1.09690
		Sixth step	5.49124
	Tenth ring zone	First step	1.09959
		Second step	1.10095
		Third step	1.10230
		Fourth step	1.10366
		Fifth step	1.10503
		Sixth step	5.53207

	TABLE 28C
	Depth [μm]
	Eleventh ring zone	First step	1.10780
		Second step	1.10918
		Third step	1.11058
		Fourth step	1.11197
		Fifth step	1.11338
		Sixth step	5.57403
	Twelfth ring zone	First step	1.11623
		Second step	1.11765
		Third step	1.11909
		Fourth step	1.12053
		Fifth step	1.12197
		Sixth step	5.61719
	Thirteenth ring zone	First step	1.12490
		Second step	1.12636
		Third step	1.12784
		Fourth step	1.12933
		Fifth step	1.13082
		Sixth step	5.66161
	Fourteenth ring zone	First step	1.13383
		Second step	1.13534
		Third step	1.13686
		Fourth step	1.13839
		Fifth step	1.13992
		Sixth step	5.70736
	Fifteenth ring zone	First step	1.14302
		Second step	1.14457
		Third step	1.14614
		Fourth step	1.14772
		Fifth step	1.14931
		Sixth step	5.75444

	TABLE 28D
	Depth [μm]
	Sixteenth ring zone	First step	1.15250
		Second step	1.15411
		Third step	1.15572
		Fourth step	1.15734
		Fifth step	1.15898
		Sixth step	5.80312
	Seventeenth ring zone	First step	1.16227
		Second step	1.16394
		Third step	1.16560
		Fourth step	1.16729
		Fifth step	1.16898
		Sixth step	5.85333
	Eighteenth ring zone	First step	1.17238
		Second step	1.17410
		Third step	1.17581
		Fourth step	1.17756
		Fifth step	1.17930
		Sixth step	5.90527
	Nineteenth ring zone	First step	1.18282
		Second step	1.18460
		Third step	1.18639
		Fourth step	1.18818
		Fifth step	1.18999
		Sixth step	5.95907
	Twentieth ring zone	First step	1.19364
		Second step	1.19548
		Third step	1.19733
		Fourth step	1.19919
		Fifth step	1.20107
		Sixth step	6.01479

	TABLE 28E
	Depth [μm]
	Twenty-first ring zone	First step	1.20485
		Second step	1.20676
		Third step	1.20868
		Fourth step	1.21062
		Fifth step	1.21256
		Sixth step	6.07258
	Twenty-second ring zone	First step	1.21648
		Second step	1.21848
		Third step	1.22047
		Fourth step	1.22248
		Fifth step	1.22450
		Sixth step	6.13270
	Twenty-third ring zone	First step	1.22859
		Second step	1.23065
		Third step	1.23274
		Fourth step	1.23483
		Fifth step	1.23693
		Sixth step	6.19527
	Twenty-fourth ring zone	First step	1.24118
		Second step	1.24334
		Third step	1.24551
		Fourth step	1.24769
		Fifth step	1.24988
		Sixth step	6.26044
	Twenty-fifth ring zone	First step	1.25432
		Second step	1.25657
		Third step	1.25883
		Fourth step	1.26111
		Fifth step	1.26339
		Sixth step	6.32854

	TABLE 28F
	Depth [μm]
	Twenty-sixth ring zone	First step	1.26804
		Second step	1.27039
		Third step	1.27276
		Fourth step	1.27514
		Fifth step	1.27754

Tables 29A and 29B show step heights of the stair-like diffraction structure provided on the second region of Example 4. In one cycle of the stair-like diffraction structure, the height of each of the first to fifth steps is set such that a phase difference of 0.84 wavelength is provided to the light of the designed wavelength for BD, and the height of the sixth step is set such that a phase difference of 4.20 wavelengths is provided in the opposite direction.

	TABLE 29A
	Depth [μm]
	First ring zone	First step	0.81238
		Second step	0.81451
		Third step	0.81673
		Fourth step	0.81903
		Fifth step	0.82142
		Sixth step	4.11949
	Second ring zone	First step	0.82647
		Second step	0.82912
		Third step	0.83186
		Fourth step	0.83468
		Fifth step	0.83760
		Sixth step	4.20298
	Third ring zone	First step	0.84367
		Second step	0.84683
		Third step	0.85006
		Fourth step	0.85337
		Fifth step	0.85676
		Sixth step	4.30105
	Fourth ring zone	First step	0.86373
		Second step	0.86732
		Third step	0.87096
		Fourth step	0.87467
		Fifth step	0.87842
		Sixth step	4.41118
	Fifth ring zone	First step	0.88611
		Second step	0.89002
		Third step	0.89399
		Fourth step	0.89799
		Fifth step	0.90205
		Sixth step	4.53079

	TABLE 29B
	Depth [μm]
	Sixth ring zone	First step	0.91030
		Second step	0.91449
		Third step	0.91873
		Fourth step	0.92301
		Fifth step	0.92734
		Sixth step	4.65856
	Seventh ring zone	First step	0.93614
		Second step	0.94061
		Third step	0.94513
		Fourth step	0.94971
		Fifth step	0.95434
		Sixth step	4.79518

It should be noted that although not shown, the sawtooth-like diffraction structure is provided on the outer region of Example 4.

Table 30 shows diffraction efficiencies at the twenty-sixth ring zone of the first region and at the eighth ring zone of the second region. The eighth ring zone of the second region is an outermost region which contributes to formation of a spot of light for DVD in the present example.

	TABLE 30
	Diffraction
	efficiency (%)
BD	Inner region	First ring zone	67
		Twenty-sixth ring zone	46
	Middle region	Eighth ring zone	63
DVD	Inner region	First ring zone	75
		Twenty-sixth ring zone	54
	Middle region	Eighth ring zone	14

The ring zone cycle of the twenty-sixth ring zone of the first region is about 13 μm, and the diffraction efficiency of the light for BD is about 46%. Meanwhile, the ring zone cycle of the first ring zone of the first region is about 136 μm, and the diffraction efficiency of the light for BD is about 67%. Thus, the diffraction efficiency of the light for BD at the twenty-sixth ring zone of the first region is much lower than the diffraction efficiency at the first ring zone. When the present invention is not applied, an amount of light in reproducing/recording on BD is insufficient with this diffraction efficiency.

In contrast, the ring zone cycle of the eighth ring zone of the second region is about 13 μm. The diffraction efficiency of the light for BD at the eighth ring zone of the second region is about 63% and is greatly improved as compared to the diffraction efficiency at the twenty-sixth ring zone of the first region. Thus, insufficiency of the light amount in reproducing/recording on BD is suppressed.

Example 5

The first surface of an objective lens element according to Example 5 is divided into a first region including a symmetry axis, a second region surrounding the first region, and an outer region surrounding the second region. An 8-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases is provided on the first region of the first surface. A 7-level stair-like diffraction structure whose height monotonically increases step by step as distance from the optical axis of the objective lens element increases is provided on the second region. A sawtooth-like diffraction structure is provided on the outer region. The second surface is provided with an aspheric surface. The objective lens element according to Example 4 is a BD/DVD compatible lens. With regard to designed values for BD, the wavelength is 408 nm; the focal length is 1.09 mm; and the protective layer thickness of an information storage medium is 0.1 mm. With regard to designed values for DVD, the wavelength is 660 nm; the focal length is 1.32 mm; and the protective layer thickness of an information storage medium is 0.6 mm.

Tables 31 and 32 show construction data of the objective lens element according to Example 5.

	TABLE 31
		BD	DVD
	Wavelength	0.408	0.660
	Effective diameter	1.68	1.68
	Working distance (WD)	0.40	0.40
	Disc thickness (DT)	0.10	0.60
	Focal length	1.09	1.32
	First surface, First	2	−2
	region
	Diffraction order
	First surface, Second	5	−1
	region
	Diffraction order
	First surface,_Outer	3	—
	region
	Diffraction order
	Object point (OP)	∞	∞
	Radius of
	curvature at
Surface No.	the top	Thickness	Material	Remarks
0		OP
1	0.72399814	1.125319	n1	First region
				(Diffractive surface),
				Second region
				(Diffractive surface),
				Outer region
				(Diffractive surface)
2	−1.738506	WD		Aspheric surface
3	∞	DT	disc	Planar
4	∞			Planar
	Wavelength	0.408	0.660
	n1	1.52505	1.50711
	disc	1.61642	1.57815

TABLE 32
	First region		Second region
First surface	Diffractive surface	First surface	Diffractive surface	Second surface	Aspherical constant
Region	0 mm-0.69 mm	Region	0.69 mm-0.84 mm	RD	−1.738506
	Aspherical constant		Aspherical constant	k	8.591035
RD	0.72399814	RD	0.15483052	A0	0
k	−0.80565617	k	−1.4090207	A2	0
A0	0	A0	−2.39E−01	A4	2.1245151
A2	0	A2	0	A6	−7.4983864
A4	0.2220808	A4	−1.0390761	A8	−66.676749
A6	−0.54716594	A6	0.25086182	A10	1288.8438
A8	3.0481241	A8	−0.018007854	A12	−5781.1044
A10	−7.4996866	A10	2.3606036	A14	−13127.483
A12	7.6042347	A12	16.318267	A16	196387.57
A14	2.8548542	A14	−44.689687	A18	−611177.23
A16	−7.5452063	A16	29.696786	A20	639839.55
	Phase function		Phase function
P2	−247.8751	P2	−490.42048
P4	30.793671	P4	646.10183
P6	−97.513681	P6	−342.89035

Tables 33A-33E shows ring zone cycles of the stair-like step structure provided on the first region of the first surface, and cycles of steps arranged in each ring zone.

	TABLE 33A
	Cycle [μm]		Cycle [μm]
First ring zone	158.94	First step	56.30
		Second step	23.34
		Third step	17.92
		Fourth step	15.11
		Fifth step	13.32
		Sixth step	12.05
		Seventh step	11.09
		Eighth step	10.32
Second ring zone	66.32	First step	9.70
		Second step	9.18
		Third step	8.73
		Fourth step	8.35
		Fifth step	8.01
		Sixth step	7.71
		Seventh step	7.44
		Eighth step	7.20
Third ring zone	51.00	First step	6.98
		Second step	6.78
		Third step	6.59
		Fourth step	6.42
		Fifth step	6.27
		Sixth step	6.12
		Seventh step	5.98
		Eighth step	5.86
Fourth ring zone	43.04	First step	5.74
		Second step	5.62
		Third step	5.52
		Fourth step	5.41
		Fifth step	5.32
		Sixth step	5.23
		Seventh step	5.14
		Eighth step	5.06

	TABLE 33B
	Cycle [μm]		Cycle [μm]
Fifth ring zone	37.90	First step	4.98
		Second step	4.91
		Third step	4.83
		Fourth step	4.76
		Fifth step	4.70
		Sixth step	4.63
		Seventh step	4.57
		Eighth step	4.51
Sixth ring zone	34.21	First step	4.46
		Second step	4.40
		Third step	4.35
		Fourth step	4.30
		Fifth step	4.25
		Sixth step	4.20
		Seventh step	4.15
		Eighth step	4.11
Seventh ring zone	31.36	First step	4.06
		Second step	4.02
		Third step	3.98
		Fourth step	3.94
		Fifth step	3.90
		Sixth step	3.86
		Seventh step	3.82
		Eighth step	3.79
Eighth ring zone	29.07	First step	3.75
		Second step	3.72
		Third step	3.68
		Fourth step	3.65
		Fifth step	3.62
		Sixth step	3.58
		Seventh step	3.55
		Eighth step	3.52

	TABLE 33C
	Cycle [μm]		Cycle [μm]
Ninth ring zone	27.15	First step	3.49
		Second step	3.46
		Third step	3.43
		Fourth step	3.41
		Fifth step	3.38
		Sixth step	3.35
		Seventh step	3.33
		Eighth step	3.30
Tenth ring zone	25.51	First step	3.27
		Second step	3.25
		Third step	3.22
		Fourth step	3.20
		Fifth step	3.18
		Sixth step	3.15
		Seventh step	3.13
		Eighth step	3.11
Eleventh ring zone	24.08	First step	3.08
		Second step	3.06
		Third step	3.04
		Fourth step	3.02
		Fifth step	3.00
		Sixth step	2.98
		Seventh step	2.96
		Eighth step	2.94
Twelfth ring zone	22.80	First step	2.92
		Second step	2.90
		Third step	2.88
		Fourth step	2.86
		Fifth step	2.84
		Sixth step	2.82
		Seventh step	2.80
		Eighth step	2.79

	TABLE 33D
	Cycle [μm]		Cycle [μm]
Thirteenth ring zone	21.66	First step	2.77
		Second step	2.75
		Third step	2.73
		Fourth step	2.72
		Fifth step	2.70
		Sixth step	2.68
		Seventh step	2.67
		Eighth step	2.65
Fourteenth ring zone	20.62	First step	2.63
		Second step	2.62
		Third step	2.60
		Fourth step	2.59
		Fifth step	2.57
		Sixth step	2.55
		Seventh step	2.54
		Eighth step	2.52
Fifteenth ring zone	19.67	First step	2.51
		Second step	2.49
		Third step	2.48
		Fourth step	2.47
		Fifth step	2.45
		Sixth step	2.44
		Seventh step	2.42
		Eighth step	2.41
Sixteenth ring zone	18.80	First step	2.40
		Second step	2.38
		Third step	2.37
		Fourth step	2.36
		Fifth step	2.34
		Sixth step	2.33
		Seventh step	2.32
		Eighth step	2.30

	TABLE 33E
	Cycle [μm]		Cycle [μm]
Seventeenth ring zone	17.99	First step	2.29
		Second step	2.28
		Third step	2.27
		Fourth step	2.25
		Fifth step	2.24
		Sixth step	2.23
		Seventh step	2.22
		Eighth step	2.21
Eighteenth ring zone	17.24	First step	2.20
		Second step	2.18
		Third step	2.17
		Fourth step	2.16
		Fifth step	2.15
		Sixth step	2.14
		Seventh step	2.13
		Eighth step	2.12
Nineteenth ring zone	16.54	First step	2.10
		Second step	2.09
		Third step	2.08
		Fourth step	2.07
		Fifth step	2.06
		Sixth step	2.05
		Seventh step	2.04
		Eighth step	2.03
Twentieth ring zone	15.89	First step	2.02
		Second step	2.01
		Third step	2.00
		Fourth step	1.99
		Fifth step	1.98
		Sixth step	1.97
		Seventh step	1.96
		Eighth step	1.95

On the first region of Example 5, one ring zone cycle is composed of consecutive 8-level stair-like steps whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases. Each ring zone cycle in Tables 33A-33E indicate the width of a ring zone in a radial direction (in a direction perpendicular to the optical axis) as indicated by an arrow in FIG. 12. On the first region, a first ring zone, a second ring zone, a third ring zone, . . . , and a twentieth ring zone are provided in order from the optical axis side toward the outer periphery side of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow in FIG. 12. In each ring zone, the steps are referred to as a first step, a second step, a third step, . . . , and an eighth step in order from the optical axis side toward the outer periphery side.

Tables 34A and 34B show ring zone cycles of the stair-like step structure provided on the second region of the first surface, and cycles of steps arranged in each ring zone.

	TABLE 34A
	Cycle [μm]		Cycle [μm]
First ring zone	22.81	First step	3.78
		Second step	3.12
		Third step	3.14
		Fourth step	3.16
		Fifth step	3.18
		Sixth step	3.20
		Seventh step	3.22
Second ring zone	23.06	First step	3.24
		Second step	3.26
		Third step	3.28
		Fourth step	3.30
		Fifth step	3.31
		Sixth step	3.33
		Seventh step	3.34
Third ring zone	23.64	First step	3.35
		Second step	3.36
		Third step	3.37
		Fourth step	3.38
		Fifth step	3.39
		Sixth step	3.39
		Seventh step	3.39
Fourth ring zone	23.64	First step	3.39
		Second step	3.39
		Third step	3.39
		Fourth step	3.38
		Fifth step	3.37
		Sixth step	3.36
		Seventh step	3.35

	TABLE 34B
	Cycle [μm]		Cycle [μm]
	Fifth ring zone	22.92	First step	3.33
			Second step	3.32
			Third step	3.30
			Fourth step	3.28
			Fifth step	3.26
			Sixth step	3.23
			Seventh step	3.21

On the second region of Example 5, one ring zone cycle is composed of consecutive 7-level stair-like steps whose height monotonically increases step by step as distance from the optical axis of the objective lens element increases. Each ring zone cycle in Tables 34A and 34B indicate the width of a ring zone in the radial direction (in the direction perpendicular to the optical axis) as indicated by an arrow in FIG. 12. On the second region, a first ring zone, a second ring zone, a third ring zone, . . . , and a fifth ring zone are provided in order from the optical axis side toward the outer periphery side of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow in FIG. 12. In each ring zone, the steps are referred to as a first step, a second step, a third step, . . . , and a seventh step in order from the optical axis side toward the outer periphery side.

Tables 35A-35E show step heights of the stair-like diffraction structure provided on the first region of Example 5. In one cycle of the stair-like diffraction structure, the height of each of the first to seventh steps is set such that a phase difference of 1.25 wavelengths is provided to light of the designed wavelength for BD, and the height of the eighth step is set such that a phase difference of 8.75 wavelengths is provided in the opposite direction.

	TABLE 35A
	Depth [μm]
	First ring zone	First step	0.97255
		Second step	0.97376
		Third step	0.97499
		Fourth step	0.97621
		Fifth step	0.97746
		Sixth step	0.97871
		Seventh step	0.97996
		Eighth step	6.86866
	Second ring zone	First step	0.98251
		Second step	0.98380
		Third step	0.98509
		Fourth step	0.98639
		Fifth step	0.98770
		Sixth step	0.98901
		Seventh step	0.99034
		Eighth step	6.94173
	Third ring zone	First step	0.99301
		Second step	0.99436
		Third step	0.99573
		Fourth step	0.99709
		Fifth step	0.99846
		Sixth step	0.99985
		Seventh step	1.00124
		Eighth step	7.01846
	Fourth ring zone	First step	1.00404
		Second step	1.00545
		Third step	1.00688
		Fourth step	1.00830
		Fifth step	1.00974
		Sixth step	1.01119
		Seventh step	1.01265
		Eighth step	7.09879

	TABLE 35B
	Depth [μm]
	Fifth ring zone	First step	1.01559
		Second step	1.01706
		Third step	1.01855
		Fourth step	1.02005
		Fifth step	1.02156
		Sixth step	1.02308
		Seventh step	1.02460
		Eighth step	7.18296
	Sixth ring zone	First step	1.02768
		Second step	1.02923
		Third step	1.03079
		Fourth step	1.03236
		Fifth step	1.03394
		Sixth step	1.03553
		Seventh step	1.03713
		Eighth step	7.27116
	Seventh ring zone	First step	1.04035
		Second step	1.04198
		Third step	1.04361
		Fourth step	1.04526
		Fifth step	1.04691
		Sixth step	1.04858
		Seventh step	1.05025
		Eighth step	7.36356
	Eighth ring zone	First step	1.05364
		Second step	1.05534
		Third step	1.05705
		Fourth step	1.05878
		Fifth step	1.06050
		Sixth step	1.06225
		Seventh step	1.06400
		Eighth step	7.46034

	TABLE 35C
	Depth [μm]
	Ninth ring zone	First step	1.06753
		Second step	1.06931
		Third step	1.07110
		Fourth step	1.07290
		Fifth step	1.07471
		Sixth step	1.07654
		Seventh step	1.07838
		Eighth step	7.56149
	Tenth ring zone	First step	1.08206
		Second step	1.08393
		Third step	1.08580
		Fourth step	1.08769
		Fifth step	1.08958
		Sixth step	1.09149
		Seventh step	1.09340
		Eighth step	7.66728
	Eleventh ring zone	First step	1.09726
		Second step	1.09921
		Third step	1.10116
		Fourth step	1.10314
		Fifth step	1.10511
		Sixth step	1.10710
		Seventh step	1.10911
		Eighth step	7.77788
	Twelfth ring zone	First step	1.11315
		Second step	1.11519
		Third step	1.11724
		Fourth step	1.11930
		Fifth step	1.12138
		Sixth step	1.12345
		Seventh step	1.12555
		Eighth step	7.89364

	TABLE 35D
	Depth [μm]
	Thirteenth ring zone	First step	1.12979
		Second step	1.13193
		Third step	1.13408
		Fourth step	1.13624
		Fifth step	1.13841
		Sixth step	1.14060
		Seventh step	1.14280
		Eighth step	8.01509
	Fourteenth ring zone	First step	1.14724
		Second step	1.14949
		Third step	1.15175
		Fourth step	1.15401
		Fifth step	1.15630
		Sixth step	1.15860
		Seventh step	1.16091
		Eighth step	8.14275
	Fifteenth ring zone	First step	1.16559
		Second step	1.16795
		Third step	1.17033
		Fourth step	1.17271
		Fifth step	1.17511
		Sixth step	1.17754
		Seventh step	1.17998
		Eighth step	8.27689
	Sixteenth ring zone	First step	1.18489
		Second step	1.18736
		Third step	1.18986
		Fourth step	1.19236
		Fifth step	1.19489
		Sixth step	1.19743
		Seventh step	1.19999
		Eighth step	8.41785

	TABLE 35E
	Depth [μm]
	Seventeenth ring zone	First step	1.20514
		Second step	1.20774
		Third step	1.21034
		Fourth step	1.21296
		Fifth step	1.21560
		Sixth step	1.21825
		Seventh step	1.22093
		Eighth step	8.56520
	Eighteenth ring zone	First step	1.22629
		Second step	1.22898
		Third step	1.23169
		Fourth step	1.23441
		Fifth step	1.23714
		Sixth step	1.23988
		Seventh step	1.24263
		Eighth step	8.71763
	Nineteenth ring zone	First step	1.24814
		Second step	1.25090
		Third step	1.25368
		Fourth step	1.25645
		Fifth step	1.25923
		Sixth step	1.26201
		Seventh step	1.26479
		Eighth step	8.87303
	Twentieth ring zone	First step	1.27035
		Second step	1.27313
		Third step	1.27590
		Fourth step	1.27866
		Fifth step	1.28141
		Sixth step	1.28416
		Seventh step	1.28690

Tables 36A and 36B show step heights of the stair-like diffraction structure provided on the second region of Example 5. In one cycle of the stair-like diffraction structure, the height of each of the first to sixth steps is set such that a phase difference of 0.26 wavelength is provided to the light of the designed wavelength for BD, and the height of the seventh step is set such that a phase difference of 1.56 wavelengths is provided in the opposite direction.

	TABLE 36A
	Depth [μm]
	First ring zone	First step	0.26920
		Second step	0.27021
		Third step	0.27127
		Fourth step	0.27238
		Fifth step	0.27353
		Sixth step	0.27473
		Seventh step	1.65591
	Second ring zone	First step	0.27730
		Second step	0.27867
		Third step	0.28011
		Fourth step	0.28161
		Fifth step	0.28319
		Sixth step	0.28484
		Seventh step	1.71949
	Third ring zone	First step	0.28841
		Second step	0.29034
		Third step	0.29238
		Fourth step	0.29453
		Fifth step	0.29681
		Sixth step	0.29923
		Seventh step	1.81075
	Fourth ring zone	First step	0.30452
		Second step	0.30743
		Third step	0.31054
		Fourth step	0.31385
		Fifth step	0.31740
		Sixth step	0.32120
		Seventh step	1.95168

	TABLE 36B
	Depth [μm]
	Fifth ring zone	First step	0.32965
		Second step	0.33435
		Third step	0.33939
		Fourth step	0.34481
		Fifth step	0.35064
		Sixth step	0.35689
		Seventh step	2.18174

It should be noted that although not shown, the sawtooth-like diffraction structure is provided on the outer region of Example 5.

Table 37 shows diffraction efficiencies at the twentieth ring zone of the first region and at the sixth ring zone of the second region. The sixth ring zone of the second region is an outermost region which contributes to formation of a spot of light for DVD in the present example.

	TABLE 37
	Diffractive
	efficiency (%)
BD	Inner region	First ring zone	77
		Twentieth ring zone	46
	Middle region	Sixth ring zone	9
DVD	Inner region	First ring zone	75
		Twentieth ring zone	22
	Middle region	Sixth ring zone	85

The ring zone cycle of the twentieth ring zone of the first region is about 16 μm, and the diffraction efficiency of the light for DVD is about 22%. Meanwhile, the ring zone cycle of the first ring zone of the first region is about 159 μm, and the diffraction efficiency of the light for DVD is about 75%. Thus, the diffraction efficiency of the light for DVD at the twentieth ring zone of the first region is much lower than the diffraction efficiency at the first ring zone. When the present invention is not applied, the diameter of a spot on a recording surface is increased with this diffraction efficiency, and recording/reproducing performance of DVD deteriorates.

In contrast, the ring zone cycle of the sixth ring zone of the second region is about 22 μm. The diffraction efficiency of the light for DVD at the sixth ring zone of the second region is about 85% and is greatly improved as compared to that at the twentieth ring zone of the first region. Thus, enlargement of a beam spot formed when the light for DVD is incident is suppressed. As a result, the recording/reproducing performance improves.

Table 38 shows the relationship between diffraction orders of the objective lens elements according to Examples 1 to 5.

	TABLE 38
	1	2	3	4	5
	BD-	BD-	BD-	BD-	BD-	BD-	BD-
Example	DVD	DVD	CD	DVD	CD	DVD	DVD
A1	1	−1	−2	−2	−3	−1	2
B1	−1	2	2	1	1	2	−2
A2	2	−2	−3	−3	−4	−3	5
B2	−1	2	2	1	1	1	−1
|A1-B1|	2	3	4	3	4	3	4
|A2-B2|	3	4	5	4	5	4	6
|B1|	1	2	2	1	1	2	2
|B2|	1	2	2	1	1	1	1

FIGS. 13 and 14 are partially enlarged views of diffraction structures of objective lens elements according to Examples 6 and 7, respectively. Specifically, FIGS. 13 and 14 each are an enlarged view of a compatible region composed of a first region and a second region. In FIGS. 13 and 14, a portion below a diffraction shape represented by a broken line is a lens material, and a portion above the diffraction shape is air.

Example 6

Example 6 corresponds to the third embodiment. The first surface of an objective lens element according to Example 2 is divided into a first region including a symmetry axis, a second region surrounding the first region, a third region surrounding the second region, and an outer region surrounding the third region. A 6-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases is provided on the first region of the first surface. An 8-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases is provided on the second region. A 4-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases is provided on the third region. A sawtooth-like diffraction structure is provided on the outer region. The second surface is an aspheric surface. The objective lens element according to Example 6 is a BD/DVD/CD compatible lens. With regard to designed values for BD, the wavelength is 405 nm; the focal length is 1.20 mm; and the protective layer thickness of an information storage medium is 0.085 mm. With regard to designed values for DVD, the wavelength is 650 nm; the focal length is 1.45 mm; and the protective layer thickness of an information storage medium is 0.6 mm. With regard to designed values for CD, the wavelength is 780 nm; the focal length is 1.64 mm; and the protective layer thickness of an information storage medium is 1.2 mm.

Tables 39 and 40 show construction data of the objective lens element according to Example 6.

	TABLE 39
		BD	DVD	CD
	Wavelength	0.405	0.650	0.780
	Effective diameter	1.66	1.66	1.66
	Working distance (WD)	0.47	0.44	0.30
	Disc thickness (DT)	0.085	0.60	1.20
	Focal length	1.2	1.45	1.64
	First surface, First region	2	−1	−2
	Diffraction order
	First surface, Second region	2	−2	−3
	Diffraction order
	First surface, Third region	1	−1	—
	Diffraction order
	First surface, _Outer region	3	—	—
	Diffraction order
	Object point (OP)	∞	−200	100
	Radius of
Surface	curvature at
No.	the top	Thickness	Material	Remarks
0		OP
1	0.80094682	1.16723	n1	First region
				(Diffractive surface),
				Second region
				(Diffractive surface),
				Third region
				(Diffractive surface),
				Outer region
				(Diffractive surface)
2	−2.36979	WD		Aspheric surface
3	∞	DT	disc	Planar
4	∞			Planar
	Wavelength	0.405	0.650	0.780
	n1	1.52550	1.50746	1.50385
	disc	1.61913	1.57881	1.57180

TABLE 40
               First region
First surface  Diffractive surface
Region         0 mm-0.76 mm
               Aspherical constant
RD             0.79379153
k              −1.0089037
A0             0
A2             0
A4             0.15630796
A6             0.1234095
A8             −0.041797152
A10            −1.9732875
A12            9.3786807
A14            −15.409691
A16            9.1687136
               Phase function
P2             −302.57753
P4             −39.744514
P6             45.905067
               Second Region
First surface  Diffractive surface
Region         0.76 mm-0.82 mm
               Aspherical constant
RD             0.37514459
k              −1.7682624
A0             −0.13107122
A2             0
A4             −0.016040955
A6             0.97078969
A8             −1.9579984
A10            −2.5798109
A12            17.700268
A14            −25.617042
A16            12.626725
A18            0.12252176
               Phase function
P2             −328.02751
P4             −89.723276
P6             209.39281
Second surface Aspherical constant
RD             −2.433225
k              18.18902
A0             0
A2             0
A4             1.458337
A6             −10.079282
A8             32.941344
A10            561.91403
A12            −7183.1577
A14            37242.998
A16            −94611.808
A18            97317.354

Tables 41A-41F show ring zone cycles of the stair-like step structure provided on the first region of the first surface, and cycles of steps arranged in each ring zone.

	TABLE 41A
	Cycle [μm]		Cycle [μm]
	First ring zone	143.91	First step	58.82
			Second step	24.34
			Third step	18.67
			Fourth step	15.73
			Fifth step	13.85
			Sixth step	12.51
	Second ring	59.36	First step	11.50
	zone		Second step	10.70
			Third step	10.04
			Fourth step	9.49
			Fifth step	9.02
			Sixth step	8.61
	Third ring zone	45.39	First step	8.26
			Second step	7.94
			Third step	7.66
			Fourth step	7.40
			Fifth step	7.17
			Sixth step	6.96
	Fourth ring	38.15	First step	6.77
	zone		Second step	6.59
			Third step	6.42
			Fourth step	6.27
			Fifth step	6.12
			Sixth step	5.99
	Fifth ring zone	33.52	First step	5.86
			Second step	5.74
			Third step	5.63
			Fourth step	5.53
			Fifth step	5.43
			Sixth step	5.33

	TABLE 41B
	Cycle [μm]		Cycle [μm]
	Sixth ring zone	30.23	First step	5.24
			Second step	5.15
			Third step	5.07
			Fourth step	4.99
			Fifth step	4.92
			Sixth step	4.85
	Seventh ring	27.74	First step	4.78
	zone		Second step	4.71
			Third step	4.65
			Fourth step	4.59
			Fifth step	4.53
			Sixth step	4.48
	Eighth ring	25.78	First step	4.42
	zone		Second step	4.37
			Third step	4.32
			Fourth step	4.27
			Fifth step	4.22
			Sixth step	4.18
	Ninth ring zone	24.18	First step	4.13
			Second step	4.09
			Third step	4.05
			Fourth step	4.01
			Fifth step	3.97
			Sixth step	3.93
	Tenth ring zone	22.84	First step	3.89
			Second step	3.86
			Third step	3.82
			Fourth step	3.79
			Fifth step	3.76
			Sixth step	3.72

	TABLE 41C
	Cycle [μm]		Cycle [μm]
Eleventh ring zone	21.71	First step	3.69
		Second step	3.66
		Third step	3.63
		Fourth step	3.60
		Fifth step	3.57
		Sixth step	3.55
Twelfth ring zone	20.73	First step	3.52
		Second step	3.49
		Third step	3.47
		Fourth step	3.44
		Fifth step	3.42
		Sixth step	3.39
Thirteenth ring zone	19.88	First step	3.37
		Second step	3.35
		Third step	3.32
		Fourth step	3.30
		Fifth step	3.28
		Sixth step	3.26
Fourteenth ring zone	19.13	First step	3.24
		Second step	3.22
		Third step	3.20
		Fourth step	3.18
		Fifth step	3.16
		Sixth step	3.14
Fifteenth ring zone	18.46	First step	3.12
		Second step	3.10
		Third step	3.09
		Fourth step	3.07
		Fifth step	3.05
		Sixth step	3.03

	TABLE 41D
	Cycle [μm]		Cycle [μm]
Sixteenth ring zone	17.87	First step	3.02
		Second step	3.00
		Third step	2.99
		Fourth step	2.97
		Fifth step	2.95
		Sixth step	2.94
Seventeenth ring zone	17.34	First step	2.92
		Second step	2.91
		Third step	2.90
		Fourth step	2.88
		Fifth step	2.87
		Sixth step	2.85
Eighteenth ring zone	16.85	First step	2.84
		Second step	2.83
		Third step	2.82
		Fourth step	2.80
		Fifth step	2.79
		Sixth step	2.78
Nineteenth ring zone	16.42	First step	2.77
		Second step	2.75
		Third step	2.74
		Fourth step	2.73
		Fifth step	2.72
		Sixth step	2.71
Twentieth ring zone	16.02	First step	2.70
		Second step	2.69
		Third step	2.68
		Fourth step	2.66
		Fifth step	2.65
		Sixth step	2.64

	TABLE 41E
	Cycle [μm]		Cycle [μm]
Twenty-first ring zone	15.66	First step	2.63
		Second step	2.62
		Third step	2.61
		Fourth step	2.60
		Fifth step	2.60
		Sixth step	2.59
Twenty-second ring zone	15.33	First step	2.58
		Second step	2.57
		Third step	2.56
		Fourth step	2.55
		Fifth step	2.54
		Sixth step	2.53
Twenty-third ring zone	15.02	First step	2.52
		Second step	2.52
		Third step	2.51
		Fourth step	2.50
		Fifth step	2.49
		Sixth step	2.48
Twenty-fourth ring zone	14.75	First step	2.48
		Second step	2.47
		Third step	2.46
		Fourth step	2.45
		Fifth step	2.45
		Sixth step	2.44
Twenty-fifth ring zone	14.49	First step	2.43
		Second step	2.43
		Third step	2.42
		Fourth step	2.41
		Fifth step	2.40
		Sixth step	2.40

	TABLE 41F
	Cycle [μm]		Cycle [μm]
Twenty-sixth ring zone	14.26	First step	2.39
		Second step	2.39
		Third step	2.38
		Fourth step	2.37
		Fifth step	2.37
		Sixth step	2.36
Twenty-seventh ring zone	14.04	First step	2.35
		Second step	2.35
		Third step	2.34
		Fourth step	2.34
		Fifth step	2.33
		Sixth step	2.33
Twenty-eighth ring zone	13.85	First step	2.32
		Second step	2.32
		Third step	2.31
		Fourth step	2.30
		Fifth step	2.30
		Sixth step	2.29

On the first region of Example 6, one ring zone cycle is composed of consecutive 6-level stair-like steps whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases. Each ring zone cycle in Tables 41A-41F indicate the width of a ring zone in a radial direction (in a direction perpendicular to the optical axis) as indicated by an arrow in FIG. 13. On the first region, a first ring zone, a second ring zone, a third ring zone, . . . , and a twenty-eighth ring zone are provided in order from the optical axis side toward the outer periphery side of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow in FIG. 13. In each ring zone, the steps are referred to as a first step, a second step, a third step, . . . , and a sixth step in order from the optical axis side toward the outer periphery side.

Table 42 shows ring zone cycles of the stair-like step structure provided on the second region of the first surface, and cycles of steps arranged in each ring zone.

	TABLE 42
	Cycle [μm]		Cycle [μm]
	First ring zone	22.55	First step	2.25
			Second step	2.26
			Third step	2.28
			Fourth step	2.30
			Fifth step	2.31
			Sixth step	2.33
			Seventh step	2.35
			Eighth step	2.37
	Second ring	21.15	First step	2.40
	zone		Second step	2.42
			Third step	2.44
			Fourth step	2.47
			Fifth step	2.50
			Sixth step	2.53
			Seventh step	2.56
			Eighth step	2.59

On the second region of Example 6, one ring zone cycle is composed of consecutive 8-level stair-like steps whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases. Each ring zone cycle in Table 42 indicates the width of a ring zone in the radial direction (in the direction perpendicular to the optical axis) as indicated by an arrow in FIG. 13. On the second region, a first ring zone and a second ring zone are provided in order from the optical axis side toward the outer periphery side of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow in FIG. 13. In each ring zone, the steps are referred to as a first step, a second step, a third step, . . . , and an eighth step in order from the optical axis side toward the outer periphery side.

Tables 43A-43F show step heights of the stair-like diffraction structure provided on the first region of Example 6. In one cycle of the stair-like diffraction structure, the height of each of the first to fifth steps is set such that a phase difference of 1.33 wavelengths is provided to light of the designed wavelength for BD, and the height of the sixth step is set such that a phase difference of 6.65 wavelengths is provided in the opposite direction.

	TABLE 43A
	Depth [μm]
	First ring zone	First step	1.02856
		Second step	1.02978
		Third step	1.03101
		Fourth step	1.03224
		Fifth step	1.03347
		Sixth step	5.17357
	Second ring zone	First step	1.03597
		Second step	1.03722
		Third step	1.03849
		Fourth step	1.03974
		Fifth step	1.04102
		Sixth step	5.21143
	Third ring zone	First step	1.04358
		Second step	1.04486
		Third step	1.04615
		Fourth step	1.04746
		Fifth step	1.04875
		Sixth step	5.25035
	Fourth ring zone	First step	1.05138
		Second step	1.05271
		Third step	1.05403
		Fourth step	1.05536
		Fifth step	1.05671
		Sixth step	5.29028
	Fifth ring zone	First step	1.05940
		Second step	1.06076
		Third step	1.06213
		Fourth step	1.06349
		Fifth step	1.06488
		Sixth step	5.33127

	TABLE 43B
	Depth [μm]
	Sixth ring zone	First step	1.06764
		Second step	1.06904
		Third step	1.07044
		Fourth step	1.07185
		Fifth step	1.07326
		Sixth step	5.37339
	Seventh ring zone	First step	1.07610
		Second step	1.07753
		Third step	1.07897
		Fourth step	1.08041
		Fifth step	1.08186
		Sixth step	5.41658
	Eighth ring zone	First step	1.08478
		Second step	1.08625
		Third step	1.08773
		Fourth step	1.08921
		Fifth step	1.09070
		Sixth step	5.46097
	Ninth ring zone	First step	1.09369
		Second step	1.09519
		Third step	1.09671
		Fourth step	1.09823
		Fifth step	1.09976
		Sixth step	5.50649
	Tenth ring zone	First step	1.10283
		Second step	1.10439
		Third step	1.10594
		Fourth step	1.10751
		Fifth step	1.10908
		Sixth step	5.55328

	TABLE 43C
	Depth [μm]
	Eleventh ring zone	First step	1.11224
		Second step	1.11384
		Third step	1.11544
		Fourth step	1.11704
		Fifth step	1.11867
		Sixth step	5.60147
	Twelfth ring zone	First step	1.12193
		Second step	1.12357
		Third step	1.12523
		Fourth step	1.12688
		Fifth step	1.12856
		Sixth step	5.65119
	Thirteenth ring zone	First step	1.13192
		Second step	1.13362
		Third step	1.13533
		Fourth step	1.13705
		Fifth step	1.13878
		Sixth step	5.70257
	Fourteenth ring zone	First step	1.14226
		Second step	1.14402
		Third step	1.14579
		Fourth step	1.14758
		Fifth step	1.14938
		Sixth step	5.75589
	Fifteenth ring zone	First step	1.15301
		Second step	1.15483
		Third step	1.15667
		Fourth step	1.15852
		Fifth step	1.16039
		Sixth step	5.81135

	TABLE 43D
	Depth [μm]
	Sixteenth ring zone	First step	1.16416
		Second step	1.16607
		Third step	1.16799
		Fourth step	1.16993
		Fifth step	1.17188
		Sixth step	5.86920
	Seventeenth ring zone	First step	1.17581
		Second step	1.17781
		Third step	1.17981
		Fourth step	1.18184
		Fifth step	1.18388
		Sixth step	5.92965
	Eighteenth ring zone	First step	1.18801
		Second step	1.19009
		Third step	1.19220
		Fourth step	1.19431
		Fifth step	1.19645
		Sixth step	5.99303
	Nineteenth ring zone	First step	1.20078
		Second step	1.20297
		Third step	1.20518
		Fourth step	1.20740
		Fifth step	1.20966
		Sixth step	6.05962
	Twentieth ring zone	First step	1.21420
		Second step	1.21651
		Third step	1.21884
		Fourth step	1.22119
		Fifth step	1.22356
		Sixth step	6.12973

	TABLE 43E
	Depth [μm]
	Twenty-first ring zone	First step	1.22836
		Second step	1.23079
		Third step	1.23325
		Fourth step	1.23573
		Fifth step	1.23822
		Sixth step	6.20378
	Twenty-second ring zone	First step	1.24332
		Second step	1.24589
		Third step	1.24849
		Fourth step	1.25113
		Fifth step	1.25378
		Sixth step	6.28230
	Twenty-third ring zone	First step	1.25918
		Second step	1.26192
		Third step	1.26470
		Fourth step	1.26750
		Fifth step	1.27034
		Sixth step	6.36607
	Twenty-fourth ring zone	First step	1.27611
		Second step	1.27905
		Third step	1.28203
		Fourth step	1.28505
		Fifth step	1.28810
		Sixth step	6.45599
	Twenty-fifth ring zone	First step	1.29433
		Second step	1.29752
		Third step	1.30074
		Fourth step	1.30401
		Fifth step	1.30734
		Sixth step	6.55356

	TABLE 43F
	Depth [μm]
	Twenty-sixth ring zone	First step	1.31414
		Second step	1.31762
		Third step	1.32116
		Fourth step	1.32476
		Fifth step	1.32843
		Sixth step	6.66080
	Twenty-seventh ring zone	First step	1.33597
		Second step	1.33984
		Third step	1.34380
		Fourth step	1.34782
		Fifth step	1.35194
		Sixth step	6.78077
	Twenty-eighth ring zone	First step	1.36045
		Second step	1.36485
		Third step	1.36934
		Fourth step	1.37395
		Fifth step	1.37867

Table 44 shows step heights of the stair-like diffraction structure provided on the second region of Example 6. In one cycle of the stair-like diffraction structure, the height of each of the first to seventh steps is set such that a phase difference of 1.25 wavelengths is provided to the light of the designed wavelength for BD, and the height of the eighth step is set such that a phase difference of 8.75 wavelengths is provided in the opposite direction.

	TABLE 44
	Depth [μm]
	First ring zone	First step	1.33470
		Second step	1.33995
		Third step	1.34543
		Fourth step	1.35111
		Fifth step	1.35706
		Sixth step	1.36328
		Seventh step	1.36976
		Eighth step	9.63594
	Second ring zone	First step	1.38370
		Second step	1.39120
		Third step	1.39909
		Fourth step	1.40740
		Fifth step	1.41618
		Sixth step	1.42546
		Seventh step	1.43530
		Eighth step	10.12016

It should be noted that although not shown, the 4-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element increases is provided on the third region of Example 6. The sawtooth-like diffraction structure is provided on the outer region of Example 6.

Table 45 shows diffraction efficiencies at the twenty-eighth ring zone of the first region and at the second ring zone of the second region. The second ring zone of the second region is an outermost region which contributes to formation of a spot of light for CD in the present example.

	TABLE 45
	Diffractive
	efficiency (%)
BD	Inner region	First ring zone	67
		Twenty-eighth ring zone	45
	Middle region	Second ring zone	57
DVD	Inner region	First ring zone	71
		Twenty-eighth ring zone	49
	Middle region	Second ring zone	30
CD	Inner region	First ring zone	65
		Twenty-eighth ring zone	32
	Middle region	Second ring zone	17

The ring zone cycle of the twenty-eighth ring zone of the first region is about 14 μm, and the diffraction efficiency of the light for BD is about 45%. Meanwhile, the ring zone cycle of the first ring zone of the first region is about 144 μm, and the diffraction efficiency of the light for BD is about 67%. Thus, the diffraction efficiency of the light for BD at the twenty-eighth ring zone of the first region is much lower than the diffraction efficiency at the first ring zone. When the present invention is not applied, an amount of light in reproducing/recording on BD is insufficient with this diffraction efficiency.

In contrast, the ring zone cycle of the second ring zone of the second region is about 21 μm. The diffraction efficiency of the light for BD at the second ring zone of the second region is about 57% and is greatly improved as compared to the diffraction efficiency at the twenty-eighth ring zone of the first region. Thus, insufficiency of the light amount in reproducing/recording on BD is suppressed.

Example 7

Example 7 corresponds to the third embodiment. The first surface of an objective lens element according to Example 3 is divided into a first region including a symmetry axis, a second region surrounding the first region, and an outer region surrounding the second region. An 8-level stair-like diffraction structure whose height monotonically decreases step by step as distance from the optical axis of the objective lens element decreases is provided on the first region of the first surface. A 4-level stair-like diffraction structure whose height monotonically increases step by step as distance from the optical axis of the objective lens element increases is provided on the second region. A sawtooth-like diffraction structure is provided on the outer region. The second surface is an aspheric surface. The objective lens element according to Example 7 is a BD/DVD/CD compatible lens. With regard to designed values for BD, the wavelength is 408 nm; the focal length is 1.80 mm; and the protective layer thickness of an information storage medium is 0.0875 mm. With regard to designed values for DVD, the wavelength is 658 nm; the focal length is 2.0 mm; and the protective layer thickness of an information storage medium is 0.6 mm. With regard to designed values for CD, the wavelength is 785 nm; the focal length is 2.1 mm; and the protective layer thickness of an information storage medium is 1.2 mm.

Tables 46 and 47 show construction data of the objective lens element according to Example 7.

TABLE 46
	BD	DVD	CD
Wavelength	0.408	0.658	0.785
Effective diameter	3.09	2.41	2.06
NA	0.86	0.6	0.47
Working distance (WD)	0.53	0.44	0.3
Disc thickness (DT)	0.0875	0.6	1.2
Focal length	1.8	2.0	2.1
First surface, First region	2	−2	−3
Diffraction order
First surface, Second region_Diffraction	1	−1	—
order
First surface, Outer region	3	—	—
Diffraction order
Object point (OP)	∞	−100	100
	Radius of curvature at
Surface No.	the top	Thickness	Material
0		OP
1	1.211178	2.256248	n1
2	−1.6991606	WD
3	∞	DT	disc
4	∞
	Wavelength	0.408	0.658	0.785
	n1	1.52173	1.50389	1.50072
	disc	1.61642	1.57829	1.57203

TABLE 47
First surface  First region
               Diffractive surface
Region         0 mm-1.06 mm
               Aspherical constant
RD             1.211178
CC             −0.4471445
A0             0
A2             0
A4             0.003042451
A6             −0.002139362
A8             −0.000916782
               Diffractive surface
P2             −95.525682
P4             10.679966
P6             −11.044402
P8             4.2227878
First surface  Second region
               Diffractive surface
Region         1.06 mm-1.22 mm
               Aspherical constant
RD             1.2175459
CC             −0.62312929
A0             0.003755648
A2             0
A4             0.0096913
A6             0.018056743
A8             −0.007521892
A10            −0.006034092
A12            0.006817604
A14            −0.001205342
A16            −0.000204353
               Diffractive surface
P2             −222.89625
P4             36.546342
P6             −10.136124
First surface  Outer region
               Diffractive surface
Region         1.22 mm-1.54 mm
               Aspherical constant
RD             1.2118213
CC             −0.63998677
A0             −0.0027581
A2             0
A4             0.013731193
A6             0.016620479
A8             −0.005058364
A10            −0.002588911
A12            0.000276612
A14            0.00109062
A16            −2.11E−04
A18            2.72E−05
A20            −3.38E−05
               Diffractive surface
P2             −274.96935
P4             117.11955
P6             −40.810453
Second surface First region
Region         0 mm-0.65 mm
               Aspherical constant
RD             −1.6991606
CC             −17.689289
A0             0
A2             0
A4             0.44868592
A6             −1.838644
A8             4.607409
A10            −4.4442043
A12            −2.9706261
A14            6.2221748
Region         0.65 mm-1.22 mm
               Aspherical constant
RD             −1.7038237
CC             −24.181952
A0             0.00145644
A2             0
A4             0.15414236
A6             −0.12617431
A8             −0.006173137
A10            0.014786931
A12            0.029242605
A14            −0.013049098
A16            −0.012637288
A18            0.005136492
A20            0.003026593
A22            −0.001344723

Tables 48A-48D show ring zone cycles of the stair-like step structure provided on the first region of the first surface, and cycles of steps arranged in each ring zone.

	TABLE 48A
	Cycle [μm]		Cycle [μm]
First ring zone	256.86	First step	90.30
		Second step	37.63
		Third step	28.92
		Fourth step	24.41
		Fifth step	21.53
		Sixth step	19.49
		Seventh step	17.94
		Eighth step	16.72
Second ring zone	364.57	First step	15.72
		Second step	14.89
		Third step	14.17
		Fourth step	13.56
		Fifth step	13.01
		Sixth step	12.54
		Seventh step	12.11
		Eighth step	11.72
Third ring zone	83.23	First step	11.37
		Second step	11.05
		Third step	10.75
		Fourth step	10.48
		Fifth step	10.23
		Sixth step	10.00
		Seventh step	9.78
		Eighth step	9.57
Fourth ring zone	70.52	First step	9.38
		Second step	9.20
		Third step	9.03
		Fourth step	8.87
		Fifth step	8.72
		Sixth step	8.57
		Seventh step	8.44
		Eighth step	8.30

	TABLE 48B
	Cycle [μm]		Cycle [μm]
Fifth ring zone	62.34	First step	8.18
		Second step	8.06
		Third step	7.94
		Fourth step	7.83
		Fifth step	7.73
		Sixth step	7.63
		Seventh step	7.53
		Eighth step	7.44
Sixth ring zone	56.46	First step	7.34
		Second step	7.26
		Third step	7.17
		Fourth step	7.09
		Fifth step	7.01
		Sixth step	6.94
		Seventh step	6.86
		Eighth step	6.79
Seventh ring zone	51.96	First step	6.72
		Second step	6.65
		Third step	6.59
		Fourth step	6.52
		Fifth step	6.46
		Sixth step	6.40
		Seventh step	6.34
		Eighth step	6.28
Eighth ring zone	48.36	First step	6.23
		Second step	6.17
		Third step	6.12
		Fourth step	6.07
		Fifth step	6.02
		Sixth step	5.97
		Seventh step	5.92
		Eighth step	5.87

	TABLE 48C
	Cycle [μm]		Cycle [μm]
Ninth ring zone	45.39	First step	5.83
		Second step	5.78
		Third step	5.74
		Fourth step	5.69
		Fifth step	5.65
		Sixth step	5.61
		Seventh step	5.57
		Eighth step	5.53
Tenth ring zone	42.90	First step	5.49
		Second step	5.45
		Third step	5.41
		Fourth step	5.38
		Fifth step	5.34
		Sixth step	5.31
		Seventh step	5.27
		Eighth step	5.24
Eleventh ring zone	40.77	First step	5.21
		Second step	5.17
		Third step	5.14
		Fourth step	5.11
		Fifth step	5.08
		Sixth step	5.05
		Seventh step	5.02
		Eighth step	4.99
Twelfth ring zone	38.94	First step	4.96
		Second step	4.93
		Third step	4.91
		Fourth step	4.88
		Fifth step	4.85
		Sixth step	4.83
		Seventh step	4.80
		Eighth step	4.78

	TABLE 48D
	Cycle [μm]		Cycle [μm]
Thirteenth ring zone	37.37	First step	4.75
		Second step	4.73
		Third step	4.70
		Fourth step	4.68
		Fifth step	4.66
		Sixth step	4.64
		Seventh step	4.61
		Eighth step	4.59
Fourteenth ring zone	36.01	First step	4.57
		Second step	4.55
		Third step	4.53
		Fourth step	4.51
		Fifth step	4.49
		Sixth step	4.47
		Seventh step	4.45
		Eighth step	4.43
Fifteenth ring zone	34.85	First step	4.42
		Second step	4.40
		Third step	4.38
		Fourth step	4.36
		Fifth step	4.35
		Sixth step	4.33
		Seventh step	4.31
		Eighth step	4.30
Sixteenth ring zone	33.87	First step	4.28
		Second step	4.27
		Third step	4.25
		Fourth step	4.24
		Fifth step	4.23
		Sixth step	4.21
		Seventh step	4.20
		Eighth step	4.19

On the first region of Example 7, one ring zone cycle is composed of consecutive 8-level stair-like steps whose height monotonically increases step by step as distance from the optical axis of the objective lens element increases. Each ring zone cycle in Tables 48A-48D indicate the width of a ring zone in a radial direction (in a direction perpendicular to the optical axis) as indicated by an arrow in FIG. 14. On the second region, a first ring zone, a second ring zone, a third ring zone, . . . , and a sixteenth ring zone are provided in order from the optical axis side toward the outer periphery side of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow in FIG. 14. In each ring zone, the steps are referred to as a first step, a second step, a third step, . . . , and an eighth step in order from the optical axis side toward the outer periphery side.

Tables 49A and 49B show ring zone cycles of the stair-like step structure provided on the second region of the first surface, and cycles of steps arranged in each ring zone.

	TABLE 49A
	Cycle [μm]		Cycle [μm]
First ring zone	16.38	First step	4.13
		Second step	4.12
		Third step	4.10
		Fourth step	4.09
Second ring zone	16.14	First step	4.07
		Second step	4.06
		Third step	4.04
		Fourth step	4.03
Third ring zone	15.90	First step	4.01
		Second step	4.00
		Third step	3.98
		Fourth step	3.97
Fourth ring zone	15.67	First step	3.95
		Second step	3.94
		Third step	3.93
		Fourth step	3.91
Fifth ring zone	15.45	First step	3.90
		Second step	3.88
		Third step	3.87
		Fourth step	3.85
Sixth ring zone	15.22	First step	3.84
		Second step	3.83
		Third step	3.81
		Fourth step	3.80
Seventh ring zone	15.00	First step	3.78
		Second step	3.77
		Third step	3.76
		Fourth step	3.74
Eighth ring zone	14.78	First step	3.73
		Second step	3.72
		Third step	3.70
		Fourth step	3.69

	TABLE 49B
	Cycle [μm]		Cycle [μm]
	Ninth ring zone	14.56	First step	3.67
			Second step	3.66
			Third step	3.65
			Fourth step	3.63
	Tenth ring zone	14.35	First step	3.62
			Second step	3.61
			Third step	3.59
			Fourth step	3.58

On the second region of Example 6, one ring zone cycle is composed of consecutive 4-level stair-like steps whose height monotonically increases step by step as distance from the optical axis of the objective lens element increases. Each ring zone cycle in Tables 49A and 49B indicate the width of a ring zone in the radial direction (in the direction perpendicular to the optical axis) as indicated by an arrow in FIG. 14. On the second region, a first ring zone, a second ring zone, a third ring zone, . . . , and a tenth ring zone are provided in order from the optical axis side toward the outer periphery side of the objective lens element. Further, each step cycle indicates the width, in the radial direction (in the direction perpendicular to the optical axis), of a step provided in each ring zone, as indicated by an arrow in FIG. 14. In each ring zone, the steps are referred to as a first step, a second step, a third step, and a fourth step in order from the optical axis side toward the outer periphery side.

Tables 50A-50D shows step heights of the stair-like diffraction structure provided on the first region of Example 7. In one cycle of the stair-like diffraction structure, the height of each of the first to seventh steps is set such that a phase difference of 1.33 wavelengths is provided to light of the designed wavelength for BD, and the height of the eighth step is set such that a phase difference of 6.65 wavelengths is provided in the opposite direction.

	TABLE 50A
	Depth [μm]
	First ring zone	First step	0.97858
		Second step	0.97964
		Third step	0.98070
		Fourth step	0.98178
		Fifth step	0.98286
		Sixth step	0.98394
		Seventh step	0.98504
		Eighth step	6.90296
	Second ring zone	First step	0.98724
		Second step	0.98836
		Third step	0.98948
		Fourth step	0.99060
		Fifth step	0.99174
		Sixth step	0.99288
		Seventh step	0.99403
		Eighth step	6.96623
	Third ring zone	First step	0.99634
		Second step	0.99751
		Third step	0.99869
		Fourth step	0.99986
		Fifth step	1.00105
		Sixth step	1.00225
		Seventh step	1.00345
		Eighth step	7.03264
	Fourth ring zone	First step	1.00588
		Second step	1.00710
		Third step	1.00834
		Fourth step	1.00958
		Fifth step	1.01081
		Sixth step	1.01206
		Seventh step	1.01333
		Eighth step	7.10211

	TABLE 50B
	Depth [μm]
	Fifth ring zone	First step	1.01586
		Second step	1.01715
		Third step	1.01844
		Fourth step	1.01973
		Fifth step	1.02103
		Sixth step	1.02234
		Seventh step	1.02365
		Eighth step	7.17483
	Sixth ring zone	First step	1.02631
		Second step	1.02765
		Third step	1.02899
		Fourth step	1.03034
		Fifth step	1.03170
		Sixth step	1.03308
		Seventh step	1.03445
		Eighth step	7.25078
	Seventh ring zone	First step	1.03721
		Second step	1.03861
		Third step	1.04001
		Fourth step	1.04143
		Fifth step	1.04285
		Sixth step	1.04428
		Seventh step	1.04571
		Eighth step	7.33005
	Eighth ring zone	First step	1.04860
		Second step	1.05006
		Third step	1.05153
		Fourth step	1.05300
		Fifth step	1.05449
		Sixth step	1.05598
		Seventh step	1.05748
		Eighth step	7.41283

	TABLE 50C
	Depth [μm]
	Ninth ring zone	First step	1.06049
		Second step	1.06201
		Third step	1.06354
		Fourth step	1.06508
		Fifth step	1.06663
		Sixth step	1.06818
		Seventh step	1.06974
		Eighth step	7.49919
	Tenth ring zone	First step	1.07289
		Second step	1.07448
		Third step	1.07608
		Fourth step	1.07768
		Fifth step	1.07929
		Sixth step	1.08091
		Seventh step	1.08254
		Eighth step	7.58923
	Eleventh ring zone	First step	1.08583
		Second step	1.08749
		Third step	1.08915
		Fourth step	1.09083
		Fifth step	1.09251
		Sixth step	1.09420
		Seventh step	1.09590
		Eighth step	7.68329
	Twelfth ring zone	First step	1.09934
		Second step	1.10108
		Third step	1.10281
		Fourth step	1.10456
		Fifth step	1.10633
		Sixth step	1.10810
		Seventh step	1.10988
		Eighth step	7.78173

	TABLE 50D
	Depth [μm]
	Thirteenth ring zone	First step	1.11348
		Second step	1.11529
		Third step	1.11711
		Fourth step	1.11895
		Fifth step	1.12079
		Sixth step	1.12265
		Seventh step	1.12451
		Eighth step	7.88480
	Fourteenth ring zone	First step	1.12829
		Second step	1.13019
		Third step	1.13210
		Fourth step	1.13403
		Fifth step	1.13596
		Sixth step	1.13791
		Seventh step	1.13988
		Eighth step	7.99304
	Fifteenth ring zone	First step	1.14385
		Second step	1.14585
		Third step	1.14786
		Fourth step	1.14989
		Fifth step	1.15194
		Sixth step	1.15399
		Seventh step	1.15606
		Eighth step	8.10705
	Sixteenth ring zone	First step	1.16025
		Second step	1.16236
		Third step	1.16449
		Fourth step	1.16664
		Fifth step	1.16880
		Sixth step	1.17098
		Seventh step	1.17316
		Eighth step	8.22763

Tables 51A and 51B show step heights of the stair-like diffraction structure provided on the second region of Example 6. In one cycle of the stair-like diffraction structure, the height of each of the first to third steps is set such that a phase difference of 1.25 wavelengths is provided to the light of the designed wavelength for BD, and the height of the fourth step is set such that a phase difference of 8.75 wavelengths is provided in the opposite direction.

	TABLE 51A
	Depth [μm]
	First ring zone	First step	1.18405
		Second step	1.18635
		Third step	1.18866
		Fourth step	3.57300
	Second ring zone	First step	1.19335
		Second step	1.19571
		Third step	1.19810
		Fourth step	3.60154
	Third ring zone	First step	1.20294
		Second step	1.20538
		Third step	1.20783
		Fourth step	3.63094
	Fourth ring zone	First step	1.21280
		Second step	1.21531
		Third step	1.21785
		Fourth step	3.66120
	Fifth ring zone	First step	1.22298
		Second step	1.22556
		Third step	1.22816
		Fourth step	3.69236
	Sixth ring zone	First step	1.23344
		Second step	1.23610
		Third step	1.23878
		Fourth step	3.72443
	Seventh ring zone	First step	1.24420
		Second step	1.24694
		Third step	1.24969
		Fourth step	3.75739
	Eighth ring zone	First step	1.25525
		Second step	1.25806
		Third step	1.26089
		Fourth step	3.79118

	TABLE 51B
	Depth [μm]
	Ninth ring zone	First step	1.26659
		Second step	1.26948
		Third step	1.27236
		Fourth step	3.82583
	Tenth ring zone	First step	1.27820
		Second step	1.28115
		Third step	1.28410
		Fourth step	3.86123

It should be noted that although not shown, the sawtooth-like diffraction structure is provided on the outer region of Example 7.

Table 52 shows object point distances in Examples 6 and 7.

TABLE 52
unit: mm
	Example
	6	7
	L1	−200	−100
	L2	100	100

The present invention can be used for an objective lens element used for performing at least one of recording, reproducing, and erasing of information on optical discs of a plurality of standards for which light of different wavelengths is used, and for an optical head device including the objective lens element.

While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It will be understood that numerous other modifications and variations can be devised without departing from the scope of the invention.

Claims

1. An objective lens element configured for converging each of light of a first wavelength, light of a second wavelength longer than the first wavelength, and light of a third wavelength longer than the second wavelength on an information recording surface of an optical disc, the objective lens element being configured such that: where,

L1<0  (3),

L2>0  (4),

L1 is a first distance from an incident surface of the objective lens element to an object point of a light source of the second wavelength, and

L2 is a second distance from the incident surface of the objective lens element to an object point of a light source of the third wavelength.

2. An optical head device configured for converging a first incident light beam of a first wavelength through a base plate of a first thickness to form a spot, a second incident light beam of a second wavelength, longer than the first wavelength, through a base plate of a second thickness, larger than the first thickness, to form a spot, and a third incident light beam of a third wavelength, longer than the second wavelength, through a base plate of a third thickness, larger than the second thickness, to form a spot, the optical head device comprising:

a first light source configured for emitting a light beam of the first wavelength;

a second light source configured for emitting a light beam of the second wavelength;

a third light source configured for emitting a light beam of the third wavelength;

an objective lens element according to claim 1; and

a detection element configured for detecting light reflected by an information recording surface of an optical disc.