Abstract: An object of the present invention is to attain stable tracking servo performance by suppressing an offset caused by a shift of an object lens or a tilt of a disk, despite the one-beam method which does not cause reduction in light quantity of the main beam. A diffraction grating is provided between a hologram and a light receiving section, and a diffraction efficiency of the diffraction grating is varied in a grating longitudinal direction. For example, if an incident light beam on the diffraction grating is shifted in the grating longitudinal direction, the quantity of received light in each light receiving section varies to cause offset. By performing tracking servo so as to cancel the change, it is possible to correct the offset, thereby attaining stable tracking servo performance.

Abstract: An optical pickup comprises: first and second semiconductor lasers (11, 14); a hologram element (17) for diffracting light reflected by an optical information recording medium (9 or 12); and a plurality of photodetectors (18-23). The hologram element (17) has two or more different diffraction regions. The plurality of photodetectors (18-23) are provided away from the first and second semiconductor lasers (11, 14) at both sides thereof along an extended line of a line between light emission positions of the first and second semiconductor lasers (11, 14). The diffracted light generated from the light beam of the first wavelength (10) by the hologram element (17) and the diffracted light generated from the light beam of the second wavelength (13) by the hologram element (17) are collected at substantially the same position at one side, and part (18, 19) of the plurality of photodetectors (18-23) are provided at the position.

Abstract: An optical pickup apparatus includes a first light source to emit a first light beam having a predetermined wavelength, a first optical path changer to change a proceeding path of the first light beam, an objective lens to focus the first light beam on a recording medium, a diffraction member to divide the first light beam reflected by the recording medium into five light regions, the diffraction member having a first diffraction region having a wide width in a direction corresponding to a tangential direction of the recording medium and second through fifth diffraction regions sequentially arranged around the outside of the first diffraction region in a direction corresponding to a radial direction of the recording medium, and a first photodetector having first through fifth light receiving portions to receive the first light beam reflected by the recording medium.

Abstract: An optical pickup apparatus, comprising: a light source to emit a light flux; a light converging system including an objective lens to converge the light flux emitted from said light source onto an optical information recording medium; a first diffraction element placed within an optical path up to the optical information recording medium; a light flux splitting element that is placed within the optical path from said light source up to the optical information recording medium and that splits an incident light flux emitted by the light source; a monitor element that receives a light flux split by said light flux splitting element and outputs a signal according to a received light amount; a second diffraction element placed between said light flux splitting element and said monitor element; and a control section that controls the drive of said light source according to the signal from said monitor element.

Abstract: An objective lens for an optical pickup is configured such that, when a light beam having a wavelength corresponding to the optical disc requiring the higher NA is converged on the optical disc requiring the higher NA at the higher NA, the phase of light flux passed through the peripheral area is substantially the same as that passed through the central area. When a light beam having a wavelength corresponding to the optical disc requiring the lower NA is converged on the optical disc requiring the lower NA at the lower NA, the phase of light flux passed through a first annular region included in the peripheral area is different from that passed through the central area, and the phase of light flux passed through a second annular region, which is outside the first region, is substantially the same as that passed through the central area.

Abstract: Regional borderlines that are the borderlines between the first region and the second region on a diffraction grating of an optical pickup partially overlap a group of y-axis parallel borderlines that are borderlines between pattern A and pattern B that are parallel to a direction parallel to the y-axis.

Abstract: An optical pickup device includes an irradiating optical system for converging a light beam to form a spot on a recording surface via a light-transmitting layer of an optical recording medium, and a photodetection optical system for converging return light reflected and returned from the spot on a photodetector. The device detects wave aberration and focal error of the light beam. A diffractive optical element is disposed on the optical axis of return light in the photodetection optical system and provided with an annulus. The diffractive optical element annularly extracts, from return light, ray components in the vicinity of a predetermined radius on a pupil on the emitting pupil surface of the irradiating optical system, which is affected by the wave aberration generated in the optical system.

Abstract: An optical head device includes a light source for emitting light; a collection optical system for collecting the light emitted by the light source to an information memory medium including tracks having prescribed grooves; a light detector having a plurality of detection areas for receiving the light reflected by the information memory medium and outputting a signal in accordance with a light amount of the light received; a tracking error signal generator for receiving the signals output from the light detector and generating a tracking error signal based on the signals; and a light division element for dividing a reflected light reflected by the information memory medium.

Abstract: An optical head device includes: an optical element collecting light reflected from a reproduction information layer (a target information layer in an optical information recording medium) and light reflected from information layers adjacent to the reproduction information layer at different positions; a light receiving element obtaining a detection signal from the reflected light collected by this optical element; and arithmetic circuitry obtaining a reproduction signal from this detection signal.

Abstract: An optical pickup device including a dual wavelength light source module for emitting a selected one of two light beams having different wavelengths, a dual grating for splitting, into three beams, the selected light beam emitted from the dual wavelength light source, and a hologram for focusing the three beams of the selected light beam onto a photodetector having typical 8-section patterns such that the three beams of the selected light beam are focused onto the same spots as those of the other light beam, respectively.

Abstract: An optical disk device is provided that can realize favorable recording/reproduction of signals on/from an optical disk having a plurality of signal planes arranged in proximity to each other. In this optical disk device, a hologram (4) is divided into n(n?2) regions Ak(k=1, 2, . . . , n) by a straight line that intersects with an optical axis, and a photodetector (7) is divided into at least two regions A and A?. Light emitted from a light source (1) is focused on a signal plane (6a) or (6b) included in a plurality of signal planes of the optical disk by an objective lens (5). Light reflected from a first signal plane (6a) and light reflected from a second signal plane (6b) pass through the objective lens (5) to turn into light beams a and a?, respectively, that enter the hologram (4).

Abstract: Light emitted from a semiconductor laser (1) is split by a diffractive optical element (3a) into a main beam and sub-beams. The diffractive optical element (3a) is divided into four regions by a straight line parallel to a tangential direction of a disc (7) and a straight line parallel to a radial direction of the disc (7). A phase of grating in two regions arranged diagonally and a phase of grating in the other two regions arranged diagonally are different from each other by ?. A focused spot of the main beam and focused spots of the sub-beams are located on the same track. A focusing error signal is detected by a differential astigmatism method, using the main beam and the sub-beams.

Abstract: An optical head is provided, which suppresses the offset in the tracking error signal.

Abstract: In a holographic pattern provided in a holographic optical element, a pattern 1a twists a diffracted light beam in a clockwise direction, to form a semi-circular light spot Sa on photodetection parts A and B so as to extend over a dividing line LX in a four-segment photodetection part. A pattern 1b similarly twists the diffracted light beam in a clockwise direction, to form a semi-circular light spot Sb on photodetection parts C and D so as to extend over a dividing line LX in the four-segment photodetection part.

Abstract: An objective lens of an optical pickup apparatus converges a divergent light flux onto an information recording surface. The following conditional formula is satisfied: |?SA1/?U|·|?U|+|?SA2/?T|·|?T|?0.07 ?rms where ? represents a wavelength of a light source, ?SA1/?U represents a change of a spherical aberration for an object-to-image distance change ?U (|?U|?0.5 mm) and ?SA2/?T represents a change of spherical aberration for a temperature change ?T (|?T|?30° C.), the object-to-image distance is a distance between the light source (a light emitting point) and the information recording surface.

Abstract: In an optical head device and an optical information recording/reproducing device using the optical head device, by driving a liquid crystal panel 4a, the liquid crystal panel 4a produces wave aberration for light on a going path and light on a returning path likewise. Accordingly, when the substrate-thickness deviation or tilt of the disc 7 is corrected, the wave aberration due to the substrate-thickness deviation or tilt of the disc 7 and the wave aberration produced by the liquid crystal panel 4a are canceled by each other for both of the light on the going path and the light on the returning path, so that no wave aberration remains for the light on the returning path and the phase distribution is not varied.

Abstract: To stably carry out recording and reproducing to and from a high density optical disk without using a double servo in the optical disk using a high NA objective lens. A detection of a spherical aberration and a detection of a coma aberration in a radial direction are simultaneously performed, and the coma aberration generated with the offset of an objective lens 109 is corrected in real time, thus enlarging an allowable offset amount of the objective lens. In order to simultaneously detect the spherical aberration and the coma aberration, focal shift and tracking shift signals in an inside region and outside region of reflected light flux are detected respectively, and the differential signals are set as spherical aberration and coma aberration signals.

Abstract: An optical head device that converges first and second laser beams having different wavelengths on recording surfaces of first and second optical recording media through a single light converging optical system including an objective lens for recording or reproducing data on the recording surfaces. A refracting surface of the objective lens is divided into two regions, i.e., a central refracting surface region with an optical axis thereof being as a center and an outer circumferential refracting surface region that surrounds an outer circumference of the central refracting surface region. A diffraction grating is formed generally entirely over the central refracting surface region. When recording or reproducing data on the first optical recording medium using the first laser beam, a beam spot is formed with a diffracted beam obtained by the central refracting surface region.

Abstract: An optical pickup is provided for a recording medium, including: a light source generating and emitting a light beam; a light beam division and detection unit dividing a particular light beam portion of the light beam after being reflected/diffracted from the recording medium into sub-divided light beams portions, and detecting the sub-divided light beam portions; and a spherical aberration detection circuit processing the sub-divided light beam portions to detect spherical aberration caused by thickness variation of the recording medium.

Abstract: In the past there has been a problem that on a detection surface the influence of interference causes a defocusing signal to degrade, narrowing the range in which spherical aberration can be stably detected. Accordingly, a diffraction grating is used to focus the inner and outer sides of luminous flux on separate optical detectors before the optical flux is focused on an optical detector and defocusing signals are independently calculated to find the difference therebetween, thereby providing a spherical aberration signal. This makes it possible to detect spherical aberation signals more stably.

Abstract: Light from a laser diode is directed by mirrors on angled surfaces of an optical unit through a first lens at an optical disc. Light returns from the disc on a parallel path through a second lens and is directed at a photodetector. A semiconductor device controls the operation of the laser diode and receives signals form the photodetector for processing. The optical unit is formed from one or two glass elements cut from a larger glass wafer or wafers using photolithographic techniques. The mirrors are formed by thin films deposited on angled glass surfaces formed by selective plasma etching of the wafers. At least one mirror is partially reflective and is formed by depositing a thin film of titanium nitride.

Abstract: An optical pickup device is adapted to read information signals from optical recording media of two different types such as a “DVD” and a “CD” for which two light beams with different wavelengths are used and comprises a photodetector having a single light receiving section that can be shared by optical recording media of two different types. Both the light beam reflected from the signal recording surface of the “DVD” 106a and the light beam reflected from the signal recording surface of the “CD” 106b are diffracted by the diffraction element 6 and focussed to a same spot on the light receiving surface of the photodiode of a photodetector 7.

Abstract: A device for scanning a first and second type of optical record carriers (2, 40) generators a first and a second radiation beam for scanning the first and second type of record carriers, respectively. The information layers (4, 42) of the first and second type of record carriers is scanned through transparent layers (3, 41) of different thickness. The first radiation beam (17) has a first wavelength and a first numerical aperture NA?1. The second radiation beam (46) has a different, second wavelength and an effective second numerical aperture NA2 smaller NA1. The rays of the second radiation beam having an NA smaller than NA2 form a central sub-beam (48), the rays having a larger NA form an outer sub-beam (49). The device includes a non-periodic phase structure that does not affect the first radiation beam. The phase structure introduces an amount of spherical aberration in the central sub-beam (48). The phase structure is transparent for the central and outer sub-beam (48, 49).

Abstract: In a holographic pattern provided in a holographic optical element, a pattern 1a twists a diffracted light beam in a clockwise direction, to form a semi-circular light spot Sa on photodetection parts A and B so as to extend over a dividing line LX in a four-segment photodetection part. A pattern 1b similarly twists the diffracted light beam in a clockwise direction, to form a semi-circular light spot Sb on photodetection parts C and D so as to extend over a dividing line LX in the four-segment photodetection part.

Abstract: A six-segment holographic surface is divided into regions by dividing lines. A four-segment photodetection part is divided into four photodetection parts equal in area by a section line substantially parallel to the radial direction of an optical disk and a section line orthogonal thereto. A main light beam diffracted in the regions of the six-segment holographic surface are condensed as spots at positions apart from each other on opposite sides on a section line of the four photodetection parts, and the main beam diffracted in the regions is condensed as spots in the center of the photodetection parts of the four-segment photodetection part.

Abstract: An optical pickup head device includes a diffraction grating for generating zero-order diffracted light and at least first-order diffracted light and provides a tracking error signal with a DPP method. The diffraction grating includes grating patterns with a nonuniform period or phase. The size of the first-order diffracted light converged on an optical recording medium is larger in the direction parallel to a tangent to the track than in the direction perpendicular to the tangent. P1/P0>PW2/PW1 is established, where PW1 represents the power that is required to record information on the optical recording medium, PW2 represents the maximum power that allows information recorded on the optical recording medium to be reproduced without being erased, P0 represents the light amount of the zero-order diffracted light converged on the optical recording medium, and P1 represents the light amount of the at least first-order diffracted light converged on the optical recording medium.

Abstract: There are provided an information write device and an information read device enabling to generate highly accurate contrast signals. When a main light spot Pc is located at the center of the a groove G, the light spot Pc and sub light spots Psa, Psb are each adapted to radiate a disc DSC such that the sub light spots Psa, Psb radiate positions displaced from the center of the land L. The reflected beams of light from the disc DSC caused by the radiation with the light spots Pc, Psa, Psb are detected to generate push-pull signals each corresponding to the light spots Pc, Psa, Psb, respectively, in accordance with each of the detected signals. Furthermore, a signal to be obtained by amplifying an addition signal, given by adding the push-pull signals each corresponding to the sub light spots Psa, Psb, with a predetermined amplification factor K/n, and a push-pull signal corresponding to the main light spot Pc are added to thereby generate a contrast signal.

Abstract: There is disclosed an optical device in which a first light source for outputting a first wavelength light is apart from a second light source for outputting a second wavelength light by a predetermined distance. An information recording medium is irradiated with the first and second wavelength lights transmitted through a holographic optical element having first and second diffraction areas. The first and second diffraction areas are provided with grating arrangements in which grating axis directions are parallel to each other and grating pitches are different from each other. The first and second wavelength lights reflected by the information recording medium are transmitted through the holographic optical element and diffracted by the first and second diffraction areas.

Abstract: An optical pickup device has a semiconductor laser for emitting a light beam, a hologram, a light-concentrating optical system, a collimation lens and a measuring unit for measuring quantity of light. The hologram generates a first light beam by splitting the light beam, which has passed through the light-concentrating optical system, along a first straight line that is perpendicular to the optical axis of the light beam and serves as a boundary, and then guides the light beam to the measuring unit. The measuring unit is provided with photosensors, which are provided linearly symmetrical each other with respect to an axis of symmetry of a straight line that extends through the optical axis of the first light beam and corresponds to the first straight line and arranged in positions located apart from the optical axis of the first light beam. An aberration signal is detected by a difference between electric signals from two of the photosensors.

Abstract: An optical pickup device is equipped with a first light source that emits first light, a second light source that emits second light having a wavelength different from a wavelength of the first light, and a diffraction element that deflects the first light or the second light to match optical axes of the lights. The diffraction element is a step-like diffraction element in which one of an incident face and an emitting face thereof has a step-like grating face. A step difference of the step-like grating face is set to have a measurement that generates a phase difference of one wavelength of one of the first light and the second light, and the number of steps of the step-like grating face is set to maximize a (+) first order diffraction efficiency or a (?) first order diffraction efficiency for the other light.

Abstract: A slim optical pickup having a light source that emits light onto an optical disc and an optical bench on which the light source is mounted. The optical bench includes a plane tilted at a predetermined angle in an optical path in front of the light source and a monitor photodetector that is disposed along the plane and receives the light emitted by the light source in order to monitor the amount of light. The slim optical pickup is advantageous in improving light reception efficiency and achieving a slimmer design.

Abstract: A pickup for a magneto-optical recording medium is provided, which improves quality of reproduced signals by eliminating a phase difference between the polarizations and is capable of coping with magneto-optical recording media of different recording/reproducing systems. A return light beam of light reflected by a magneto-optical recording medium is polarized and separated into ± first-order diffracted lights through a light-branching diffraction element provided between a light source and an objective lens. The ± first-order diffracted lights are compensated for their phases through first and second phase compensation elements so that there is no phase difference between the polarizations, and are, further, polarized and separated through the first and second polarization/separation elements, and are received by first and second light detectors.

Abstract: This invention is directed to an optical pickup apparatus which includes a first laser source which emits a first beam, a second laser source which emits a second beam having a polarization plane substantially perpendicular to a polarization plane of the first beam, a polarization diffraction element which selectively diffracts one of the first beam and the second beam in accordance with polarized states thereof, and an objective lens which records and/or reproduces information by focusing the first beam which has passed through the polarization diffraction element onto an information recording surface of a first optical information recording medium, and records and/or reproduces information by focusing the second beam which has passed through the polarization diffraction element onto an information recording surface of a second optical information recording medium.

Abstract: To stably carry out recording and reproducing to and from a high density optical disk without using a double servo in the optical disk using a high NA objective lens. A detection of a spherical aberration and a detection of a coma aberration in a radial direction are simultaneously performed, and the coma aberration generated with the offset of an objective lens 109 is corrected in real time, thus enlarging an allowable offset amount of the objective lens. In order to simultaneously detect the spherical aberration and the coma aberration, focal shift and tracking shift signals in an inside region and outside region of reflected light flux are detected respectively, and the differential signals are set as spherical aberration and coma aberration signals.

Abstract: An optical pickup apparatus having compatibility with a recodable compact disk (CD-R) and a digital video disk (DVD), which uses a wavelength of one of a first light beam and a second light beam according to the recording medium to be used. In the optical pickup apparatus, laser light sources emit a first light beam having a relatively shorter wavelength for the. DVD and a second light beam having a longer wavelength for the CD-R, respectively. An objective lens has a predetermined focal length in accordance with the position of an information recording surface in the DVD. An optical path control unit controls the path of light beams so that the light beam emitted from one of the laser light sources is directed to the objective lens and the light output from the objective lens is directed to the optical detection unit.

Abstract: In an optical head of a DVD drive, a light-emitting device and light-receiving devices are disposed on one face of a substrate, and on the other face of the substrate, a diffraction light separating device, a &lgr;/4 plate, and a dielectric plate having a converging focus lens are sequentially, hierarchically integrated. With the configuration, a laser beam (transmission beam) emitted from the front light-outgoing end of the semiconductor laser device sequentially passes through the diffraction light separating device, &lgr;/4 wave plate, and dielectric plate having the converging focus lens to form an image on a recording face of a recording medium (optical recording medium). A reflected beam reflected by the recording face returns reversely along the optical path through which the transmission beam has traveled, the phase of the reflected beam is changed by the &lgr;/4 wave plate, and the phase-changed reflected beam is separated into two primary diffracted beams in two directions.

Abstract: An object of the present invention is to attain stable tracking servo performance by suppressing an offset caused by a shift of an object lens or a tilt of a disk, despite the one-beam method which does not cause reduction in light quantity of the main beam. A diffraction grating is provided between a hologram and a light receiving section, and a diffraction efficiency of the diffraction grating is varied in a grating longitudinal direction. For example, if an incident light beam on the diffraction grating is shifted in the grating longitudinal direction, the quantity of received light in each light receiving section varies to cause offset. By performing tracking servo so as to cancel the change, it is possible to correct the offset, thereby attaining stable tracking servo performance.

Abstract: To provide a light receiving device, a light detecting device, and an optical signal reproducing device each of which allows one to perform many different computations in detecting aberration amounts and focus error quantities without requiring exact position relations between laser light to be received and light receiving elements, and between the light receiving elements.

Abstract: Disclosed herein is an optical pick-up apparatus using a holographic optical element and method of forming holographic gratings of the element. The apparatus includes a light emitting element for generating three beams with different wavelengths, a multiplexed holographic optical element provided with three holographic gratings for receiving beams reflected from an optical disc and diffracting the received beams according to wavelengths of the received beams, and a light receiving element for receiving beams diffracted while passing through the multiplexed holographic optical element. The method includes the steps of forming a first holographic grating on a transparent substrate, forming a first transparent layer on the substrate on which the first holographic grating is formed, forming a second holographic grating on the first transparent layer, forming a second transparent layer on the first layer on which the second holographic grating is formed, and forming a third holographic grating on the second layer.

Abstract: An optical device includes a light-emitting element for irradiating light onto an information recording medium, a diffraction grating for splitting light emitted from said light-emitting element into a plurality of beams, a focussing member for focussing the plurality of beams onto the information recording medium, a deflection member for deflecting the plurality of beams after they have been reflected from the information recording medium; and a photodetector for receiving the plurality of beams after they have been deflected by the deflection member. The diffraction grating has a first grating region and a second grating region, which have different diffraction efficiencies. The zero-order diffraction light in the first grating region is used as the main beam for reproducing the information signal, and the +1-order or −1-order diffraction light in the second grating regions is used as sub-beams for reproduction of the tracking error signal.

Abstract: To implement light quantity monitoring with high frequency responsivity and correction of astigmatic differences of a semiconductor laser with a simple configuration with a fewer parts.

Abstract: An optical pickup assembly includes an optical pickup unit (OPU), a thermally conductive plate, and a flex circuit. The flex circuit is mounted atop the plate. The OPU is also mounted atop the plate through a cutout of the flex circuit. Alternatively, the flex circuit is mounted below the plate and the OPU is mounted atop the plate. The optical pickup assembly further includes an actuator arm. The OPU is mounted on the actuator arm with a portion of the plate contacting the actuator arm.

Abstract: An object of the present invention is to provide an optical head apparatus which are capable of detecting correctly a radial tilt of an optical recording medium, without allowing offsets in radial tilt signals, even when an objective lens is shifted in a radial direction. A beam emitted from a semiconductor laser is divided into 0th order main beam and ±1st order sub beams. The three beams are shifted in the radial direction of the disc. The three beams reflected from the disc are diffracted by a holographic element, and are received by a photo detector. With respect to the main beam and the sub beams, a radial tilt of the disc is detected on the basis of a difference between intensities of the main and sub beams diffracted from a plurality of regions of the holographic element.

Abstract: Substantially all of the laser light emitted from laser chip(s) 11 toward optical disk(s) 17 may be transmitted through polarizing hologram(s) 13 and may thereafter be transmitted through quarter-wave plate(s) 15, and after being reflected at optical disk(s) 17, may again be transmitted through quarter-wave plate(s) 15 and be incident on polarizing hologram(s) 13. Accordingly, where laser light is transmitted twice through quarter-wave plate(s) 15, the direction(s) of polarization thereof may be rotated by 90 degrees before it is incident on polarizing hologram(s) 13. For this reason, even if laser light is p-polarized when it is transmitted through polarizing hologram(s) 13 from laser chip(s) 11, it may be s-polarized when it is again transmitted through polarizing hologram(s) 13 after being reflected from optical disk(s) 17. Laser light, after being made s-polarized, may be diffracted by polarizing hologram(s) 13, which diffracts only s-polarized light.

Abstract: The invention concerns a recording medium for optically readable data of the multilayer type, a method for making same and an optical system for reproducing said data for reading the medium.

Abstract: An optical pickup device is adapted to read information signals from optical recording media of two different types such as a “DVD” and a “CD” for which two light beams with different wavelengths are used and comprises a photodetector having a single light receiving section that can be shared by optical recording media of two different types. Both the light beam reflected from the signal recording surface of the “DVD” 106a and the light beam reflected from the signal recording surface of the “CD” 106b are diffracted by the diffraction element 6 and focussed to a same spot on the light receiving surface of the photodiode of a photodetector 7.

Abstract: A polarization diffraction grating includes two media having different orientation states arranged alternately and cyclically, wherein each boundary between the media forms an oblique rectangular shape.

Abstract: An optical disk device has an aperture of an objective lens in an incoming path of a beam from a semiconductor laser to an optical disk formed larger than an aperture in a return path from the optical disk or an aperture is varied in recording and in reproduction. This configuration improves recording/reproducing ability since light is focused on an optical disk with high numerical aperture. In addition, since reflected light from the optical disk is detected with low numerical aperture, margins for tilt and defocus are not reduced. Furthermore, since unnecessary signal components contained in the reflected light can be eliminated, a SIN (signal-to-noise ratio) of an information signal also increases. Thus, a high-performance optical disk device can be obtained. Alternatively, by varying the aperture of an objective lens in recording and in reproduction, an optical disk device in which recording density and recording quality are increased without deteriorating reproduction quality can be obtained.

Abstract: An optical disk device has an aperture of an objective lens in an incoming path of a beam from a semiconductor laser to an optical disk formed larger than an aperture in a return path from the optical disk or an aperture is varied in recording and in reproduction. This configuration improves recording/reproducing ability since light is focused on an optical disk with high numerical aperture. In addition, since reflected light from the optical disk is detected with low numerical aperture, margins for tilt and defocus are not reduced. Furthermore, since unnecessary signal components contained in the reflected light can be eliminated, a S/N (signal-to-noise ratio) of an information signal also increases. Thus, a high-performance optical disk device can be obtained. Alternatively, by varying the aperture of an objective lens in recording and in reproduction, an optical disk device in which recording density and recording quality are increased without deteriorating reproduction quality can be obtained.

Abstract: A compound objective lens is composed of a hologram lens or transmitting a part of incident light without any diffraction to form a beam of transmitted light and diffracting a remaining part of the incident light to form a beam of first-order diffracted light, and an objective lens for converging the transmitted light to form a first converging spot on a front surface of a thin type of first information medium and converging the diffracted light to form a second converging spot on a front surface of a thick type of second information medium. Because the hologram selectively functions as a concave lens for the diffracted light, a curvature of the transmitted light differs from that of the diffracted light.