Abstract: A diffraction grating with periodically arranged protrusions and grooves at a relatively narrow pitch and improved diffraction efficiency is disclosed. The protrusions of the grating are made of a material whose index of refraction is greater than that of the grooves, and the ratio of the width D of the protrusion to the pitch &Lgr; of the protrusion is set equal to or less than 0.4 (D/&Lgr;≦0.4).

Abstract: A hybrid achromatic optical lens having a high numerical aperture whose chromatic aberration is removed and a method for manufacturing the same are provided. The hybrid achromatic optical lens includes a first optical member of a low index of refraction and a second optical member of a high index of refraction. The second optical member is formed on a depressed portion of the first optical member and has a diffractive surface, which is a contact surface of the second optical member with the first optical member and has a plurality of pitches formed on a refractive surface.

Abstract: Microparticles 8 includes an optical substrate 10 having at least one diffraction grating 12 disposed therein. The grating 12 having a plurality of colocated pitches &Lgr; which represent a unique identification digital code that is detected when illuminated by incident light 24. The incident light 24 may be directed transversely from the side of the substrate 10 with a narrow band (single wavelength) or multiple wavelength source, in which case the code is represented by a spatial distribution of light or a wavelength spectrum, respectively. The code may be digital binary or may be other numerical bases. The micro-particles 8 can provide a large number of unique codes, e.g., greater than 67 million codes, and can withstand harsh environments. The micro-particles 8 are functionalized by coating them with a material/substance of interest, which are then used to perform multiplexed experiments involving chemical processes, e.g., DNA testing and combinatorial chemistry.

Abstract: A diffractive optical element for guiding a light having a color spectrum characterized by a plurality of wavelengths longer than a shortest wavelength, &lgr;B, and shorter than a longest wavelength, &lgr;R, the light striking the diffractive optical element at an angle greater than a first field-of-view angle, &agr;−FOV, and smaller than a second field-of-view angle, &agr;+FOV. The diffractive optical element comprising a linear grating being formed in a light-transmissive substrate. The linear grating is characterized by a pitch, d, selected so as to allow total internal reflection of a light having wavelength of &lgr;B and a striking angle of &agr;−FOV. The light-transmissive substrate is characterized by an index of refraction, ns, larger than a minimal index of refraction, nMIN, which is selected so as to allow total internal reflection of a light having wavelength of &lgr;R and a striking angle of &agr;+FOV.

Abstract: In order to obtain a diffractive optical element with which a high diffraction efficiency can be obtained in a wide wavelength range and color flare is not noticeable and a picture taking apparatus using this diffractive optical element, an image is formed on a picture taking unit by using a diffractive optical element in an optical system. The diffractive optical element has a grating structure obtained by stacking a plurality of diffraction gratings made of at least two types of materials with different dispersion properties to enhance diffraction efficiency for a particular design order throughout a predetermined wavelength region and a plurality of design wavelengths which exist in a predetermined design order and at which a maximum optical path length difference in the grating structure becomes an integer multiple of a wavelength, the diffractive optical element having a design wavelength &lgr;0 satisfying predetermined conditions.

Abstract: A diffraction grating includes a metallic base layer and layers of dielectric materials of varying refractive index, where a bottom interface of the layers is adherent to the metallic base layer. The dielectric layers are periodically spaced on top of the metallic base layer, leaving the metallic base layer exposed in regions. This grating allows for the polarization insensitive reflective properties of the base metallic layer to operate in conjunction with the polarization sensitive diffraction properties of the multilayer grating structure to provide near 100% diffraction efficiency over a reasonable wavelength bandwidth, independent of the polarization of the incident beam.

Abstract: A grating fabrication process utilizes real-time measurement of a grating characteristic (such as, for example, grating period chirp, reflectivity, group delay) as a feedback error signal to modify the writing process and improve the characteristics of the finished grating. A test beam is launched through the optical medium during the writing process (or at the end of an initial writing process) and a particular characteristic is measured and used to generate a “corrective” apodization refractive index profile that can be incorporated with the grating to improve its characteristics. The improvements may be applied to a phase (or amplitude) mask used to write the grating (etching, local deformation, coating changes, for example), or the grating itself may be corrected using additional UV exposure, non-uniform annealing, non-uniform heating, and/or non-uniform tension—these techniques applied separately or in an intermittent sequence.

Abstract: An enhanced volume phase diffraction grating provides high dispersion, uniformly high diffraction efficiency and equal diffraction efficiencies for all polarizations across a wide range of wavelengths. The thickness of the volume phase material and the modulation of its refractive index are jointly established to provide equalization of diffraction efficiencies for all polarizations over a wide range of wavelengths. The equalization occurs where the S and P diffraction efficiencies are both at a maximum.

Abstract: A phase mask, for writing fiber Bragg gratings (FBG) in an optical fiber, adjusts the amplitude and the phase of the FBG, while maintaining a constant mean index of refraction of the fiber in a single pass. Specifically, the phase mask embodies the amplitude information so that the amplitude information is an integral part of the phase mask and preferably cannot be separated from the phase information. A first embodiment employs a reflective or opaque surface, defining a window, on the substrate of a phase mask controlling the amplitude of light passing through the phase mask. Another embodiment employs a polygonal shaped grating region on a clear substrate. A third embodiment interleaves regions of grating and smooth substrate surface. Preferred embodiments employ two areas of gratings with the areas disposed: perpendicularly, out of bandwidth or out of phase, relative to each other or additive.

Abstract: The present invention provides a scanning device with the following features: the system is capable of writing data to record carriers of a first format because it is corrected for chromatic aberration resulting from fast wavelength variations during write operation; it has a diffractive element with a generally sawtooth-like pattern, such that it may for example be manufactured in a single-step replication process; the system has high efficiency for scanning first optical record carriers (e.g. DVDs) and acceptable efficiency for scanning second optical record carriers (e.g. CDs); and the lens provides limited spherochromatism and the system is thus able to cope with wavelength variations.

Abstract: A reflective lamellar diffraction grating is provided that is suitable for a variety of applications, including applications related to C-band telecommunication functions. The average efficiency of the diffraction grating in S- and P-polarization states exceeds 90% while simultaneously providing a PDL less than 0.2 dB over the entire wavelength range used for C-band telecommunication functions. The diffraction grating is thus suitable for incorporation into various telecommunication systems, including a wavelength router configured for routing signals having a plurality of spectral bands.

Abstract: A diffractive element used in conjunction with a grating to move a desired order beam off-axis, thereby reducing interference by undesired orders. Use of the off-axis grating allows a more uniform beam in the presence of manufacturing defects, such as etch depth errors. The diffractive element used with the off-axis grating may include a beam shaper, a one-dimensional beam splitter, or a two-dimensional beam splitter.

Abstract: A first diffractive optical element pattern with a pattern pitch that is no greater than a wavelength of incident light is formed on a first main surface of substrate such as a glass plate. Second diffractive optical element patterns are formed at positions that are respectively incident to positive first-order diffracted light and negative first-order diffracted light produced by the first diffractive optical element pattern. Negative first-order diffracted light produced by each second diffractive optical element pattern is incident upon a boundary face of the substrate at an angle that is smaller than the critical angle, and so exits the substrate.

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: A method for manufacturing an optical waveguide refractive index grating having a desired grating pitch &Lgr;. The method includes the step of providing a photosensitive waveguide and a writing beam of actinic radiation, the writing beam having an intensity. The waveguide is translated relative to the writing beam at a velocity v(t). The intensity of the writing beam is modulated as a function of time at a frequency f(t), wherein v ⁢ ( t ) f ⁢ ( t ) ≈ Λ . The step of modulating the intensity of the writing beam as a function of time f(t) at a frequency f(t) including the step of varying &Lgr;.

Abstract: A diffractive optical element has a plurality of diffraction structures for a certain wavelength. These each have a width measured in the plane of the diffractive optical element and a height measured perpendicularly thereto. The widths and the heights of the diffraction structures vary over the area of the diffractive optical element. An optical arrangement comprising such a diffractive optical element has, in addition, a neutral filter. The efficiency of such a diffractive optical element and of such an arrangement can be optimized locally for usable light.

Abstract: A lamellar volume grating operating in reflection and transmission and providing a substantially wavelength- and polarization-independent diffraction efficiency is disclosed. The lamellar grating has an approximately rectangular grating profile with a height-to-width ratio of the grooves or “teeth” advantageously greater than 2. The grating operates preferably in first order and can be implemented as an immersion grating. In particular, the lamellar volume grating described herein can be employed in an optical multiplexer/demultiplexer arrangement with small overall dimensions.

Abstract: Diffraction optical element used in a scanning optical apparatus as a scanning optical element has a diffraction grating formed on a substrate. The diffraction grating has a tilt portion for generating a power and a wall portion connecting one end portion of the tilt portion to the substrate. The wall portion is tilted within a predetermined range with respect to a normal of the substrate surface. The tilt angle of the wall portion of the diffraction grating with respect to the normal of the substrate continuously changes to increase as a distance away from an optical axis of the diffraction optical element becomes large.

Abstract: The invention relates to a beam shaper, which is intended for use in connection with a quasi-monochromatic light source (1) and which is fabricatedf rom a substantially transparent material as a transmission element (3) guiding the propagation of light for rounding, making elliptical, collimating, diverging, converging and/or for the like application of a light beam/beams (R). The beam shaper has its transmission element (3′) guiding the light beam/beams (R) provided with a strucutre which at least partially consists of binary, surface relief type of diffractive patterns, having local grating periods thereof optimized with respect to longitudinal and transverse directions, as well as with respect to an optical axis, essentially in accordance with the Bragg diffraction geometry for providing a maximum diffraction efficiency.

Abstract: The invention provides a method for producing a diffractive optical element, including forming first and second gratings, substantially in mutual registration, of at least first and second optical materials, such that for predetermined two or more wavelengths, the diffractive optical element has desired phase retardations, thereby avoiding the generation of chromatic aberrations. The invention further provides a diffractive optical element.

Abstract: A lens optical system is provided with a cemented lens element formed by cementing two constituent lens elements made of different optical materials together, with a diffractive optical surface formed at the cementing interface between the two constituent lens elements. The two constituent lens elements have at their respective interfaces with air a radius of curvature different from the radius of curvature that they have at the cementing interface.

Abstract: A sub-wavelength anti-reflective diffractive structure is incorporated with a base diffractive structure having a small period to form a high efficiency diffractive structure. In the high efficiency diffractive structure, the anti-reflective structure and/or the base diffractive structure are altered from their ideal solo structure to provide both the desired performance and minimize reflections.

Abstract: In order to improve a structure arrangement comprising a plurality of portions having a relief structure with an optical-diffraction effect, in particular for visually identifiable optical security elements for value-bearing documents, for example banknotes, credit cards, passes or cheque documents, or other articles to be safeguarded, in such a way that it can give a viewer an image impression which in particular is more homogenous and more brilliant than is possible with known structure arrangements, it is proposed that a predominant number of the portions is of strip or band configuration.

Abstract: An imaging element includes a diffraction optical element on which an off-axis beam with a field angle is incident and a stop. Of beams having different field angles and incident on the diffraction optical element, beams having incident angles larger than the incident angle of a principal light ray of the beams incident on the grating surface of the diffraction optical element are partially shielded by a light-shielding member, so that the diffraction efficiencies of orders adjacent to a design order are kept low and hence imaging performance almost free from flare can be achieved even if a beam with a field angle is incident.

Abstract: Filter gratings are formed in optical waveguides having photosensitive cores by exposing the cores to actinic radiation in the form of interfering beams having peak intensities that are relatively displaced along an optical axis of the waveguides. Each of the interfering beams has a single-lobed intensity profile and a degree of spatial coherence required to achieve a desired fringe contrast between the two relatively displaced beams. Index modulations in the photosensitive core match the illumination (interference) pattern of the radiation. The relative displacement of the interfering beams reduces side lobes of the gratings' spectral responses by leveling the average refractive index of the index modulations. A second exposure with the two beams but without the beams' interference effects further levels the average refractive index.

Abstract: A light dispersing device for dispersing a polychromatic input beam into a plurality of narrow-band output beams such that the output beams are directed in a wavelength dependent manner and such that the directions of the output beams are less affected by a change in temperature. The preferred embodiment of the device comprises a diffraction grating mounted to a surface of a prism. The grating and prism have dispersing characteristics that make the output of the device substantially insensitive to a change in temperature. In one embodiment, the device is adapted to reduce the angles between the input beam and the output beams at the grating, and thereby improve throughput efficiency.

Abstract: Fluoropolymers consisting of alternating perfluorocyclobutane and aryl ether linkages possess suitable properties for optical waveguides and other devices using refractive properties of the polymers. Perfluorocyclobutane (PFCB) polymers having aryl groups alternating on an ether chain have shown useful physical properties for optical waveguide applications. Processes for micromolding such polymeric films by replicating a pattern or image directly from a silicon master, rather than from a polydimethyl siloxane (PDMS) mold) are disclosed.

Abstract: A grating structure with a dielectric coating is disclosed that operates efficiently away from the blaze angle in low order with a high diffraction efficiency and high wavelength dispersion. Such grating structure can be employed in Littrow configuration to provide, for example, cavity feedback in excimer lasers.

Abstract: An optical device scans record carriers with radiation of two wavelengths. The device has an objective lens with a diffractive part with a pattern of elements of stepped profile. The total of the step heights are substantially equal to multiples of height h1 with h 1 = λ 1 ( n - n 0 ) , where &lgr;1 is the first wavelength, n is the refractive index of the diffractive part, and n0 is the refractive index of an adjacent medium. The radiation of the first wavelength is transmitted through the diffractive part substantially without diffraction and the objective lens has a first focusing characteristic for the first wavelength.

Abstract: A lens system includes a lens, a grating and a non-periodic phase structure. The lens system is a-thermalised by a compensation of the temperature-dependence of the spherical aberration of the lens by the temperature-dependence of the spherical aberration of the non-periodic phase structure and the grating. The lens system is also a-spherochromatised by a compensation of the wavelength-dependence of the spherical aberration of the grating by the wavelength-dependence of the spherical aberration of the non-periodic phase structure.

Abstract: Optical gratings are fabricated on a scale that may be smaller than the resolution of the lithographic system used to generate the grating pattern. Parallel ridges are formed using lithographic techniques. A conformal deposition and anisotropic etch are then used to form sidewalls on the sides of the ridges. After removing the ridges, the remaining sidewalls are used as a mask to etch the substrate. Removing the sidewalls leaves the desired grating pattern, with a pitch spacing of one-half that of the original ridges.

Abstract: A laminate-type diffractive optical element which is easily producible and which allows high optical performance to be easily achieved, and an optical system using the same, are disclosed. In the diffractive optical element formed by laminating a plurality of diffraction gratings, when the distance between the uppermost portion of one diffraction grating and the lowermost portion of the other diffraction grating in two diffraction gratings adjacent to each other along the incident direction of a light is represented by D (&mgr;m), these two diffraction gratings are arranged to satisfy the following relation: 10 &mgr;m<D<40 &mgr;m.

Abstract: The invention concerns a light reflecting element comprising a substrate (10), a multilayer mirror (20) and optical coupling means (32, 40) comprising a diffraction grating (40); whereby the reflection coefficient of one polarization is damped without damping the reflection coefficient of the orthogonal polarization over a broad wavelength range and with a large tolerance on the optogeometrical parameters of the device.

Abstract: An optical element is provided with one or more sets of diffractive elements. Individual diffractive element transfer functions collectively yield corresponding overall transfer functions between corresponding entrance and exit ports. Diffractive elements are defined by contours that include diffracting region(s) altered to diffract, reflect, and/or scatter incident optical fields (altered index, surface, etc). Element transfer functions are determined by: fraction of contour filled by diffracting region(s) (partial-fill grayscale); and/or the spatial profile of the diffracting region(s) (profile-based grayscale). Optical elements may be configured: as planar or channel waveguides, with curvilinear diffracting segments; to support three-dimensional propagation with surface areal diffracting segments; as a diffraction grating, with grating groove segments.

Abstract: A method of preparing a composite micro relief element. The micro relief element comprises a first layer of a first substrate and a second layer of a relief forming polymer comprising an overlay and at least one relief feature portion. The method includes the steps of forming a line of contact between the receptive surface and at least one mold feature formed in a flexible dispensing layer, applying a quantity of resin to fill the mold feature along the line of contact. Moving the line of contact across the receptive surface whereby sufficient resin is captured by the mold feature and a quantity of resin passes the line of contact to form the overlay portion. The resin is then cured and the flexible dispensing layer is released.

Abstract: A diffractive optical element for white light is composed of a plurality of layers of optical materials and has a relief pattern constituting a diffraction grating formed at least at one cementing interface between the two different optical materials. The grating height of the relief pattern constituting the diffraction grating is defined by the following formula: h=&lgr;/|n−n′| where h represents the grating height of the relief pattern constituting the diffraction grating; &lgr; represents the wavelength (here it is assumed that &lgr;≦450 (nm)); n represents the refractive index of the optical material abutting the interface from the object side for light of the wavelength &lgr;; and n′ represents the refractive index of the optical material abutting the interface from the image side for light of the wavelength &lgr;.

Abstract: A layered diffractive optical element is disclosed, with which a high diffraction efficiency is attained over a broad region of used wavelengths, and with which diffraction light of unnecessary diffraction orders can be suppressed. A diffractive optical element of the present invention includes a plurality of diffraction gratings made of materials with different Abbe numbers, the plurality of diffraction gratings being layered with each other. Moreover, the partial dispersion ratio with respect to a g-line and a F-line of the material constituting at least one of the plurality of diffraction gratings satisfies the condition represented by a specified expression.

Abstract: An auxiliary light projection apparatus used for focus detection has a laser beam generating device and a diffraction plate for diffracting a laser light from said laser beam generating device. The diffraction plate has micro lenses regularly arranged in vertical and horizontal directions. The curvature of each micro lens in the vertical direction is different from that in the horizontal direction. The pitch of arrangement of the micro lenses in the vertical direction is different from that in the horizontal direction. In addition, the height of the micro lenses is varied cyclically. The auxiliary light projection apparatus projects a diffraction light pattern of the laser light emergent from the diffraction plate to an object to be photographed, as a pattern used for focus detection.

Abstract: A method for designing a diffractive lens that separates an incident light beam into a plurality of beams and condenses the respective separated beams is provided. The method includes: defining an expression of a diffractive surface having a beam converging function based on given focal length; defining an expression of a diffractive surface having a beam separating function that separates a light beam into plural orders of diffractive light based on said given focal length, a given number of separated light and a given distance between focusing points; and superimposing the defined expressions of the diffractive surface having the beam converging function and the diffractive surface having the beam separating function to determine a shape of the diffractive lens having a plurality of optical functions.

Abstract: A diffractive optical element includes a diffraction optical part provided with a phase-type diffraction grating. The diffraction grating of the diffraction optical part has on the surface thereof minute uneven structure of which the period is substantially constant. The period is smaller than a wavelength used.

Abstract: In one embodiment of the invention, a grating structure etched on a mirror substrate has a grating period causing diffracting, out of an optical path, a first incident radiation within a first band around a first wavelength. A multi-layer coating deposited on the grating structure reflects the first incident radiation, in the optical path, within the first band and a second incident radiation within a second band around a second wavelength. In another embodiment, a first multi-layer coating deposited on a mirror substrate reflects a first incident radiation within a first band around a first wavelength and a second incident radiation, in an optical path, within a second band around a second wavelength. A grating structure is deposited on the first multi-layer coating. The grating structure is etched to have a grating period causing diffracting, out of the optical path, the second incident radiation within the second band.

Abstract: A variable spacing diffraction grating is fabricated using micro-electromechanical (MEMS) technology. An array of interconnected beam elements is fabricated into a diffraction grating and mounted in such a manner that one or both ends of the array of beam elements may be actuated using a mechanical actuator. The beam elements may be linear, spiral shaped or arranged in a staircase structure. Applying a force to one or both ends of the array of beam elements changes the spacing of the grating, and presents a different ruling spacing distribution to incoming radiation, thus altering the diffracted angle among individual diffraction orders of the wavelength. Controlling the diffracted signal in this way allows for specific diffraction pass bands to be fixed on a particular detector or a particular area of a detector or optical relay lens or lenses.

Abstract: A diffractive optical element for guiding a light having a color spectrum characterized by a plurality of wavelengths longer than a shortest wavelength, &lgr;B, and shorter than a longest wavelength, &lgr;R, the light striking the diffractive optical element at an angle greater than a first field-of-view angle, &agr;−FOV, and smaller than a second field-of-view angle, &agr;+FOV. The diffractive optical element comprising a linear grating being formed in a light-transmissive substrate. The linear grating is characterized by a pitch, d, selected so as to allow total internal reflection of a light having wavelength of &lgr;B and a striking angle of &agr;−FOV. The light-transmissive substrate is characterized by an index of refraction, ns, larger than a minimal index of refraction, nMIN, which is selected so as to allow total internal reflection of a light having wavelength of &lgr;R and a striking angle of &agr;+FOV.

Abstract: The methods and apparatus according to the invention equalize the power of at least one frequency in a multi-wavelength optical signal, or limit the power contained in a single or multi-frequency signal. More particularly, the optical power equalizer according to the invention is a filter with separably variable wavelength dependent transmission coefficients, wherein each coefficient decreases with increasing power for each respective wavelength coupled to the equalizer. Thus, the highest power wavelength output from an EDFA will be filtered more than the lower power wavelengths, making the output power from the EDFA more evenly distributed among the wavelengths. Such an equalizer can be placed downstream from each EDFA without destabilizing the optical network so that no changes need to be made to the EDFA or to the other components in the system.

Abstract: A small-sized and low-cost wavelength division multiplexer having little insertion loss, little polarization dependence and a broad wavelength bandwidth, the wavelength division multiplexer adopting a grating configuration in which an incident light is retroreflected, exit lights from respective grooves are enhanced by interference effect in the incident direction of the light, wave surfaces of evanescent waves in the grooves are parallel to the normal direction of the grating and phases of the evanescent waves in the respective grooves agree with each other. The wavelength division multiplexer has high diffraction efficiency in each of TM and TE polarized lights at a several-order diffraction order and accordingly has a broad wavelength bandwidth and remarkably low polarization dependence.

Abstract: A beam generator, or beam shaping system, for example for use in an optical scanner, creates a non-Gaussian beam which provides improved indicia-reading characteristics. In one embodiment, diffractive optical elements are used to create a Bessel-Gaussian scanning beam, which comprises a coherent combination of a Gaussian beam and a Bessel beam.

Abstract: A diffractive optical element in which eclipse of rays at a grating side surface of a diffraction grating can be reduced is disclosed. In a diffractive optical element comprises a plurality of diffraction gratings made of different materials are laminated, at least one of the plurality of diffraction gratings includes a region in which the grating side surface inclines or a region in which edges thereof are shifted with respect to the edge of another diffraction grating adjacent thereto. The inclination or shift amount is different according to the positions thereof in a surface of the diffractive optical element in accordance with the incident angle or emergent angle of a specified ray such as a ray determined on the basis of an average value, gravity value, maximum value and minimum value obtained from the distribution of incident angle or emergent angle or the like, rays, etc.

Abstract: A renewable liquid reflection grating. Electrodes are operatively connected to a conducting liquid in an arrangement that produces a reflection grating and driven by a current with a resonance frequency. In another embodiment, the electrodes create the grating by a resonant electrostatic force acting on a dielectric liquid.

Abstract: A grating fabrication process utilizes real-time measurement of a grating characteristic (such as, for example, grating period chirp, reflectivity, group delay) as a feedback error signal to modify the writing process and improve the characteristics of the finished grating. A test beam is launched through the optical medium during the writing process (or at the end of an initial writing process) and a particular characteristic is measured and used to generate a “corrective” apodization refractive index profile that can be incorporated with the grating to improve its characteristics. The improvements may be applied to a phase (or amplitude) mask used to write the grating (etching, local deformation, coating changes, for example), or the grating itself may be corrected using additional UV exposure, non-uniform annealing, non-uniform heating, and/or non-uniform tension—these techniques applied separately or in an intermittent sequence.

Abstract: An optical system includes, in order from an object side to an image side, a first lens unit of positive optical power, and a second lens unit of negative refractive power, wherein focusing from an infinitely distant object to a closest object is effected by moving the second lens unit toward the image side along an optical axis, and the second lens unit consists of one negative lens. Further, it is desirable that the following conditions are satisfied: d2L1(&lgr;)/d&lgr;2>0 d2T1(&lgr;)/d&lgr;2<0 &ngr;2>30 where L1(&lgr;) is an aberration coefficient of longitudinal chromatic aberration of the first lens unit, T1(&lgr;) is an aberration coefficient of lateral chromatic aberration of the first lens unit, &lgr; is a wavelength of light, and &ngr;2 is an Abbe number of material of the negative lens of the second lens unit.