Abstract: A diffractive optical element is positioned in an optical path between a coherent light source and a surface. The diffractive optical element is designed to disperse the light across one or more spots on the surface in a manner designed to create speckle patterns having substantially uniform intensities.

Abstract: This invention is an optical pickup device having a composite optical element (32) which has a first diffraction grating (45) for splitting a light beam emitted from a light source (31) into zeroth-order light, plus-first-order light and minus-first-order light, a second diffraction grating (46) for diffracting the optical path of a return light beam from an optical disc (2), and a split prism (47) arranged at a position where the minus-first-order light diffracted by the second diffraction grating (46) is incident and adapted for splitting the minus-first-order light into a plurality of light beams. It also has a light receiving unit (35) for acquiring a focusing error signal FE by receiving each return light beam split by the split prism (47) and for acquiring a tracking error signal by receiving return light beams from the optical disc (2) of the plus-first-order light and the minus-first-order light split by the first diffraction grating (45).

Abstract: A quadrant detector includes a holographic optical element and a plurality of detector elements. The holographic optical element is designated to have four quadrants, each of which directs radiation incident thereon to an associated detector element. Signals from the detector elements are processed individually to optimise the performance of each detector element.

Abstract: The invention relates to an optical element with variable optical properties, which ensures that the amount of light available is increased, and an optical device using the same. In a liquid crystal variable hologram element 3, polymer layers 18 and liquid crystal layers 19 are alternately arranged between transparent substrates 16 and 17, with liquid crystal particles 20 lining up in each liquid crystal layer 19. When voltages are applied on transparent electrodes 21 and 22, liquid crystal molecules 23 have their longitudinal directions oriented vertically with respect to the electrodes, so that the refractive index of the polymer layers 18 is substantially equal to that of the liquid crystal layers 19, resulting in no development of hologram. When the voltages are held off, the refractive index of the polymer layers 18 is different from that of the liquid crystal layers 19. This repetition gives rise to an interference fringe action, by which a hologram can be developed.

Abstract: An electro-holographic lens device comprising: an active layer made from a transparent photo-refractive material, whose refractive index may be locally modified by locally applying an electric field; an array of electrodes in a predetermined density, provided adjacent the active layer for locally providing electric fields across an array of predetermined locations in the active layer; and a control unit for applying and controlling application of electric fields in a dynamically controllable predetermined locations for creating a holographic fringe pattern across the active layer.

Abstract: A filtering technique for a free space communication that features encoding and decoding of signals employing a filtering apparatus that includes a bulk holographic transform function. Employing the encoding and decoding technique facilitates providing a great number of channels of communication in a unit volume while preventing unwanted cross-talk between the communication channels. In addition, secure communication links between transmitters and receivers may be provided.

Abstract: Astigmatism and coma are suppressed in an optical pickup device such that light from a single real laser light source is diffracted and divided by a hologram module into a plurality of light spots which are focused upon a plurality of tracks of an optical disk to read data on the tracks at the same time. A non-diffraction end of fraction hologram patterns of the hologram module are determined so that light diffracted by the hologram module is given inverse aberration of that to be caused by optical elements in optical path from the real laser light source to the optical disk. Another hologram pattern provides a uniform intensity of a light spot formed of a laser light by an optical element.

Abstract: A part (1) made of transparent material that allows visible light waves to pass therethrough, said part comprising outer faces (11, 12) defining a solid body (10) made of transparent material, the part being characterized in that it further comprises light diffracting means (14) situated inside the solid body (10) at a distance from the outer faces (11, 12).

Abstract: This invention is an optical pickup device having a composite optical element (32) which has a first diffraction grating (45) for splitting a light beam emitted from a light source (31) into zeroth-order light, plus-first-order light and minus-first-order light, a second diffraction grating (46) for diffracting the optical path of a return light beam from an optical disc (2), and a split prism (47) arranged at a position where the minus-first-order light diffracted by the second diffraction grating (46) is incident and adapted for splitting the minus-first-order light into a plurality of light beams. It also has a light receiving unit (35) for acquiring a focusing error signal FE by receiving each return light beam split by the split prism (47) and for acquiring a tracking error signal by receiving return light beams from the optical disc (2) of the plus-first-order light and the minus-first-order light split by the first diffraction grating (45).

Abstract: Optical transceivers include a diffractive optical element (DOE) attached to a surface of a prism or other optical support. The DOE is configured to direct an input optical signal to a planar or curved reflective surface, or receive an output optical signal from the planar or curved reflective surface at angles greater than a critical angle in the prism. In some examples, the optical support includes one or more curved reflective surfaces and the DOE is a hologram. Such optical transceivers include a reflective surface that is rotatable with respect to the DOE, or with respect to a selected communication direction and the DOE for selection of a transmission or reception direction. The optical supports of such optical transceivers can be mounted to a window, and include a reflective region configured to total internally reflect optical signals. Selection of a communication direction is based on a rotation of the rotatable reflective surface.

Abstract: Optical transceivers include a diffractive optical element (DOE) attached to a surface of a prism or other optical support. The DOE is configured to direct an input optical signal to a curved reflective surface or receive an output optical signal from the curved reflective surface at angles greater than a critical angle. In some examples, the optical support includes one or more curved reflective surfaces and the DOE is a hologram. Such optical transceivers can be provided with optical mounts configured for attachment to a building window.

Abstract: A hologram screen 1 displays an image by diffracting and scattering image light 31 projected from an image projection apparatus 2. An upward/downward light scattering device 13, which scatters light incident from an upward/downward specific angle range spreading obliquely upward or downward, is placed on the image projection apparatus side of a hologram device 12 in the hologram screen 1. A leftward/rightward light scattering device 14, which scatters light incident from a leftward/rightward specific angle range spreading obliquely leftward and rightward, is placed between the hologram device 12 and the scattering device 13. The construction is such that the upward/downward specific angle range contains the incidence angle of the image light 31 on the hologram screen 1, and thus provides a hologram screen having good color reproducibility and which permits viewing of the background.

Abstract: An image display apparatus has an image display device for displaying an image, an eyepiece optical system, including a holographic magnifying optical element, for magnifying and projecting the image, and a holographic filter, disposed either as part of the image display device or in the optical path between the image display device and the eyepiece optical system, for restricting the wavelength range of the image light.

Abstract: A holographic optical element (HOE) device is mounted in wireless optical telecommunication system transceiver. The HOE device includes a developed emulsion material having an interference pattern recorded thereon, sandwiched between a pair of elements, such as a pair of clear glass plates. In operation, the HOE device uses the recorded interference pattern to diffract received light rays towards an optical processing unit of the system receiver. A transmitter unit can be positioned at least in part behind the HOE device. An opening in the emulsion material allows a light signal transmitted from the transmitter unit to be substantially unaffected by the recorded interference pattern. The transceiver can also include a spotting scope and an alignment beacon.

Abstract: A light transmission system includes a laser, an optical fiber, and a transfer lens. The transfer lens transfers light emitted by the laser into the optical fiber. The transfer lens includes a diffractive surface for receiving and collimating the light originating form the laser. The diffractive surface is defined by a surface function that includes a first phase function having angular symmetry and a second phase function having radial symmetry. The second phase function includes a cusp region with a discontinuous slope therein. The transfer lens provides reflection management so that light reflected from the end of the optical fiber is not focused at a location at which light is emitted by the laser and also favorable launch conditions so that light launched into the optical fiber avoids index anomalies along the axis of the optical fiber.

Abstract: A filtering technique for free space communication that features an improved extinction ratio by providing a filter that employs a bulk holographic transform function and a polarizing film. In this manner, a greater number of channels of communication may be provided in a unit volume while preventing unwanted cross-talk between the communication channels. To that end, the system includes a source of energy to direct energy along a path, a detector disposed in the path, and a filter. The filter has a surface upon which a polarizing film is disposed and a holographic transform function recorded throughout a volume thereon.

Abstract: An object beam divided by a light dividing means 12 is caused to enter a photo-refractive polymer layer 6 through an optical fiber 23. On the other hand, a reference beam divided by the light dividing means 12 is irradiated on the photo-refractive polymer layer 6 through a lens 13 from the opposite side. In this manner, the object beam is superimposed on the reference beam in the photo-refractive polymer layer 6 to provide an interference fringe or pattern. Since a photo-refractive polymer has a specific characteristic whereby the refractive index changes according to the intensity of irradiation light and the change is fixed after the irradiation light is stopped, a diffraction grating 5 corresponding to the interference fringe is recorded on the photo-refractive polymer layer 6. This diffraction grating 5 exhibits a specific characteristic whereby the object beam emitted from the optical fiber 23 is emitted in the direction of the reference beam.

Abstract: A rain sensor for a motor vehicle, having a transmitter emitting radiation, a receiver, and a holographically embodied diffractive element that has a linear-grating-like structure.

Abstract: The invention relates to an optical element with variable optical properties, which ensures that the amount of light available is increased, and an optical device using the same. In a liquid crystal variable hologram element 3, polymer layers 18 and liquid crystal layers 19 are alternately arranged between transparent substrates 16 and 17, with liquid crystal particles 20 lining up in each liquid crystal layer 19. When voltages are applied on transparent electrodes 21 and 22, liquid crystal molecules 23 have their longitudinal directions oriented vertically with respect to the electrodes, so that the refractive index of the polymer layers 18 is substantially equal to that of the liquid crystal layers 19, resulting in no development of hologram. When the voltages are held off, the refractive index of the polymer layers 18 is different from that of the liquid crystal layers 19. This repetition gives rise to an interference fringe action, by which a hologram can be developed.

Abstract: A light transmission system includes a laser, an optical fiber, and a transfer lens. The transfer lens transfers light emitted by the laser into the optical fiber. The transfer lens includes a diffractive surface for receiving and collimating the light originating form the laser. The diffractive surface is defined by a surface function that includes a first phase function having angular symmetry and a second phase function having radial symmetry. The second phase function includes a cusp region with a discontinuous slope therein. The transfer lens provides reflection management so that light reflected from the end of the optical fiber is not focused at a location at which light is emitted by the laser and also favorable launch conditions so that light launched into the optical fiber avoids index anomalies along the axis of the optical fiber.

Abstract: A planar substrate having a first diffractive element for coupling light waves of different colors into the substrate and guiding the light waves by successive internal reflections. A second diffractive element, disposed on the substrate, causes the guided light waves to be partially transmitted out of the substrate where the light waves encounter the second diffractive element. Because light waves of each color are reflected at different reflection angles, the light waves with smaller reflection angles encounter the second diffractive element at more locations than those with larger reflection angles, resulting in color non-uniformity in the light transmitted out from the substrate surface. One or more interfaces are provided between the surfaces of the substrate to selectively reflect the light waves having larger reflection angles toward the second diffraction element, so that light waves of different colors encounter the second diffraction element substantially at the same number of locations.

Abstract: A holographic telescope (40). The novel invention is comprised of an eyepiece (44) and a first holographic optical element (42) positioned to receive incident electromagnetic energy and focus the energy on the eyepiece (44). In the preferred embodiment, the eyepiece (44) is a second holographic optical element, and both holographic optical elements are volume holograms. Each holographic optical element can be transmissive or reflective depending on packaging needs. The holographic telescope (40) can be multi-spectral by including multiple independent holograms in each holographic optical element, each hologram responding only to a particular wavelength.

Abstract: A holographic element (HOE) with multiple apertures formed in a single substrate for use an optical communication system. The multi-aperture HOE can be use instead of an optics unit having one or more separate optical elements for each aperture needed by an optical transmitter, receiver or transceiver. The single substrate reduces complexity and cost relative to conventional system that the use separate optical elements to implement the optics unit.

Abstract: An apparatus for a free space optical communication system includes an element having a front surface and a back surface. A holographic optical element (HOE) is disposed to receive a light signal from the free space optical communication system, and to angularly direct the light signal, having a certain wavelength, towards one of the surfaces of the element to allow the light signal to propagate between the front and back surfaces of the element via total internal reflection. The internally reflected light signal eventually converges and is received at an optical detector circuit. At the same time, light having other wavelengths, including visible background light, passes through the HOE, thereby giving the apparatus an appearance of being transparent and unobtrusive.

Abstract: A reflection type color display device and a reflection type of direct-view color display device are provided. The reflection type color display device includes a spatial light modulator having a collection of pixels and a controllable transmittance per pixel, with a reflection type hologram color filter located on the back side of the modulator. The reflection type of direct-view color display device includes a hologram color filter which has an array of light-collecting holograms. Each of the light-collecting holograms has its own hologram color filter, reflection type hologram, and transmission type spatial light modulator located between the hologram color filter and the reflection type hologram.

Abstract: A free space (FSO) receiver includes an illumination sensing unit with multiple illumination apertures. The illumination sensing unit is used in deriving alignment information from portion(s) of a received FSO signal that are incident on the illumination apertures. The FSO receiver provides this information to the FSO transmitter that transmitted the FSO signal so that the FSO transmitter can adjust the direction it is pointing. The illumination apertures can be formed in a holographic optical element on a single substrate. Further, transmission, reception, tracking and/or spotting scope apertures may be formed in the single substrate.

Abstract: In performing an light emitting operation using a plurality of semiconductor light-emitting devices, these semiconductor light-emitting devices are lighted up such that the driving current is lessened and the life time of the devices is prevented from being shortened and lights can be emitted to a remote site without reducing an amount of lights. Semiconductor laser devices 23a to 23c, which emit lights through independent lenses 25a to 25c, are connected in series to each other and connected to a signal generating circuit 24, serving as a power supply, so as to perform pulse lighting, whereby making it possible to light up three semiconductor laser devices simultaneously at a driving current corresponding to one semiconductor laser device. A package of semiconductor laser devices 23a to 23c comprises three lead terminals, and an electrical connection to two lead terminals, which are electrically insulated from a metallic base, is established.

Abstract: When illumination light is externally incident on a liquid crystal display, of the incident light, light of a polarized component in one direction passes through a polarizing film to reach a liquid crystal panel. After the light reaching the liquid crystal panel is optically rotated in accordance with the display pattern displayed on the liquid crystal panel, it reaches a polarizing film. The light passing through the polarizing film enters a holographic reflector. The incident light passes through a hologram without being diffracted due to the angle selectivity of the hologram. Light passing through the hologram is regularly reflected by a metal reflective layer arranged on the back surface of the hologram to be incident on the hologram again. Incident light (reflected light) is incident on the hologram at an incident angle having a plus or a minus sign reverse to the previous incident light.

Abstract: An apparatus and method for manipulating small dielectric particles. The apparatus and method involves use of a diffractive optical element which receives a laser beam and forms a plurality of light beams. These light beams are operated on by a telescope lens system and then an objective lens element to create an array of optical traps for manipulating small dielectric particles.

Abstract: A holographic optical element (HOE) device is mounted in a receiver unit, such as a wireless optical telecommunication system receiver. The HOE device includes a developed emulsion material having an interference pattern recorded thereon, sandwiched between a pair of elements, such as a pair of clear glass plates. In operation, the HOE device uses the recorded interference pattern to diffract incident light rays towards an optical processing unit of the system receiver. The optical processing unit includes a photodetector that detects the diffracted light rays. The system receiver can include various other components and/or can have various configurations. In one configuration, a plurality of mirrors is used to control the direction of the light rays coming from the HOE device, and a collimating optical assembly collimates these light rays. A beam splitting optical assembly can be used to split the light rays into a tracking channel and a communication channel.

Abstract: A system for the production of a second three-dimensional image magnified from a first three-dimensional image in substantially the same proportions, including a first active optical system for creating from the three-dimensional image, a first two-dimensional array. The array is comprised of two-dimensional elemental images. The first active optical system has a number of elements which is equal to the number of elemental images in the array. The configuration of the elements of the first active optical system corresponds to the configuration of the elemental images in the array. The first array is magnified equally in all directions to create a second two-dimensional array comprised also of two-dimensional elemental images. A second active optical system reconstructs a second three-dimensional image that is a magnification of the first three-dimensional image. The second active optical system has an F-number equal to the F-number of the first active optical system.

Abstract: By proposing a diffractive optical element having optical functions of a random phase plate and a lens array in combination, and an illumination apparatus employing this lens element, reduction of speckles, and improvements in energy efficiency and light utilization efficiency are achieved simultaneously.

Abstract: The invention provides a holographic optical device, including a light-transmissive substrate; a first holographic optical element carried by the substrate; at least one second holographic optical element carried by the substrate laterally of the first holographic optical element, and at least one third holographic optical element carried by the substrate laterally displaced from the first and second holographic optical elements; wherein the center of at least one of the first, second or third holographic optical elements is located outside a single, straight line.

Abstract: An optical system for alternative or simultaneous direction of light from two scenes to the eye of a viewer comprising (a) a first lens (42) having a first focal length; (b) a second lens (44) having a second focal length, the first and second lenses being positioned aside one another in front of one of the eyes of the viewer, such that a single light beam from any of the scenes (48, 50) passes only through one of the first and second lenses; and (c) an optical arrangement for directing incident light originating from the first scene and passing through the first lens into the eye of the viewer, and at the same time, for directing incident light originating from the second scene and passing through the second lens into the eye of the viewer, the optical arrangement being positioned between the first and second lenses and the eye of the viewer.

Abstract: A method of compensating for and correcting temperature induced focus and distortional errors within a head-up display unit is described. The head-up display is comprised of a cathode ray tube and a plurality of lenses constructed of a plastic material defining a lens train and a mounting device constructed of a metal material. The compensation for the temperature can be accomplished by measuring the difference in thermal expansion between the plastic material and the metal material and conveying the measured difference to an alignment device for an appropriate adjustment.

Abstract: An infrared lens is made from a moldable IR transmissive material and has an optically significant surface with a surface relief holographic grating. The moldable IR transmissive material is an arsenic selenide glass. The lens and the optically significant surface are manufactured as a unitary structure in a molding operation.

Abstract: A reflective liquid crystal display device (LCD) (40) includes: a substrate (60), a reflecting layer (61), a liquid crystal layer (62), an electrode layer (63), a color filter layer (64), an upper substrate (65), and a polarizer (66). The reflective liquid crystal display device further includes a holographic field lens (67). By providing the holographic field lens, the LCD can be used with a closely placed, offset light source (41), since the holographic field lens simultaneously concentrates light incident on the LCD and redirects it in a preferred direction. The holographic field lens can be provided within the structure of the LCD.

Abstract: A projection screen (30) for representation of images by back-projection comprises a plurality of holographic-optical elements (10) arranged in one plane. The holographic-optical elements are illuminated by a projector (14) comprising three projection devices (22,24,26). Each projection device (22,24,26) generates a light bundle (16,18,20) respectively including the chromatic components of an image to be projected. Each holographic-optical element (10) comprises at least one lens and one diffraction grating which are diffracted in such a manner by the different light bundles (16,18,20) that the holographic-optical element (10) will emit an exiting light bundle (28) with an identical exit angle (&bgr;) in the direction of the viewer (12). Such a projection screen will scatter the light only to a small extent so that the projection screen can be used also at daylight and for large-surface image projection.

Abstract: An anamorphic optical collimator for laser diode having 5-lens elements of positive, positive, negative, positive, and positive optical power and designed to have an input aperture angle of up to ±22° in its meridianal section and up to ±6° in its sagittal section, large exit pupil, anamorphic coefficient equal to 4, angular divergence of collimated beam not exceeding 1 mrad, and the possibility of independently adjusting the angular divergences of the exit laser beam in the meridianal and sagittal planes of the optical collimator. The optical collimator comprises a first positive spherical lens element of a meniscus shape with its concave surface faced to laser diode, a second positive spherical lens element of a plane-convex shape, a third negative cylindrical lens element of a plane-concave shape, a fourth positive cylindrical lens element of a plane-convex shape, and a fifth positive lens element of a spherical plane-convex shape.

Abstract: Provided is an optical backplane interconnect system, one embodiment of which features transceiver subsystems employing holographic optical elements (HOEs) that define, and discriminate between, differing optical channels of communication. The HOEs employ a holograph transform to concurrently refract and filter optical energy to remove optical energy having unwanted characteristics. To that end, the transceiver subsystem is mounted to an expansion card and includes a source of optical energy and an optical detector. The HOE need not be mounted to the expansion card. In one embodiment, however, the HOE is mounted to the expansion card and in optical communication with either the source of optical energy, the optical detector or both.

Abstract: An optical system and an imaging and display system using the optical system. The optical system has at least first and second optical elements aligned on a common axis. In one embodiment, each of the first and second optical elements can operate in an active state or in inactive state and comprise a photopolymer and liquid crystal combination. Each of the first and second optical elements is configured to transmit incident light substantially unaltered when each of the first and second optical elements operates in the inactive state. In the active state, however, each of the first and second optical elements does not transmit incident light substantially unaltered. Rather each of the first and second optical elements substantially alters light incident thereon when each of the first and second optical elements operates in the active state.

Abstract: The present invention provides a hologram color filter having a drastically decreased dependence of diffraction efficiency on wavelength and well corrected for a color balance among the three colors R, G and B comprising an array of converging element holograms 5′, each of which enables white light incident at a given angle &thgr; with respect to a normal line of a hologram recorded surface thereof to be spectroscopically separated by wavelength dispersion in a direction substantially along the hologram recorded surface, wherein the converging element holograms 5′ have each a plurality of hologram pieces 51 and 52 superposed on each other or multi-recorded therein, which, with respect to the white light 3 incident at the given angle &thgr;, have substantially identical spatial wavelength distributions of wavelength dispersion and different peak wavelengths of diffraction efficiency.

Abstract: An apparatus for generating a pattern on a photomask includes a beam generator generating a row of spaced apart optical beams, a blocking element downstream from the beam generator in a path of at least one of the optical beams, and a refracting element downstream from the blocking element for aligning optical beams closer together and while leaving an enlarged beam spacing downstream from the at least one blocked optical beam. The apparatus further includes a modulator downstream from the refracting element for defining the pattern to be generated on the photomask. The apparatus generates the row of spaced apart optical beams with an enlarged beam spacing between the two central beams without using high precision mirrors that are expensive to produce and requires precise alignment with respect to one another.

Abstract: Provided is an optical backplane interconnect system, one embodiment of which features transceiver subsystems employing holographic optical elements (HOEs) that define, and discriminate between, differing optical channels of communication. The HOEs employ a holograph transform to concurrently refract and filter optical energy to remove optical energy having unwanted characteristics. To that end, the transceiver subsystem is mounted to an expansion card and includes a source of optical energy and an optical detector. The HOE need not be mounted to the expansion cared. In one embodiment, however, the HOE is mounted to the expansion card and in optical communication with either the source of optical energy, the optical detector or both.

Abstract: There is provided an image projection apparatus which is small and in which a picture image can be stably observed while high resolution is maintained. The image projection apparatus is structured such that a beam of light from a light source passes through a condensing lens to be converged, and is brought into focus at a space filter. This focus is a secondary point light source, and a diverging spherical wave from this is converted into a parallel beam of light by an illuminating hologram optical element. This parallel beam of light illuminates a transmission space modulator (color liquid crystal display element), is converted into a converged beam of light by an object hologram optical element, and is brought into focus at a pupil. This point is a secondary light source image. This image becomes a Fourier transform image (diffraction image surface) of picture image information of the space modulator, and after passing through an eyeball lens, it forms a geometric image on a retina.

Abstract: A cover plate on a holographic optical element (HOE) device corrects for aberrations, such as spherical aberrations. The HOE device includes an emulsion material, having a recorded interference pattern that can perform diffraction, sandwiched between a first element and a second element, wherein the second element functions as the cover plate. One of the surfaces of the second element is aspheric to correct for aberrations. The aspheric shape of the surface adds approximately equal and approximately opposite aberrations associated with light rays diffracted by the recorded interference pattern, thereby substantially improving focus.

Abstract: The invention provides an optical lens having a combination volume holographic optical element that provides a diffractive optical power. The optical lens has a programmed activating angle in which the holographic optical element provides a diffractive optical power. The invention also provides a method for producing a multilayer holographic element suitable for the optical lens.

Abstract: A method of producing a switchable hologram generally comprises providing a substrate comprised of a liquid crystal material and having two generally opposed surfaces and recording a plurality of holographic fringes composed of liquid crystal droplets in the substrate with one or more of the plurality of holographic fringes positioned such that a surface of the fringes is angled relative to one of the substrate surfaces. The substrate is subjected to an electrical field during recording of the holographic fringes. The electrical field varies across the surface of the substrate so that the liquid crystal droplets are all oriented generally in a common direction.

Abstract: An optical device receives light from an adjustable light source, focuses part of the light into an optical waveguide, and directs another part of the light toward a control unit as monitor light for feedback control of the light source. The optical device is an optical plate with a computer-generated hologram formed on at least one surface to focus light into the optical waveguide. The monitor light may be reflected at this surface or another surface of the optical plate. The monitor light may be focused by the same or another computer-generated hologram.

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.