DESCRIPTION OF THE PREFERRED EMBODIMENT

[0060] Referring now to the accompanying drawings, the embodiments of the invention will be described.

[0061] First Embodiment

[0062] FIG. 2 shows an optical arrangement of a pick up system (hereinafter referred to simply as pick up) according to a first embodiment of the invention, and an optical information-recording medium in an optical information recording/reproducing apparatus according to the first embodiment, and FIG. 3 is a block diagram of the total system in the optical information recording/reproducing apparatus according to the first embodiment. In this case, the optical information recording/reproducing apparatus comprises an optical information recording apparatus and an optical information reproducing apparatus. In the first embodiment, a disk-like optical disk is used as an optical information-recording medium. However, a card-like recording medium may also be used in another embodiment.

[0063] the Structure of the Optical Information-Recording Medium

[0064] Referring to FIG. 2 , the optical information-recording medium according to the first embodiment will be firstly described. The optical information-recording medium 1 is constituted by sequentially laminating a quarter-wave plate (polarization changing layer) 4 , a hologram-recording layer 3 as an information-recording layer for recording information utilizing the volume holography, a reflection layer 5 and a substrate (protection layer) 8 on one side of a disk-shaped transparent base plate 2 made of polycarbonate or the like.

[0065] The quarter-wave plate 4 is used to transform the light passing therethrough from the linear polarization to the circular polarization, when such a linear polarized light as P-polarized light or S-polarized light impinges on the quarter-wave plate 4 , and when the plane of the linear polarization is orientated at 45 degrees with respect to the optical axis of a crystal in the quarter-wave plate 4 . The quarter-wave plate 4 is used either to transform the linear polarization to the circular polarization or to transform the circular polarization to the linear polarization. In the first embodiment, a recording reference light used for recording the information in the hologram-recording layer 3 and a reproducing reference light used for reproducing the information from the hologram-recording layer 3 are P-polarized light. In this case, when either the recording reference light or the reproducing reference light (P-polarized light) is incident on the quarter-wave plate 4 , the light passing therethrough becomes a circularly polarized light. Furthermore, the circularly polarized light is reflected from the reflection layer 5 in the optical information-recording medium 1 and then returns to the quarter-wave plate 4 . In this case, the circularly polarized light changes into the S-polarized light, after the reflected light again passes through the quarter-wave plate 4 .

[0066] The quarter-wave plate 4 is disposed at a position closer than the hologram-recording layer 3 , viewed from the incident side of the reproducing reference light.

[0067] In the first embodiment, as shown in FIG. 4 , the transparent base plate 2 has a thickness of, e.g., 0.4 mm, the hologram recoding layer 3 has a thickness of, e.g., 0.2 mm and the optical information-recording medium 1 has a thickness of, e.g., 1.2 mm in total. The thickness of the reflection layer 5 is of order of Angstrom, so that it is negligibly small, compared with the total thickness of the recording medium.

[0068] In the first embodiment, as shown in FIG. 4 , the optical information-recording medium is constituted so as to have a thickness of 1.2 mm, which is comparable with the thickness of CD or DVD, and therefore the hologram recording medium as the information-recording medium is compatible therewith.

[0069] The hologram-recording layer 3 is constituted by a hologram material having optical properties, such as the refractivity, dielectric constant, reflectivity and others, which are changed in response to the intensity of the illuminated light. As a hologram material, photopolymer HRF-600 (product number), Dupont Co. Ltd, can be employed.

[0070] The reflection layer 5 is a film used for reflecting light (reproducing reference light or the like). The reflection layer 5 is disposed at a position farther away from the hologram-recording layer 3 and the quarter-wave plate 4 , viewed from the incident side of light (reproducing reference light or the like). The reflection layer 5 is produced by, for instance, aluminum.

[0071] The substrate (protection layer) 8 is used as an address-including substrate which is produced by means of, for instance, the injection. In the substrate (protection layer) 8 , address servo areas 6 in the form of radially extending lines are disposed in a predetermined angular spacing to determine the position, and individual sectors between two adjacent address servo areas 6 are used as data area 7 . In the address servo areas 6 , information on the execution of the focus servo and tracking servo in the sample-hold mode and the information on the address are recorded in advance by emboss bits or the like (pre-format). In this case, the focus servo may be carried out using the reflection surface of the reflection layer 5 , and the wobble bits, for instance, may be used for the information on the execution of the tracking servo.

[0072] The Method for Manufacturing the Quarter-Wave Plate 4

[0073] FIG. 24 is a plan view of the quarter-wave plate 4 which is in contact with the transparent base plate 2 . The quarter-wave plate 4 is circular, and molecules 4 e in a phase difference-generating layer are arranged along a concentric circle 4 d in the quarter-wave plate 4 . The phase difference-generating layer is used to generate a phase difference in the light, which is incident on the quarter-wave plate 4 . The material for the phase difference-generating layer is, for instance, azobenzene. The recording and the reproduction of information are carried out in the state of rotating the optical information-recording medium 1 . For an optimal operation of the quarter-wave plate 4 , it is essential that the molecules 4 e are arranged along the concentric circle 4 d in the phase difference-generating layer.

[0074] FIG. 25 ( a ) is a front view of a quarter-wave plate 4 and FIG. 25 ( b ) is a plan view thereof, and these drawings are used to exemplarily describe a method for manufacturing the quarter-wave plate 4 . The substrate 4 a of the quarter-wave plate 4 is a transparent plate. The material for the phase difference-generating layer 4 b (for example, azobenzene) is applied to the surface of the substrate 4 a . Thereafter, the quarter-wave plate 4 is rotated in the direction indicated by arrows in FIG. 25 ( a ) (the so-called spin coating). The thickness of the phase difference-generating layer 4 b is controlled by the speed of revolution. Moreover, a linearly polarized light L, whose vibration plane is aligned in the direction of rotation radius R of the quarter-wave plate 4 , impinges on the phase difference-generating layer 4 b (see FIG. 25 ( b )) in the state of rotating the quarter-wave plate 4 . A film made of azobenzene has an optical anisotropy, and therefore has a tendency of orientating in the direction perpendicular to the polarization plane of the irradiating polarized light. Consequently, the molecules 4 e in the phase difference-generating layer are arranged along the concentric circle 4 d , as shown in FIG. 25 ( b ). In the above procedure, the linearly polarized light L is scanned in the direction of the rotation radius R of the quarter-wave plate 4 so as to illuminate the entire surface thereof.

[0075] FIG. 26 shows drawings for exemplarily describing another method for manufacturing a quarter-wave plate 4 . FIG. 26 ( a ) is a sectional view of the quarter-wave plate 4 in the manufacturing course, and FIG. 26 ( b ) is a plan view thereof. FIG. 26 ( c ) is a sectional view of a finished quarter-wave plate. A polyimide film 4 c is firstly formed on the surface of a substrate 4 a and then the quarter-wave plate 4 is rotated in the direction of arrows shown in FIG. 26 ( a ). In this case, a piece of cloth 61 made of nylon or the like is placed on the polyimide film 4 c , aligning in the direction of the rotation radius (see FIG. 26 ( b )). In this procedure, very small scratches are generated along the concentric circle 4 d , and this procedure is the so-called rubbing. Thereafter, a material for the phase difference-generating layer 4 d is applied to the surface of the substrate 4 a (polyimide film 4 c ), and then the spin coating is carried out. Molecules 4 e in the phase difference-generating layer are arranged in the fine scratches along the concentric circle 4 d.

[0076] After the spin coating, the processes, such as drying, UV light irradiation and others, are carried out. Since these processes are well known in the relating technical field, the description thereof is omitted herein.

[0077] Moreover, the thickness of the phase difference-generating layer 4 b should be preferably 2 to 10 μm.

[0078] The Structural Arrangement of an Optical Information Recording/Reproducing Apparatus

[0079] Referring now to FIG. 3 , the structural arrangement of an optical information recording/reproducing apparatus according to the first embodiment will be described. The optical information recording/reproducing apparatus 10 comprises a spindle 81 onto which an optical information-recording medium 1 is mounted; a spindle motor 82 for rotating the spindle 81 ; and a spindle servo circuit 83 for controlling the spindle motor 82 so as to maintain the optical information-recording medium 1 in a predetermined number of revolution. Moreover, the optical information recording/reproducing apparatus 10 comprises a pick up 11 for reproducing the information recorded in the optical information-recording medium 1 by irradiating a reproducing reference light onto the optical information-recording medium 1 and then by detecting the reproducing light; and a driving apparatus 84 for guiding the pick up 11 in the radial direction of the optical information-recording medium 1 .

[0080] Furthermore, the optical information recording/reproducing apparatus 10 comprises a detection circuit 85 for detecting a focus error signal FE, a tracking error signal TE and a reproduction signal RF in response to the output from the pick up 11 ; a focus servo circuit 86 for moving the objective in the thickness direction of the optical information-recording medium 1 for the focusing by driving an actuator in the pick up 11 , based on the focusing error signal FE detected by the detection circuit 85 ; a tracking servo circuit 87 for moving the objective in the radial direction of the optical information-recording medium 1 to carry out the tracking by driving the actuator in the pick up 11 , based on the tracking error signal TE detected by the detection circuit 85 ; and a slide servo circuit 88 for moving the pick up 11 in the radial direction of the optical information-recording medium 1 to carry out the slide servo by controlling the driving unit 84 , based on both the tracking error signal TE and an instruction command from a controller, which will be later described.

[0081] Furthermore, the optical information recording/reproducing apparatus 10 comprises a signal processing circuit 89 for decoding the data output from a CMOS or CCD array in the pick up 11 (which will be later described) to reproduce the data stored in a data area 7 of the optical information-recording medium 1 and/or for reproducing the basic clock on the basis of a reproducing signal RF from the detection circuit 85 to identify an address; a controller 90 for controlling the entire system of the optical information recording/reproducing apparatus 10 ; and an operation unit 91 for supplying various instructions to the controller 90 . The controller 90 receives the basic clock from the signal processing circuit 89 and/or address information to control the pick up 11 , the spindle servo circuit 83 and the slide servo circuit 88 and others. The spindle servo circuit 83 receives the basic clock output from the signal processing circuit 89 . The controller 90 includes a CPU (central processing unit), a ROM (read only memory) and a RAM (random access memory), in which case, the CPU executes the program stored in the ROM, using the RAM as a working area, in order to realize the function of the controller 90 .

[0082] Referring now to FIG. 2 , the function of the pick up 11 according to the first embodiment will be described. The pick up 11 comprises an objective 12 facing the transparent base plate 2 of the optical information-recording medium 1 , when the optical information-recording medium 1 is mounted onto the spindle 81 ; an actuator 13 enabling the objective 12 to be moved in the thickness direction of the optical information-recording medium 1 as well as in the radial direction thereof; a mirror 15 ; and a polarization beam splitter (PBS) 16 .

[0083] Moreover, the pick up 11 is equipped with a CCD or CMOS sensor (detection unit) 29 for detecting the reproducing light returned from the polarization splitting plate 16 a of the polarization beam splitter 16 on the side (the lower side of PBS 16 ) where the return light (reproducing light) is reflected therefrom. In this case, a polarizer plate (noise suppressing unit) 51 for passing only the S-polarized light is interposed between the CCD or CMOS sensor 29 and the polarization beam splitter 16 . Namely, the polarizer plate 51 serves to transmit only the linearly polarized light having the same vibration direction as the light (S-polarized light) emerged after the circularly polarized light passes through the quarter-wave plate 4 .

[0084] Moreover, a semi-transparent mirror 17 is disposed on one side of the polarized-light separating plane 16 a (right hand side of the PBS) on which the reference light or information light impinges. Furthermore, reference light generating unit comprising a convex lens 18 for defocusing, mirrors 19 and 20 , and a half-wave plate 21 is disposed in the incident direction of the light reflected from the semi-transparent mirror 17 (on the lower side of the semi-transparent mirror 17 ). The half-wave plate 21 is disposed to coincide the polarizing direction of the reference light with the polarizing direction of the information light, which will be later described. The convex lens 18 for defocusing produces reference light which is incident on the objective 12 in the form of a divergent beam by converting a parallel light beam into a divergent light.

[0085] The pick up 11 is equipped with a polarization beam splitter 22 in the incident direction of light on the half-wave plate 21 (on the right hand side of the half-wave plate 21 ). In addition, a spatial light modulator 23 , a mirror 24 and an optical shutter 25 are disposed in the incident direction of the light penetrating the semi-transparent mirror 17 (on the right hand side of the semi-transparent mirror 17 ). The spatial light modulator 23 has a plurality of pixels arranged in a lattice-shape to spatially modulate the light intensity by selecting the state of transmission/interception of the light in each of the pixels, thereby enabling the information light carrying the information to be generated. The spatial light modulator 23 is used as information light generating unit according to the present invention. As a spatial light modulator, for example, a DMD or liquid crystal can be employed.

[0086] In the pick up 11 , moreover, a half-wave plate 26 is disposed on the side of the incident surface for the beam splitter 22 (on the lower side of the PBS 22 ), and further a collimator lens 27 and a light source 28 are disposed in this order from the incident surface. In this case, the intensity ratio of the information light to the recording reference light, where these lights are incident on the optical information-recording medium 1 may be optimally adjusted by appropriately changing the inclination angle of the half-wave plate 26 . Moreover, the light source 28 is used to emit a linearly polarized light having a high coherency and can be produced by, for instance, a semiconductor laser.

[0087] In the pick up 11 , moreover, the light from a light source 32 for servo is used to irradiate the optical information-recording medium, and then the light returned therefrom arrives at a quarter divided photo-detector 35 via the objective 12 , a dichroic mirror 30 , a polarization beam splitter (a semi-transparent mirror can also be employed) 31 , a convex lens 33 and a cylindrical lens 34 .

[0088] The quarter divided photo-detector 35 has four light-receiving areas 35 a to 35 d , which are formed by dividing the optical information-recording medium 1 by a dividing line 36 a parallel to the track direction and by another dividing line 36 b perpendicular thereto, as shown in FIG. 5 . A cylindrical lens 34 is disposed such that the center axis of the cylinder surface thereof is inclined at 45° with respect to the dividing lines 36 a and 36 b for the quarter divided photo-detector 35 .

[0089] FIG. 5 shows a block diagram of the detector circuit 85 for sensing the focus error signal FE, tacking error signal TE and reproducing signal RF based on the output from the quarter divided photo-detector 35 . The detector circuit 85 includes a first adder 37 for adding the output from the light-receiving diagonal section 35 a to that from the light-receiving diagonal section 35 d in the quarter divided photo-detector 35 ; a second adder 38 for adding the outputs from the light-receiving section 35 b to that from the light-receiving section 35 c in the quarter divided photo-detector 35 ; a first subtracter 39 for determining a difference between the output from the first adder 37 and the output from the second adder 38 to generate the focus error signal FE on the basis of the astigmatic aberration method; a third adder 40 for adding the outputs from the light-receiving sections 35 a and 35 b in the quarter divided photo-detector 35 , where these sections are adjacent to each other in the track direction; a fourth adder 41 for adding the outputs from the light-receiving sections 35 c and 35 d in the quarter divided photo-detector 35 , where these sections are adjacent to each other in the track direction; a second subtracter 42 for determining a difference between the output from the third adder 40 and the output from the fourth adder 41 to generate the tracking error signal TE on the basis of the astigmatic aberration method; and a fifth adder 43 for adding the output from the third adder 40 and the output from the fourth adder 41 to generate the reproducing signal RF. In the first embodiment, the reproducing signal RF is a signal, which is reproduced from information stored in an address servo area 6 inside the optical information-recording medium 1 .

[0090] In this case, the spatial light modulator 23 and the light sources 28 , 32 in the pick up 11 are all controlled by the controller 90 shown in FIG. 3 .

[0091] In the pick up 11 according to the invention, either a phase spatial modulator can be interposed between the convex lens 18 for defocusing and the mirror 19 or a reflection-type phase spatial modulator can be disposed in the same position as that in the mirror 19 or 20 , replacing the mirror therewith, although these are not shown. In this case, the phase spatial modulator includes a plurality of pixels arranged in the form of a lattice and is capable of spatially modulating the optical phase by selecting the phase of light incident on each pixel. Such a phase spatial light modulator may be produced either by a liquid crystal element or by a micro-mirror device in which a micro-mirror may be moved in the direction parallel to the optical axis of the light leaving the device. The phase spatial modulator is also controlled by the controller 90 shown in FIG. 3 . The controller 90 includes information on a plurality of modulation patterns for spatially modulating the phase of light in the phase spatial modulator. The operation section 91 is designed such that an appropriate modulation pattern can be selected from the modulation patterns stored therein. The controller 90 supplies the information on either a modulation pattern automatically selected in accordance with predetermined conditions or a modulation pattern selected by the operation section 91 to the phase spatial modulator. In conjunction with this, the phase spatial modulator spatially modulates the phase of light with a corresponding modulation pattern in accordance with the information on the modulation pattern provided by the controller 90 .

[0092] Moreover, in the pick up 11 according to the invention, the optical system is designed such that the length of the ray path from the polarization beam splitter 22 from the semi-transparent mirror 17 via the mirror 24 and the spatial light modulator 23 is the same as the length of the ray path from the beam splitter 22 to the semi-transparent mirror 17 via the mirrors 20 , 19 , and the convex lens 18 for defocusing. Such a structural arrangement ensures that the path length of the recording reference light is the same as that of the light from an object, and further provides an advantage that the contrast of the interference fringes may be used in a highest efficiency even if the coherent distance (coherency) of the laser for the hologram recording light source is small.

[0093] In the following, the function of the optical information recording/reproducing apparatus according to the first embodiment will be described in the sequence of the servo, recording and reproducing operation modes. The optical information-recording medium 1 is rotated by the spindle motor 82 in such a manner that it always maintains a rated number of revolution in any case of the servo, recording and reproducing operation modes.

[0094] Servo Operation Mode

[0095] Referring to FIG. 6 , the function of the optical information recording/reproducing apparatus in the servo operation mode will be described. In the servo operation mode, the light source 32 for servo is used. The intensity of light emitted from the light source 32 for servo is set at a low power for reproduction. In this case, the controller 90 predicts the period during which the light leaving the objective 12 passes through the address servo area 6 , based on the basic clock reproduced from the reproducing signal RF, and maintains the setting of the above-mentioned power during the period during which the light leaving the objective 12 passes through the address servo area 6 .

[0096] The P-polarized light emitted from the light source 32 for servo is incident on the polarization beam splitter 31 , after collimated by the collimating lens 31 , and then passes through the polarized light splitting plane 31 a , and is further reflected from the dichroic mirror 30 in the form of a parallel light beam. The light reflected from the dichroic mirror 30 (the P-polarized light) impinges on the optical information-recording medium 1 in such a way that it is converged on the reflection layer 5 in the optical information-recording medium 1 by the objective 12 . In this case, the light is modulated by emboss pits in the address servo area 6 , and then is returned toward the objective 12 . Moreover, the light is converted to a circularly polarized light by the quarter-wave plate 4 , before converging on the reflection layer 5 .

[0097] The light returned from the reflection layer 5 (the circularly polarized light) is converted to the S-polarized light after its polarization direction is changed by the quarter-wave plate 4 , and then collimated by the objective 12 . The S-polarized light thus returned proceeds toward the polarization beam splitter after reflected from the dichroic mirror 30 . The dichroic mirror 30 is designed such that the light having a wavelength of, e.g., λ=655 nm is reflected and the light having a wavelength of λ=532 nm or less penetrates the mirror in a transparency of 100%. Accordingly, a red light laser having a wavelength of 655 nm can be employed as the light source 32 for servo and a green light laser light having a wavelength of, e.g., 532 nm, a green purple light laser having a wavelength of 405 nm, or another laser such as a blue light laser can be employed as a light source 28 .

[0098] The light reflected from the dichroic mirror is a S-polarized light. Accordingly, the light is incident on the polarization beam splitter in the form of a parallel light, and then reflected from the polarization splitting plane 31 a , and then further impinges on the convex lens 33 . The light incident on the convex lens 33 is converted into a convergent light beam and detected by the quarter divided photo-detector 35 after passing through the cylindrical lens 34 . Upon the basis of output from the quartered photo-detector 35 , the focus error signal FE, tracking error signal TE and reproducing signal RF are generated by the detection circuit 85 shown in FIG. 5 . In accordance with these signals, the focus servo and tracking servo are carried out, along with the reproduction of the basic clock and the identification of the address.

[0099] In the above-described servo operation mode, the structural arrangement of the pick up 11 is the same as that of the pick up for recording and reproduction, which is used for a conventional optical disk such as CD (compact disk), DVD (digital video disk or digital versatile disk), HS (hyper storage disk) or the like. Accordingly, it is possible to design the optical information recording/reproducing apparatus 10 according to the invention such that it is compatible with such a conventional optical disk apparatus.

[0100] Recording Operation Mode

[0101] In the following, the function of the optical information recording/reproducing apparatus in the recording operation mode will be described. FIG. 7 shows the structural arrangement of the pick up 11 in the recording operation mode.

[0102] The intensity of light emitted from the light source 28 is set at a high power for pulse recording. In this case, the controller 90 predicts the period during which the light leaving the objective 12 passes through the data area 7 , based on the basic clock reproduced from the reproducing signal RF, and thereby maintains the setting of the above power during the period where the light leaving the objective 12 passes through the data area 7 . The focus servo and the tacking servo are maintained in the state of the light passing through the servo area 7 during the period where the light leaving the objective 12 passes through the data area 7 , so that the objective 12 is fixed. In the following description, it is assumed that the light source 28 emits a P-polarized light toward the polarization beam splitter 22 .

[0103] In FIG. 7 , the P-polarized light emitted from the light source 28 is collimated by the collimator lens 27 and then the polarization direction of the light is changed by the half-wave plate (for instance, +22.5 degrees) 26 , thereby enabling the light having a P-polarized light component and a S-polarized light component to be generated. The light is incident on the beam splitter 22 , in which case, part of light (the P-polarized light component) passes through the polarization splitting plane 22 a and the remaining part of light (the S-polarized light component) is reflected from the polarization splitting plane 22 a . The reflected light (the S-polarized light component) is incident on the half-wave plate (+45 degrees) 21 , where the polarization direction of the S-polarized light is changed by 90 degrees to generate a P-polarized light. The S-polarized light is incident on the convex lens 18 via the mirrors 19 and 20 . Thanks to the convex lens, a divergent recording reference light beam at the objective 12 can be generated, as will be later described. The recording reference light thus generated is reflected from the semi-transparent mirror 17 .

[0104] In the case when a phase spatial light modulator is interposed between the convex lens 18 and the mirror 19 , the phase spatial light modulator spatially modulates the phase of light by selectively adding a predetermined phase difference of 0 (rad), π (rad) or a value between them to the light passing therethrough for each pixel in accordance with the predetermined modulation pattern, thereby causing a recording reference light to be generated, in which the phase of the light is spatially modulate. The controller 90 provides the information on the modulation pattern selected either automatically in accordance with a predetermined condition or by the operation section 91 to the phase spatial light modulator. Accordingly, the phase spatial light modulator spatially modulates the phase of the light passing therethrough in accordance with the information on the modulation pattern provided by the controller 90 .

[0105] On one hand, the P-polarized light penetrating the polarization splitting plane 22 a of the beam splitter 22 is reflected from the mirror 24 because the shutter 25 is opened in the recording operation mode, and therefore the reflected light impinges on the spatial light modulator. In the spatial light modulator 23 , the reflection state (hereinafter referred to as ON) or the interception state (herein after referred to as OFF) is selected for each pixel in accordance with the information to be stored in the optical information-recording medium 1 to form an information light by spatially modulating the reflected light. In accordance with the embodiment, one bit information is represented by two pixels, where one of the two pixels corresponding to the one bit information is always set ON, and the other is always set OFF. In this case, DMD can be employed as a spatial light modulator.

[0106] The information light thus generated (the P-polarized light) penetrates the semi-transparent mirror 17 , where the information light as the P-polarized light and the recording reference light as the P-polarized light are again combined with each other (the optical axes thereof are the same). The above-mentioned two types of light behave as the P-polarized light and therefore pass through the polarization beam splitter 16 . The information light behaviors as a collimated light beam whereas the recording reference light behaves as a convergent light beam converted by the convex lens for defocusing, and impinges on the polarization beam splitter 16 in the form of a convergent beam. The information light and the recording reference light are both reflected from the mirror 15 , thereby causing the proceeding direction of these light beams to be altered.

[0107] Since the information light is the light emitted from a green light laser having a wave length of, e.g., 532 nm, as described above, it penetrates the dichroic mirror 30 and then be changed from a collimated light beam to a light beam converging on the reflection layer 5 in the optical information-recording medium 1 by the objective 12 .

[0108] On the other hand, the recording reference light is once converged in an area between the mirror 15 and the objective 12 , and then impinges on the objective 12 in the form of a divergent beam. Since the recording reference light also behaves as the light emitted from, for instance, a green light laser, it penetrates the dichroic mirror 30 and impinges on the objective 12 in the form of a divergent beam, thereby focusing at a point F. In other words, the recording reference light is defocused on the reflection layer 5 in the optical information-recording medium 1 , and the light reflected from the reflection layer is focused on a focus point F′ which is conjugate to the focus point F.

[0109] In this case, a spatial filter (not shown) is interposed between the mirror 15 and the dichroic mirror 30 , so that only the information light of 0 or ±1 order passes through the filter and an extra information light of higher order is rejected from entering the filter. In the first embodiment, the reference light is not modulated by the spatial light modulator and therefore there is no light beam rejected by such a spatial filter. When, however, the reference light is generated by modulating the phase of light with a phase spatial light modulator, higher order light beams are generated in the reference light. Accordingly, only the reference light of 0 or ±1 order penetrates the spatial filter and reference light of higher order is rejected therefrom.

[0110] FIGS. 9 and 10 show ray path diagrams in the recording operation mode.

[0111] As shown in FIG. 9 , information light 61 L (the P-polarized light) is incident on the optical information-recording medium 1 via the object 12 , and is changed into a circularly polarized light after passing through the quarter-wave plate 4 . Moreover, the circularly polarized light penetrates the recording layer 3 and is converged on the reflection layer 5 in a minimum spot size. Thereafter the circularly polarized light is reflected from the reflection layer 5 . The reflected light (information light 61 R) again penetrates the recording layer 3 in a circularly polarized light, and is then converted from the circularly polarized light to the S-polarized light after passing through the quarter-wave plate 4 . Then, the S-polarized light is collimated by the object 12 . The information light 61 R has information on the page data on the left half plane, as similarly to the information light 61 L.

[0112] On the other hand, recording reference light 62 L as well as recording reference light 62 R is also a P-polarized light, and is incident on the optical information medium 1 via the objective lens 12 , and further changed into a circularly polarized light after passing through the quarter-wave plate 4 . Furthermore, the circularly polarized light beam penetrates the hologram-recording layer 3 and is reflected from the reflection layer 5 in such a way that it is defocused on the reflection layer 5 . The actual focus point for the recording reference light is located at F, as shown in FIG. 9 , and the light reflected from the reflection layer 5 is converged at F′ which is the conjugate focus point for F. The optical information-recording medium 1 is illuminated by the recording reference light under the condition that the conjugate focus point F′ is located not at the inside of the hologram-recording layer 3 , but at a point below the interface between the transparent base plate 2 and the quarter-wave plate 4 (on the side of the objective 12 ) in FIG. 9 . This is due to the fact that if the conjugate focus point F′ is located in the hologram-recording layer 3 , the light intensity becomes maximum at the conjugate focus point F′ so that the material of the hologram-recording layer 3 is burnt up and the optical information-recording medium 1 breaks down.

[0113] The conjugate focus point F′ can be situated anywhere below the interface between the hologram-recording layer 3 and the quarter-wave plate 4 . However, an increase in the departure from the optical information-recording medium 1 provides an increase in the area where the recording reference light penetrates the recording layer 3 , so that an extra area other than the portion, at which the interference fringes are generated, are exposed by the reference light. When, therefore, the conjugate focus point F′ is situated in the inside of the transparent base plate 2 , the exposure of such an extra area can be suppressed. This arrangement can be employed in a preferable case.

[0114] The circularly polarized information light 61 L passed through the quarter-wave plate 4 and the circularly polarized recording reference light 62 L passed through the quarter-wave plate 4 interfere with each other to form a transmission type interference pattern (vertical fringes) at an area X 1 , and the interference pattern is three-dimensionally recorded in the area X 1 of the hologram-recording layer 3 . Moreover, a reflection type interference pattern (horizontal fringes) is also formed in part of the area X 1 by the returned light of the recording reference light 62 L reflected from the reflection layer 5 and the information light 61 L, although these are not shown.

[0115] Moreover, the circularly polarized information light 61 L passed through the quarter-wave plate 4 and the circularly polarized recording reference light 62 R passed through the quarter-wave plate 4 interfere with each other to form a transmission type interference pattern (vertical fringes) in an area Y 1 , and the interference pattern is three-dimensionally recorded in the area Y 1 of the hologram-recording layer 3 . Moreover, a reflection type interference pattern (horizontal fringes) is also formed in part of the area Y 1 by the returned light of the recording reference light 62 R reflected from the reflection layer 5 and the information light 61 L.

[0116] As shown in FIG. 10 , the optical information-recording medium 1 is irradiated by the information light 63 R (the P-polarized light) via the objective 12 and a circularly polarized light is produced after the information light 63 R passes through the quarter-wave plate 4 . Moreover, the circularly polarized light is converged in a minimum spot size on the reflection layer 5 after passing through the recording layer 3 and then reflected from the reflection layer 5 . The reflected light (information light 63 L) again penetrates the recording layer 3 in the circularly polarized light and further penetrates the quarter-wave plate 4 to change from the circularly polarized light to the S-polarized light. Then, the S-polarized light is collimated by the objective 12 . The information light 63 L has the information of the page data on the right half plane, as similarly to the information light 63 R.

[0117] The recording reference light beams 62 L and 62 R provide the same function as that elucidated, referring to FIG. 9 , and therefore the description thereof is omitted.

[0118] The circularly polarized information light 63 R passed through the quarter-wave plate 4 and the circularly polarized recording reference light 62 R interfere with each other to form a transmission type interference pattern (vertical fringes) in an area Y 2 , and the interference pattern is three-dimensionally recorded in the area Y 2 of the hologram-recording layer 3 . Moreover, a reflection type interference pattern (horizontal fringes) is also formed in part of the area Y 2 by the returned light of the recording reference light 62 R reflected from the reflection layer 5 and the information light 63 R, although these are not shown.

[0119] Furthermore, The circularly polarized information light 63 R passed through the quarter-wave plate 4 and the circularly polarized recording reference light 62 L interfere with each other to form a transmission type interference pattern (vertical fringes) in an area X 2 , and the interference pattern is three-dimensionally recorded in the area X 2 of the hologram-recording layer 3 . Moreover, a reflection type interference pattern (horizontal fringes) is also formed in part of the area X 2 by the returned light of the recording reference light 62 L reflected from the reflection layer 5 and the information light 63 R.

[0120] Referring now to FIG. 8 , the behavior of light before and after the incidence on the quarter-wave plate 4 will be described. As shown in FIG. 8 ( a ), the information light and recording reference light are both P polarized lights, and are changed into the circularly polarized lights by the quarter-wave plate 4 . FIG. 8 ( b ) shows the behavior of the circularly polarized light. From the diagram shown in FIG. 8 ( b ), it can be recognized that a helicoide having a period of one wavelength is provided by the electric field vectors indicated by both the solid line arrow and the broken line arrow. This is the circularly polarized light. In the recording, therefore, the information light and the recording reference light are in the state of circular polarization.

[0121] As shown in FIGS. 9 and 10 , in the first embodiment, the optical axis of the information light and the optical axis of the recording reference light are positioned on a line, and the hologram-recording layer 3 is illuminated from one side thereof by both the information light and the recording reference light.

[0122] In the first embodiment, moreover, it is possible to carry out the multiple recording of the information in a portion of the hologram-recording layer 3 by the phase code multiplexing in which the recording reference light is recorded several times with varied modulation patterns in the portion of the hologram-recording layer 3 .

[0123] As described above, the transmission type hologram and the reflection type hologram are formed in the same area of the hologram-recording layer 3 according to the first embodiment. However, even when the transmission type hologram (vertical fringes) is formed, it is determined in accordance with the hologram material constituting the hologram-recording layer 3 as to whether or not the reflection type hologram (horizontal fringes) is formed and/or how much the reflection type hologram is formed. Generally, it is difficult to enhance the sensitivity for the hologram material in the reflection type hologram, compared with that in the transmission type hologram. Therefore, if a hologram material having no sensitivity to the reflection type hologram is used, the above-mentioned reflection type hologram (horizontal fringes) is formed neither in part of the area X 1 , nor in part of the areas Y 1 , X 2 andY 2 .

[0124] In the first embodiment, moreover, the ray path in the optical system for servo and that in the optical system for recording/reproduction are separated from each other, and therefore it is also possible to carry out the focus servo in the recording operation mode.

[0125] In the first embodiment, the magnitude of the area (hologram), in which an interference pattern produced by both the information light and the reference light in the hologram-recording layer 3 is three-dimensionally recorded, can be arbitrarily determined by moving the convex lens 16 in the forward/backward direction and/or by altering the magnification thereof.

[0126] The Reproducing Operation Mode

[0127] In the following, the function of the optical information recording/reproducing apparatus according to the first embodiment in the reproducing operation mode will be described. FIG. 11 is a diagram showing the operation state of the pick up 11 .

[0128] In the reproducing operation mode, a shutter 25 interposed between the mirror 24 and the polarization beam splitter 22 is turned on, so that the incidence of light onto the spatial light modulator 23 is forbidden. The light incident on the spatial light modulator 23 can be intercepted by the shutter 25 in the reproducing operation mode. However, all the pixels in the spatial light modulator 23 can also be turned on by way of precaution.

[0129] The intensity of the light emitted from the light source 28 is set at a low power for reproduction. In this case, the controller 90 predicts the period where the light passed through the objective 12 , based on the basic clock reproduced from the reproducing signal RF, and sets the intensity of the light into the low power during the period where the light passed through the objective 12 . In the below description, it is assumed that the light source 28 emits a P-polarized light to the beam splitter 22 in the reproducing operation mode, as similarly to the recoding operation mode.

[0130] As shown in FIG. 11 , the P-polarized light emitted from the light source 28 is collimated by a collimator lens 27 . Then, the polarization direction thereof is changed by the half-wave plate (+22.5 degrees) 26 to form a light beam including a P-wave component and a S-wave component with respect to the beam splitter 20 . The light beam is incident on the beam splitter 20 in such that part of the light (the P-polarized light) penetrates the polarization splitting plane 22 a and the remaining part of the light (the S-polarized light) is reflected from the polarization splitting plane 22 a . The reflected light (S) is incident on the half-wave plate (+45 degrees) 21 where the polarizing direction of the S-polarized light is altered by 90 degrees to generate P-polarized light. The P-polarized light is incident on the convex lens 18 via the mirrors 20 and 19 . The reproducing reference light converged at the objective 12 is produced by the convex lens 18 . The reproducing reference light thus produced is incident on the polarization beam splitter 16 after reflected by the semi-transparent mirror 17 . The reproducing reference light is the same as the recording reference light, which is used in the recording operation mode.

[0131] When a phase spatial modulator (not shown) is interposed between the convex lens 18 and the mirror 19 to produce a recording reference light, the controller 90 supplies the information on the modulation pattern for the recording reference light to the phase spatial light modulator in the case of recording the information to be reproduced. The phase spatial light modulator spatially modulates the phase of the transmitting light in accordance with the modulation pattern supplied by the controller 90 to generate the reproducing reference light in which the phase of light is spatially modulated.

[0132] The reproducing reference light incident on the polarization beam splitter 16 is a P-polarized light and penetrates the polarization separation plane 16 a of the polarization beam splitter 16 , and then is reflected by the mirror 15 to alter the proceeding direction of the light beam. The reproducing reference light is once converged between the mirror 15 and the objective 12 , and thereafter incident on the objective 12 in the form of a divergent light beam. Since the reproducing reference light is, for instance, light from emitted from a green laser, it penetrates the dichroic mirror 30 and is incident on the objective 12 in the form of a divergent beam, so that it focuses on the point F. In other words, the reproducing reference light is defocused on the reflection layer 5 in the optical information-recording medium 1 , and the light thus reflected by the reflection layer is converged on focus point F′ which is conjugate to the focus point F.

[0133] In this case, a spatial filter (not shown) is interposed between the mirror 15 and the dichroic mirror 30 . When, however, the reproducing reference light is generated by modulating the phase of the light with the phase spatial light modulator in the first embodiment, higher order light are also generated in the reference light. Accordingly, only 0, or +1 order reference light passes through the spatial filter and the higher order light is rejected by the spatial filter.

[0134] The reproducing light is generated by the irradiation of the reproducing reference light. The reproducing light thus generated is changed from the circularly polarized light to a S-polarized light by the quarter-wave plate 4 , and further collimated by the objective 12 . The reproducing light penetrates the dichroic mirror 30 , and is further incident on the polarization beam splitter 16 , after reflected by the mirror 15 . Since the reproducing light is a S-polarized light, it is reflected by the polarization separation plane 16 a , so that a reproduced image is detected by a CCD or CMOS sensor 29 . In this case, stray light generated by optical elements, such as the base plate, objective 12 and/or the like closer to the recording layer 3 on the incident side is a P-polarized light, so that it is intercepted so as not to enter the CCD or CMOS sensor 29 by the polarizer plate 51 . The reproduced image thus detected are subject to signal processes, such as the error correction, required decoding and others, and then reproduced in accordance with the data stored in the optical information-recording medium 1 . A series of such signal processes is carried out in the signal processing circuit 89 in FIG. 3 . FIGS. 13 and 14 show the behavior of light in the reproducing operation mode.

[0135] As shown in FIG. 13 , the reproducing reference light 64 L impinges on the optical information-recording medium 1 via the objective 12 , and in changed into a circularly polarized light after passing through the quarter-wave plate 4 . Moreover, the circularly polarized light passes through the hologram-recording layer 3 and is reflected by the reflection layer 5 , so that it is converged in a minimum spot size at a focus point F′ which is conjugate to the focus point F in the case of no reflection layer 5 . The reproducing reference light reflected by the reflection layer 5 again passes through the hologram-recording layer 3 . In accordance with such a reproducing reference light, the reproducing light 65 R corresponding to the information light 61 L (left half plane image on DMD=let half page data) in the record mode is generated from the area X 1 of the hologram-recording layer 3 . The reproducing light 65 R is the light emerged from the vertical fringes generated in X 1 . The reproducing light 65 R thus produced is changed from the circularly polarized light to the S-polarized light after passing through the right hand side of the quarter-wave plate 4 .

[0136] The reproducing reference light 64 R impinges on the optical information medium 1 via the objective 12 , and is changed from the P-polarized light to the circularly polarized light, after passing through the right side of the quarter-wave plate 4 . Thereafter, the circularly polarized light penetrates the hologram-recording layer 3 and then is reflected by the reflection layer 5 to converge in a minimum spot size at a focus point F′ which is conjugate to the focus point in the case of no reflection layer 5 . The reproducing reference light 64 R thus reflected by the reflection layer 5 again penetrates the hologram-recording layer 3 . In accordance with such illumination of the reproducing reference light, a reproducing light 65 R′ corresponding to the information light 61 L (left half plane image=left half page data) in the record mode is generated in the area Y 1 of the hologram-recording layer 3 . The reproducing light 65 R′ is the light which is emerged from the horizontal fringes generated in Y 1 . The reproducing light 65 R′ thus generated is changed from the circularly polarized light to the S-polarized light after passing through the right hand side of the quarter-wave plate 4 , as similarly to the reproducing light 65 R.

[0137] The reproducing light 65 R and reproducing light 65 R′ are the images corresponding to the information light 61 L (the left half plane image on DMD), so that they provide no ghost image and can be clearly detected by the CCD or CMOS sensor 29 .

[0138] On the other hand, as shown in FIG. 14, a reproducing light 66 L corresponding to the information light 63 R (right half plane image on DMD=right half page data) in the record mode is generated from the area Y 2 of the hologram-recording layer 3 in accordance with the illumination of the reproducing reference light 64 R. The reproducing light 66 L is the light, which is emerged from the vertical fringes generated in Y 2 . The reproducing light 66 L thus generated is changed from the circularly polarized light to the S-polarized light after passing through the left side of the quarter-wave plate 4 .

[0139] Similarly, a reproducing light 66 L′ corresponding to the information light 63 R (right half plane image on DMD=right half page data) in the recording operation mode is generated from the area X 2 of the hologram-recording layer 3 in accordance with the illumination of the reproducing reference light 64 L. The reproducing light 66 L′ is the light emerged from the horizontal fringes generated in X 2 . The reproducing light 66 L′ thus generated is also changed from the circularly polarized light to the S-polarized light after passing through the left side of the quarter-wave plate 4 , as similarly to the reproducing light 66 L.

[0140] The reproducing light 66 L and reproducing light 66 L′ provide an image corresponding to the information light 63 R (left half plane image on DMD), so that they provide no ghost image and can be clearly detected by the CCD or CMOS sensor 29 .

[0141] Referring to FIG. 12 , the behavior of light before and after the incidence on the quarter-wave plate 4 in the reproducing operation mode will be described. As shown in FIG. 12 ( a ), the reproducing reference light is a P-polarized light and it is converted to the circularly polarized light by the quarter-wave plate 4 . FIG. 12 ( b ) shows the behavior of the circularly polarized light. From the diagram shown in FIG. 12 ( b ), it can be recognized that the electric field vectors indicated by the solid line arrow and the broken line arrow provide a helicoide having a period of one wavelength. This is the circularly polarized light. Accordingly, the reproducing reference light behaves as a circularly polarized light in the reproducing operation mode.

[0142] In the first embodiment, the polarization of the reproducing reference light and the polarization of the reproducing light are the S polarization after passing through the quarter-wave plate 4 . As a result, the reproducing reference light is also detected by the CCD or CMOS sensor 29 , and therefore prevents the reproducing image from detecting. In view of this fact, the reference light is spatially separated, using a mask, as shown in FIG. 15 .

[0143] FIG. 15 shows the schematic view of the optical elements arranged from the optical information-recording medium 1 to the CCD or CMOS sensor 29 . In FIG. 15 , the same reference numerals are used for the same functional elements. In