JP2009282141A - Hologram reproducing device and hologram reproducing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hologram reproducing device and a hologram reproducing method, obtaining a reproducing signal of high quality and reducing in size and thickness of the device. <P>SOLUTION: Incident light L1 of p-polarized light is caused to enter a hologram record medium 6, and transmitted light L2 at this time is reversed to generate reproducing reference light L3. The polarized light of the reproducing reference light 3 is converted to s-polarized light by a quarter-wave plate 10, so that reproducing signal light L4 generated from a hologram 9 also becomes s-polarized light. A sheet polarizer 15 separates reflected light of p-polarized light obtained by reflecting the incident light L1 by the hologram record medium 6 from the reproducing signal light L4 of s-polarized light. As a result, SN ratio of a reproducing signal can be improved. Further, the sheet polarizer 15 can be disposed just before a light-receiving surface 16a of an image pickup element 16, so that the hologram reproducing device 100 can be reduced in size and thickness. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a reproducing apparatus and a reproducing method for reproducing a hologram recorded on a recording medium.

  In recent years, volume holographic memories capable of high-density optical recording have attracted attention. In general, in volume holography, signal light modulated with information to be recorded and reference light are caused to interfere inside a hologram recording medium. At this time, interference fringes generated three-dimensionally by actively utilizing the thickness direction of the recording medium are recorded as holograms. A thick hologram recorded three-dimensionally has a high diffraction efficiency. Further, it is possible to increase the recording density by using the angle multiplex recording method.

  FIG. 10 is a schematic diagram showing a configuration of a conventional hologram recording / reproducing apparatus 200. The hologram recording / reproducing apparatus 200 records a hologram by causing the spherical wave signal light and the plane wave reference light to interfere with each other in the hologram recording medium, and reproduces the recorded hologram. A hologram recording / reproducing apparatus having the same configuration is disclosed in, for example, Japanese Patent Application Laid-Open No. 2006-317886 (Patent Document 1). In FIG. 10, the horizontal direction in the drawing is the X direction, the vertical direction in the drawing is the Z direction, and the direction perpendicular to the paper surface is the Y direction. The hologram recording medium 206 is arranged along the X and Y directions.

  Referring to FIG. 10, a laser light source 201 is a laser device that emits laser light having a single wavelength such as blue or green. The laser beam generated by the laser light source 201 is expanded in diameter by the beam expander 203 and emitted as parallel light. The parallel light is split by a polarization beam splitter (PBS) 225 into P-polarized signal light P2 and S-polarized reference light P1.

  After the signal light P <b> 2 is reflected by the mirror 226, the diameter of the signal light P <b> 2 is expanded by the beam expander 227. Thereafter, the signal light P <b> 2 is intensity-modulated by the spatial light modulator 228 and then passes through the PBS 215. The signal light P <b> 2 is collected by the objective lens 214 and enters the hologram recording medium 206.

  On the other hand, the reference light P <b> 1 is converted to P-polarized light by the half-wave plate 204 and then reflected by the mirror 205. Thereafter, the reference light P1 passes through the irradiation position adjusting lens 212 (212a, 212b) and enters the hologram recording medium 206 in a parallel light state. Thus, the reference light P 1 interferes with the signal light P 2 in the hologram recording medium 206.

  The irradiation position adjusting lenses 212a and 212b, the mirror 205, and the hologram recording medium 206 constitute a so-called 4f optical system. Therefore, by rotating the mirror 205 to change the reflection direction, the incident angle can be changed without changing the incident position of the reference light P1 on the hologram recording medium 206.

  When the incident angle of the reference light incident on the hologram recording medium is changed in this way, the inclination of the interference fringes formed by the interference with the signal light changes. As a result, different holograms are formed according to the incident angle of the reference light. At this time, if the information of the signal light is changed every time the incident angle of the reference light is changed, a plurality of pieces of information can be written in the same location. As described above, the method of multiplexing and recording the hologram 209 at the same location by changing the incident angle of the reference light incident on the hologram recording medium 206 is called an angle multiplexing recording method.

  Next, a procedure for reproducing a hologram recorded by the hologram recording / reproducing apparatus 200 will be described.

  At the time of reproduction, first, incident light P1 enters the hologram recording medium 206 from the same direction as the reference light at the time of recording. The transmitted light P3 obtained by transmitting the incident light P1 through the hologram recording medium 206 is converted into circularly polarized light by the quarter wavelength plate 210. Circularly polarized light enters the mirror 211. The mirror 211 reflects the circularly polarized light by 180 ° in the original direction by performing both the rotation operation and the operation shifting in the Z direction.

  The reflected circularly polarized light is incident on the quarter-wave plate 210 again and converted to S-polarized reproduction reference light P4. The S-polarized reproduction reference beam P4 enters the hologram recording medium 206 from the opposite direction to that during recording. The incident reproduction reference light P4 is diffracted by the hologram 209, whereby reproduction signal light P5 is generated.

  The generated reproduction signal light P5 is incident on a reproduction optical system including the objective lens 214, the PBS 215, and the image sensor 216. At this time, the S-polarized reproduction signal light P5 is reflected by the PBS 215 and guided to the image sensor 216. By decoding the image data captured by the image sensor 216, the reproduction of the information recorded in the hologram is completed.

In order to reproduce a multiplex hologram by the angle multiplex recording method, it is necessary to change the incident angle of the reproduction reference light P4 with respect to the hologram recording medium as in the case of recording. In the case of FIG. 10, the incident light P1 is made incident on the hologram recording medium 206 at the same incident angle as the reference light at the time of recording by adjusting the deflection angle of the mirror 205 on the front side according to the target hologram to be reproduced. At this time, the reproduction reference light P4 is generated by inverting the transmitted light P3 by 180 ° on the back side of the hologram recording medium 206. In this way, by using the method in which the reproduction reference light P4 is incident on the hologram recording medium 206 from the direction opposite to that at the time of recording, the laser light source 201 and the reproduction optical system can be arranged on the same side with respect to the hologram recording medium 206. Therefore, the apparatus can be reduced in size and thickness.
JP 2006-317886 A

  However, when the hologram is reproduced by the above method, there are the following problems.

  The reproduction reference light P4 in FIG. 10 is generated by inverting the transmitted light P3 obtained by transmitting the incident light P1 through the hologram recording medium 206 on the back surface side of the hologram recording medium 206. In general, a hologram recording medium has a structure in which a recording material is sandwiched between a glass substrate, a resin substrate, and the like (hereinafter referred to as a substrate). Further, the entire hologram recording medium may be built in the cartridge. When the incident light P1 is incident on the hologram recording medium having such a structure, a part of the light is reflected on the substrate surface to become reflected light. When the reflected light enters the light receiving surface of the image sensor, noise is generated. In some cases, the reflected light is further scattered by the aperture of the objective lens, and the scattered light is incident on the light receiving surface of the image sensor. Although it is possible to reduce the intensity of reflected light and scattered light incident on the light receiving surface to some extent by performing antireflection treatment on the substrate surface and limiting the incident angle to the hologram recording medium, it is not sufficient.

  In particular, the influence of noise becomes a problem when the diffraction efficiency of signal light is reduced in order to increase the recording density. At present, a maximum of several tens of M numbers representing the ability to multiplex hologram recording media are obtained. Since the M number is proportional to the route of diffraction efficiency, the multiplex number can be increased as the diffraction efficiency is lower. However, when the hologram diffraction efficiency is reduced in order to increase the recording density, the reproduced signal light becomes weak, so the influence of noise due to reflected light and scattered light cannot be ignored.

  Further, it is necessary to use an objective lens having the same characteristics as the objective lens used for condensing the signal light at the time of recording as the objective lens for imaging the reproduction signal light on the image sensor. In order to improve the recording density, it is conceivable to use an objective lens having a high NA during recording. When an objective lens with a high NA is used, since the focal length of the objective lens is generally shortened, the distance between the objective lens and the hologram recording medium is reduced. As a result, stray light such as reflected light or scattered light from the hologram recording medium is likely to enter the objective lens during reproduction, and the quality of the reproduced signal may be reduced.

  In addition, when PBS is used for polarization separation as in the conventional example of FIG. 10, the size of PBS increases in proportion to the beam diameter of the reproduction signal light, which is disadvantageous for downsizing and thinning of the apparatus. is there. In addition, since it is necessary to place an image sensor at the imaging position of the objective lens, it becomes difficult to design the objective lens when the size of the PBS is increased. Further, the optical axis of the reproduction signal light after passing through the PBS may be shifted due to the rotation or tilt of the PBS. In particular, when the S-polarized reproduction signal light is reflected by the reflection surface inside the PBS, the optical axis shift becomes a problem. Therefore, when PBS is used, it is necessary to align the PBS in the in-plane direction with respect to the image sensor.

  The present invention has been made in view of the above problems. An object of the present invention is to provide a hologram reproducing apparatus and a hologram reproducing method capable of obtaining a high-quality reproduction signal and capable of reducing the size and thickness of the apparatus.

  In summary, the present invention is a hologram reproducing apparatus for reproducing a transmission type hologram recorded on a plate-shaped recording medium having first and second surfaces, and includes an incident light generation unit and a reproduction reference light optical system. And a light receiving portion and a polarizing plate. The incident light generation unit generates coherent linearly polarized light and makes it incident on the recording medium as incident light from the first surface. The reproduction reference light optical system uses the transmitted light that has been transmitted through the recording medium, and the incident light enters the recording medium from the second surface, and generates a linearly-polarized reproduction reference light that is 90 ° different from the polarization direction of the transmitted light. To do. The light receiving unit receives the reproduction signal light emitted from the first surface when the reproduction reference light is diffracted by the hologram. The polarizing plate is provided on the optical path of the reproduction signal light between the first surface and the light receiving unit, and selectively transmits light in the polarization direction of the reproduction signal light.

Preferably, the polarizing plate is provided in close contact with the light receiving surface of the light receiving unit that receives the reproduction signal light.
Preferably, the reproduction reference light optical system includes a quarter-wave plate that converts transmitted light into circularly polarized light, and a reflecting unit that reflects circularly polarized light. The quarter-wave plate further converts the reflected circularly polarized light into reproduction reference light.

Preferably, the incident light is P-polarized light.
More preferably, the hologram reproducing device further includes a control unit. The control unit controls the incident light generation unit to change the incident angle of the incident light on the first surface according to each of the plurality of holograms recorded on the recording medium by the angle multiplexing method. Here, the change range of the incident angle of the incident light includes the Brewster angle. The control unit further controls the reflection direction of the reflection unit so that the reproduction reference light travels in a direction 180 ° opposite to the traveling direction of the transmitted light.

  Alternatively, more preferably, the incident angle of the incident light on the first surface is fixed to the Brewster angle. Then, the hologram reproducing device controls the reflection direction of the reflecting portion so as to change the incident angle of the reproduction reference light to the second surface according to each of the plurality of holograms recorded on the recording medium by the angle multiplexing method. And a control unit.

  In another aspect of the present invention, the present invention is a hologram reproducing method for reproducing a transmission type hologram recorded on a plate-shaped recording medium having first and second surfaces. The hologram reproducing method of the present invention generates a coherent linearly polarized light and makes it incident on the recording medium as incident light from the first surface, and transmits the incident light transmitted through the recording medium. And a step of generating linearly polarized reproduction reference light that is 90 ° different from the direction of polarization of transmitted light, and reproduction reproduced from the first surface by diffracting the reproduction reference light by the hologram. The method includes a step of selectively transmitting signal light using a polarizing plate and a step of receiving reproduction signal light transmitted through the polarizing plate.

  Preferably, the step of entering as incident light includes the step of generating laser light, the step of converting the generated laser light into P-polarized light with respect to the incident surface of the incident light, and a plurality of multiple recordings recorded on the recording medium Irradiating the first surface with laser light converted to P-polarized light at an incident angle corresponding to each of the holograms. The steps of generating the reproduction reference light include the steps of converting the transmitted light into circularly polarized light using a quarter wavelength plate, reflecting the circularly polarized light in the reverse direction by 180 °, and reflecting the reflected light. Converting circularly polarized light into reproduction reference light using a quarter-wave plate.

  Alternatively, preferably, the step of entering as incident light includes the step of generating laser light, the step of converting the generated laser light into P-polarized light with respect to the incident surface of the incident light, and the laser converted into P-polarized light Irradiating the first surface with light at a Brewster angle. The step of generating reproduction reference light includes a step of converting the transmitted light into circularly polarized light using a quarter wavelength plate, and a reflection angle corresponding to each of a plurality of holograms recorded in a multiplexed manner on the recording medium. Reflecting the circularly polarized light and converting the reflected circularly polarized light into reproduction reference light using a quarter-wave plate.

  According to the present invention, since a light source for generating incident light and a light receiving unit for receiving reproduction signal light can be arranged on the same side with respect to the recording medium, a hologram reproducing apparatus and method that can be reduced in size and thickness are provided. can do. Moreover, since the incident light is separated from the reflected light reflected from the surface of the recording medium and the reproduction signal light by the polarizing plate, a high-quality reproduction signal can be obtained. Further, since the polarizing plate used for separation of the reproduction signal light can be disposed immediately before the light receiving surface of the light receiving unit, the reproducing apparatus can be made smaller and thinner.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The same or corresponding parts are denoted by the same reference numerals, and the description thereof will not be repeated.

[Embodiment 1]
FIG. 1 is a schematic diagram showing a configuration of a hologram reproducing device 100 according to Embodiment 1 of the present invention. The hologram reproducing apparatus 100 in FIG. 1 reproduces a transmission type hologram that is multiplexed and recorded on the plate-like hologram recording medium 6 by an angle multiplexing method. The hologram reproducing device 100 includes a laser light source 1, a beam expander 3, a polarizer 4, a front side mirror 5, a quarter wavelength plate 10, a back side mirror 11, an objective lens 14, and a polarizing plate 15. And an imaging device 16 and an aperture 19.

  The hologram recording medium 6 includes a front side substrate 7a and a back side substrate 7b (hereinafter collectively referred to as a substrate 7), and a recording layer 8 sandwiched between both the substrates 7 to form interference fringes. That is, the hologram recording medium 6 has a sandwich structure in which the recording layer 8 is sandwiched between the front substrate 7a and the back substrate 7b. As a material of the substrate 7, a light-transmitting synthetic resin such as polycarbonate or acrylic, glass or the like is used. The recording layer 8 is made of, for example, a synthetic resin material such as a photopolymer whose refractive index changes with light irradiation.

  In the case of a transmission type hologram, signal light and reference light modulated by information to be recorded are incident from the same side of the plate-like hologram recording medium 6. The interference fringes of the signal light and the reference light generated at this time are recorded on the hologram recording medium 6. In the following description, the surface on which signal light and reference light are incident during recording is referred to as the front surface 6a of the hologram recording medium 6, and the opposite surface is referred to as the back surface 6b of the hologram recording medium 6.

  When reproducing a transmission type hologram, if reproduction reference light is incident from the front side in the same direction as the reference light during recording, reproduction signal light is obtained on the back side. On the other hand, when reproduction reference light is incident from the back side opposite to the reference light during recording, reproduction signal light is obtained on the front side. When the reproduction reference light is incident from the back side, the reproduction signal light has a phase conjugate relationship with the signal light at the time of recording.

  Hereinafter, for convenience of explanation, directions along the front surface (front surface 6a and back surface 6b) of the hologram recording medium 6 are referred to as an X direction and a Y direction. The + X direction is a direction from the left to the right of the drawing, and the + Y direction is a direction from the back to the front of the drawing. The thickness direction of the hologram recording medium 6 is referred to as the Z direction. The + Z direction is a direction from the top to the bottom of the drawing. In the case of FIG. 1, the front side of the hologram recording medium 6 is the −Z direction, and the back side is the + Z direction.

  The laser light source 1 is a laser device that emits blue or green laser light having a single wavelength. The light emitted from the laser light source 1 is expanded in diameter by the beam expander 3 and emitted as parallel light. The intensity distribution of the parallel light has a Gaussian shape. At this time, if there is a difference in the light intensity distribution in the hologram to be reproduced, this may cause unevenness in the intensity of the reproduced image. Therefore, it is preferable to use parallel light having a uniform intensity distribution with respect to the size of the hologram. For example, it is effective to insert an apodizer (not shown) that flattens a Gaussian-shaped beam light distribution into the optical path of the laser light. Alternatively, light in the vicinity of the center may be used by removing a peripheral portion of the laser beam having a sufficiently wide beam diameter with an aperture (not shown) by the beam expander 3.

  The polarizer 4 is provided on the output side of the beam expander 3 and extracts linearly polarized light from the laser light. In the first embodiment, the direction of the transmission axis of the polarizer 4 is aligned with the X direction so that the incident light L1 becomes P-polarized with respect to the incident surface when entering the hologram recording medium 6. Here, P-polarized light refers to linearly polarized light whose electric field component is parallel to the incident surface (paper surface). On the other hand, S-polarized light refers to linearly polarized light whose electric field component is perpendicular to the incident surface. As the polarizer 4, for example, a birefringent crystal plate such as calcite or a polarizing prism can be used. When the laser light source 1 that generates linearly polarized laser light is used as in the semiconductor laser device, the semiconductor laser device is arranged so that the polarization direction of the generated laser light matches the direction of the transmission axis of the polarizer 4. Adjust the direction. Alternatively, the polarizer 4 need not be used if the polarization direction of the incident light L1 is determined by the arrangement direction of the semiconductor laser device.

  The front-side mirror 5 reflects the P-polarized laser light transmitted through the polarizer 4 in the direction of the hologram recording medium 6. The front-side mirror 5 performs both the angular displacement operation and the shift operation in the Z direction according to the control of the control unit 20. As a result, the laser beam is deflected at a desired angle corresponding to the multiplex-recorded hologram without changing the irradiation position on the hologram recording medium 6, and is incident on the hologram recording medium 6 as incident light L1. In such a configuration, since the distance between the hologram recording medium 6 and the front-side mirror 5 can be set freely, the restriction on the arrangement of members is small and the degree of freedom in design is high. In addition, the deflection angle of the incident light L1 can be increased. For example, a galvanometer mirror that is angularly displaced by microstep drive using a stepping motor can be used as the front-side mirror 5.

  The laser light source 1, the beam expander 3, the polarizer 4, and the front side mirror 5 constitute the incident light generation unit 30 of the present invention. The incident light generation unit 30 generates coherent linearly polarized light (P-polarized light) and enters the generated light as incident light L1 at a desired position on the front surface 6a of the hologram recording medium 6.

The reason why the polarization of the incident light L1 is changed to P polarization will be described next with reference to FIG.
FIG. 2 is a graph showing the relationship between the incident angle of incident light and the reflectance at the surface of the substrate 7 of the hologram recording medium 6. The graph of FIG. 2 shows a case where the material of the substrate 7 is glass and the wavelength of the laser light is 532 nm. The horizontal axis represents the incident angle of incident light, and the vertical axis represents the reflectance at the surface. Since the refractive index of glass at the same wavelength is 1.51, the Brewster angle is 56.5 °.

  As shown in FIG. 2, the reflectance of the P-polarized component is 0 at the Brewster angle. The reflectance of P-polarized light near the Brewster angle is smaller than that of S-polarized light. Therefore, in the first embodiment, when the hologram is multiplexed and recorded on the hologram recording medium 6, the range of the incident angle of the reference light is set to a range of ± 10 ° with the Brewster angle as the center. As a result, the range of the incident angle of the incident light L1 at the time of reproducing the hologram is within a range of ± 10 ° with the Brewster angle as the center, so that the reflected light on the surface of the hologram recording medium 6 can be suppressed. When the reflected light is taken into the image sensor 16, noise is generated, and the S / N ratio of the reproduction signal can be improved by suppressing the reflected light.

  Referring to FIG. 1 again, a part of the incident light L1 is diffracted by the hologram 9. Since the diffracted light at this time is emitted to the back surface 6b side of the hologram recording medium 6, it is not used in the hologram reproducing apparatus 100 of the first embodiment. Most of the other incident light L1 passes through the hologram recording medium 6 as transmitted light L2.

  The quarter wavelength plate 10 is provided on the optical path between the hologram recording medium 6 and the rear surface side mirror 11. The quarter-wave plate 10 gives a 90 ° phase difference to the polarization components in the principal axis (fast axis, delay axis) directions orthogonal to each other. The linearly polarized light inclined by 45 ° with respect to the main axis of the quarter-wave plate 10 is converted into circularly polarized light, and the circularly polarized light is converted into linearly polarized light. In the case of FIG. 1, the quarter wavelength plate 10 converts the P-polarized transmitted light L2 into circularly polarized light. The quarter-wave plate 10 can be formed by cleaving anisotropic crystals such as mica and quartz to a desired thickness.

  The back surface side mirror 11 as the reflecting portion shifts in the linear direction and angularly displaces in accordance with the displacement amount of the front side mirror 5 according to the command of the control portion 20. Thereby, the back surface side mirror 11 reflects the light converted into the circularly polarized light by the quarter wavelength plate 10 in a direction opposite to 180 °. The reflected circularly polarized light is incident on the quarter-wave plate 10 again. The quarter-wave plate 10 converts circularly polarized light into S-polarized reproduction reference light L3. In this way, S-polarized reproduction reference light L3 that travels 180 ° opposite to the P-polarized transmitted light L2 is generated. The quarter-wave plate 10 and the back surface side mirror 11 constitute the reproduction reference light optical system 40 of the present invention.

  The generated S-polarized reproduction reference light L3 is incident on the back surface 6b of the hologram recording medium 6. The incident S-polarized reproduction reference light L3 is diffracted by the target hologram 9 to be reproduced and emitted from the front surface 6a of the hologram recording medium 6 as S-polarized reproduction signal light L4.

  The objective lens 14 images the reproduction signal light L4 generated from the hologram 9. Here, it is desirable for the objective lens 14 to have the same optical characteristics as at the time of recording. By using such an objective lens, the influence of wavefront aberration caused by the objective lens is canceled, so that the distortion of the reproduced image can be reduced. As the objective lens 14, for example, a lens having a numerical aperture NA of 0.5 and a focal length of 4 mm is used.

  The aperture 19 is provided between the hologram recording medium 6 and the objective lens 14 so that stray light other than the reproduction signal light L4 does not enter the objective lens 14 as much as possible.

  The image sensor 16 as a light receiving unit is disposed on the image forming surface of the objective lens 14 and receives the reproduction signal light L4. The image sensor 16 reproduces two-dimensional image data corresponding to intensity modulation applied to the received reproduction signal light L4. As the imaging device 16, for example, a charge coupled device (CCD) image sensor or a complementary metal-oxide semiconductor (CMOS) image sensor can be used.

  The polarizing plate 15 is provided on the optical path of the reproduction signal light L4 between the objective lens 14 and the image sensor 16. The direction of the transmission axis of the polarizing plate 15 is set in the Y direction so as to selectively transmit the reproduction signal light L4 that is S-polarized light. For the polarizing plate 15, for example, a polarizer made of a polyvinyl alcohol film dyed with iodine is used. A protective film such as a triacetyl cellulose film is bonded to one side or both sides of the polarizer. The effect of providing such a polarizing plate 15 will now be described with reference to FIG.

  FIG. 3 is a schematic diagram for explaining reflected light and scattered light generated during reproduction of a hologram. FIG. 3 is a diagram in which a part of the hologram reproducing apparatus 100 of FIG. 1 is taken out and paths of reflected light L5 and scattered light L6 are added. 3 that are the same as those in FIG. 1 are denoted by the same reference numerals and description thereof will not be repeated.

  Referring to FIG. 3, a part of the P-polarized incident light L1 incident on the hologram recording medium 6 is reflected by the front substrate 7a to become P-polarized reflected light L5. Further, a part of the reflected light L5 is scattered by the aperture 19 to become P-polarized scattered light L6.

  Here, if the P-polarized incident light L1 is incident near the Brewster angle, the intensity of the reflected light L5 is reduced to 0.1% or less. Actually, since it is necessary to change the incident angle of the incident light L1 in order to reproduce the hologram recorded by the angle multiplex recording method, the intensity of the reflected light becomes larger than that when the incident light is incident at the Brewster angle. Even if the intensity of the reflected light L5 is suppressed to about 0.1%, the diffraction efficiency of the reproduced signal light L4 is about 0.1%, so the reflected light L5 and the reproduced signal light L4 are The light intensity is almost the same. Therefore, if the polarizing plate 15 is not provided, the reflected light L5 and the scattered light L6 having substantially the same intensity as the reproduction signal light L4 are superimposed on the reproduction signal light L4 and enter the image pickup device 16. Therefore, the quality of the reproduced image is deteriorated.

  Therefore, in the first embodiment, by providing the quarter wavelength plate 10, the direction of polarization of the reproduction signal light L4 (S-polarized light) is made different from the direction of polarization of the reflected light L5 and the scattered light L6 (P-polarized light). . Further, a polarizing plate 15 is provided immediately before the light receiving surface 16a of the image sensor 16 so as to selectively transmit the S-polarized reproduction signal light L4. As a result, it is possible to avoid a decrease in the S / N ratio of the reproduction signal.

  Here, the reflection direction of the reflected light L5 depends on the incident angle of the incident light L1 to the hologram recording medium 6. Therefore, the arrangement of the hologram recording medium 6 and the aperture 19 is devised, and the incident angle of the incident light L1 is limited so that the reflected light L5 does not enter the objective lens 14 even if the polarizing plate 15 is not provided. This is considered possible. However, if the incident angle of the incident light L1 is limited, the degree of freedom in design becomes small, and it becomes difficult to reduce the size and thickness. Furthermore, if the incident angle of the incident light L1 is limited, an increase in recording density by the angle multiplex recording method is also restricted. Furthermore, when the beam diameter of the incident light L1 is large or when the distance between the front substrate 7a and the objective lens 14 is short, a part of the reflected light or scattered light is applied to the objective lens 14 even if the aperture 19 is provided. Incident light is inevitable.

  On the other hand, in the case of the hologram reproducing apparatus 100 of FIG. 1, the polarizing plate 15 can be disposed immediately before the light receiving surface 16a of the image sensor 16, so that the reproducing apparatus can be easily reduced in size and thickness. Further, the increase in design freedom and recording density is not hindered. Furthermore, the beam diameter of the incident light L1 and the distance between the front substrate 7a and the objective lens 14 can be freely set without limitation.

  In order to further reduce the size and thickness of the hologram reproducing apparatus 100, the polarizing plate 15 is preferably provided in close contact with the light receiving surface 16a by a method such as sticking to the light receiving surface 16a of the image sensor 16. In addition, when the polarizing plate 15 is brought into close contact with the light receiving surface 16a in this way, the reflected light or scattered light transmitted through the objective lens 14 is further reflected or scattered by the peripheral member, and the imaging device 16 enters the polarizing plate 15 to enter. Can be prevented from entering the light receiving surface 16a.

  Next, with reference to FIG. 4 and FIG. 5, the effect of the hologram reproducing apparatus 100 according to the first embodiment will be further supplementarily described.

  FIG. 4 is a diagram schematically showing the reproduction signal 21 when the SN ratio is deteriorated by the incidence of reflected light and scattered light. FIG. 4 shows a reproduction signal imaged by the image sensor 16 when the beam diameter of the incident light L1 (reproduction reference light L3) is 2 mm and the incident angle of the incident light L1 (reproduction reference light L3) is 60 °. Show.

  FIG. 5 is a diagram schematically showing a reproduction signal 24 when only the reproduction signal light L4 is selectively imaged by providing the polarizing plate 15. The beam diameter and incident angle of the incident light L1 (reproduction reference light L3) in the case of FIG. 5 are the same as those in FIG.

  4 and FIG. 5, in the case shown in FIG. 4, since the noise signals 22 and 23 caused by some reflected light and scattered light are superimposed on the reproduction signal, the SN ratio of the reproduction signal 21 is It turns out that it falls. In particular, since the positions of the noise signals 22 and 23 vary depending on the incident angle of the incident light L1 (reproduction reference light L3), when the incident angle is smaller than 60 degrees, the reflection direction of the reflected light L5 is the reproduction signal. It comes closer to the traveling direction of the light L4. For this reason, the quality of the reproduction signal may be further deteriorated. On the other hand, in the case of FIG. 5, only the reproduction signal light L4 is selectively imaged on the light receiving surface 16a of the image sensor 16, so that the imaged reproduction signal 24 has noise as shown in FIG. No signal appears.

  FIG. 6 is a flowchart showing the operation of the hologram reproducing apparatus 100 of FIG. Hereinafter, the operation of the hologram reproducing apparatus 100 will be described generally with reference to FIGS. 1 and 6.

  In step S1 of FIG. 6, the laser light source 1 generates a coherent laser beam. The generated laser light is expanded in diameter by the beam expander 3 and emitted as parallel light.

  In the next step S2, the polarizer 4 converts the laser light emitted from the beam expander 3 into P-polarized light.

  In the next step S3, the front-side mirror 5 reflects the laser beam converted to P-polarized light and irradiates the target hologram to be reproduced as incident light L1. At this time, the control unit 20 sets the deflection angle of the front mirror 5 so that the incident angle of the incident light L1 is equal to the incident angle of the reference light when the target hologram is recorded.

  Part of the incident light L1 passes through the hologram recording medium 6. In the next step S4, the quarter wavelength plate 10 converts the transmitted incident light of P-polarized light (transmitted light L2) into circularly-polarized light.

  In the next step S5, the back surface side mirror 11 reflects the circularly polarized light in the reverse direction of 180 °. At this time, the control unit 20 sets the declination of the back-side mirror 11 according to the declination of the front-side mirror 5.

  The circularly polarized light reflected by the back side mirror 11 is converted from circularly polarized light to S-polarized reproduction reference light L3 by the quarter wavelength plate 10 in the next step S6. Then, the S-polarized reproduction reference light L3 is diffracted by the hologram 9 to generate the S-polarized reproduction signal light L4. The reproduction signal light L4 is emitted from the front side of the hologram recording medium 6 and passes through the objective lens 14.

  In the next step S7, the polarizing plate 15 selectively transmits the S-polarized reproduction signal light L4 transmitted through the objective lens 14. At this time, the P-polarized reflected light generated by the P-polarized incident light L 1 reflected by the front surface 6 a of the hologram recording medium 6 is blocked by the polarizing plate 15.

  In the next step S8, the image sensor 16 receives the reproduction signal light L4 that has passed through the polarizing plate 15 and outputs a reproduction signal. Thus, the hologram reproduction procedure by the hologram reproduction apparatus 100 is completed.

  As described above, the hologram reproducing apparatus 100 according to Embodiment 1 has the following operational effects. First, the hologram reproducing device 100 generates the reproduction reference light L3 using the incident light (transmitted light L2) transmitted through the hologram recording medium 6. Then, the reproduction signal light L4 is obtained by irradiating the reproduction reference light L3 from the back surface 6b side of the hologram recording medium 6. Therefore, since the laser light source 1 and the image sensor 16 can be arranged on the same side (front side) of the hologram recording medium 6, a small and thin hologram reproducing device 100 can be realized.

  Further, in the hologram reproducing device 100, the polarization of the transmitted light L2 transmitted through the hologram recording medium 6 and the polarization of the reproduction reference light L3 are different from each other by 90 °, and the reproduction signal light L4 is immediately before the light receiving surface 16a of the image sensor 16. A polarizing plate 15 that selectively transmits polarized light is disposed. As a result, the reflected light L5 and scattered light L6 that cause noise and the reproduction signal light L4 can be separated by the polarizing plate 15, so that the quality of the reproduction signal can be improved. Furthermore, since a high-quality reproduction signal can be obtained even when the diffraction efficiency of the hologram is reduced, the number of multiplexing can be increased, and as a result, high-density recording can be realized. Furthermore, since there is no restriction on the positional relationship between the hologram recording medium 6 and the objective lens 14 and the incident angle of the incident light L1 with respect to the hologram recording medium 6, the hologram reproducing apparatus 100 can be easily reduced in size and thickness.

  Further, by adopting the quarter wavelength plate 10, it is possible to generate the reproduction reference light L3 obtained by rotating the polarization direction of the transmitted light L2 by 90 degrees with a simple configuration. The quarter-wave plate 10 can be made small and thin, and position adjustment in the in-plane direction of the back side mirror 11 is unnecessary. For this reason, the hologram reproducing apparatus 100 can be easily reduced in size and thickness.

  Further, since the polarizing plate 15 that selectively transmits the reproduction signal light L4 can be disposed immediately before the light receiving surface 16a of the image sensor 16, the hologram reproduction apparatus 100 can be easily reduced in size and thickness. Further, by bringing the polarizing plate 15 and the light receiving surface 16a into close contact with each other, light other than the reproduction signal light L4 is reflected or scattered by the peripheral member so that it does not go around the polarizing plate 15 and enter the light receiving surface 16a. it can. As a result, a high quality reproduction signal can be obtained. In addition, unlike the polarization beam splitter used in the conventional example, the change in the relative position of the polarizing plate 15 with respect to the in-plane direction of the image sensor 16 has little influence on the signal quality. This eliminates the need for signal processing for correcting the deviation of the optical axis.

Moreover, the hologram reproducing apparatus 100 according to the first embodiment can be modified as follows.
First, in the first embodiment, laser light converted into P-polarized light by the polarizer 4 is incident on the hologram recording medium 6 as incident light L1. On the other hand, laser light converted into S-polarized light by the polarizer 4 may be incident on the hologram recording medium 6 as incident light L1. In the case of the S-polarized incident light L1, the reproduction signal light L4 becomes P-polarized light, and the reflected light L5 and the scattered light L6 become S-polarized light. Therefore, if only the P-polarized reproduction signal light L4 is selectively transmitted by the polarizing plate 15, the S-polarized reflected light L5 and the scattered light L6 can be prevented from entering the image sensor 16, and the SN of the reproduction signal can be prevented. The ratio can be improved. However, the polarization of the incident light L1 is preferably P polarization as in the first embodiment. This is because the reflectance of the P-polarized light on the surface of the hologram recording medium 6 is smaller than that of the S-polarized light, so that the intensity of the reflected light L5 and the scattered light L6 can be reduced.

  The polarization of the incident light L1 is not limited to P-polarized light or S-polarized light, and may be linearly polarized light in any direction. In this case, the reflectance and transmittance on the surface of the hologram recording medium 6 are different between the electric field component in the X direction and the electric field component in the Y direction. Therefore, the polarization direction of the incident light L1 and the polarization direction of the transmitted light L2 are different, and the polarization direction of the reproduction reference light L3 and the polarization direction of the reproduction signal light L4 are different. However, in this case as well, the SN ratio of the reproduction signal can be improved by adjusting the direction of the transmission axis of the polarizing plate 15 so as to selectively transmit the light in the polarization direction of the reproduction signal light L4.

  Further, instead of the quarter wavelength plate 10, the optical path of the transmitted light L2 and the optical path of the reproduction reference light L3 may be separated and a half wavelength plate may be provided on either one of the optical paths. Also in this case, the direction of polarization of the transmitted light L2 when emitted from the hologram recording medium 6 and the direction of polarization of the reproduction reference light L3 when incident on the hologram recording medium 6 can be different by 90 °. .

  Further, the incident light L1 may be incident from the back surface 6b side of the hologram recording medium 6 with the front and back surfaces of the hologram recording medium 6 reversed. In this case, when the reproduction reference light L3 is incident on the front surface 6a side of the hologram recording medium 6, the reproduction signal light L4 is emitted from the rear surface 6b side. Therefore, if the reproduction signal light L4 is imaged by the objective lens 14 provided on the back surface 6b side and imaged by the imaging element 16, the hologram can be reproduced.

[Embodiment 2]
FIG. 7 is a schematic diagram showing the configuration of the hologram reproducing apparatus 101 according to the second embodiment of the present invention. The incident light generation unit 30a in FIG. 7 differs from the incident light generation unit 30 in FIG. 1 in that it further includes an irradiation position adjustment lens 31 (31a, 31b). Further, the reproduction reference light optical system 40a of FIG. 7 is different from the reproduction reference light optical system 40 of FIG. 1 in that it further includes an irradiation position adjusting lens 41 (41a, 41b). With respect to other points, hologram reproducing apparatus 101 in FIG. 7 is common to hologram reproducing apparatus 100 in the first embodiment in FIG. 1, and therefore, common portions are denoted by the same reference numerals and description thereof will not be repeated. In FIG. 7, the objective lens 14, the polarizing plate 15, the image sensor 16, and the aperture 19 that are the same as those in FIG. 1 are not shown.

  Referring to FIG. 7, the front side mirror 5, the irradiation position adjusting lens 31, and the hologram recording medium 6 of the incident light generating unit 30a constitute a 4f optical system. That is, the front side mirror 5, the irradiation position adjustment lens 31a, the focal plane 32, the irradiation position adjustment lens 31b, and the hologram recording medium 6 are arranged in this order at intervals of the focal length of the irradiation position adjustment lenses 31a and 31b. . In the case of the 4f optical system, the control unit 20a can change the incident angle without changing the irradiation position on the hologram recording medium 6 by adjusting only the deflection angle of the front-side mirror 5. That is, since it is not necessary to displace the front side mirror 5 in the Z direction, the control speed of the front side mirror 5 can be increased.

  Similarly, the back side mirror 11, the irradiation position adjusting lens 41, and the hologram recording medium 6 of the reproduction reference light optical system 40a constitute a 4f optical system. That is, the rear surface side mirror 11, the irradiation position adjustment lens 41b, the focal plane 42, the irradiation position adjustment lens 41a, and the hologram recording medium 6 are arranged in this order at intervals of the focal length of the irradiation position adjustment lenses 41a and 41b. The In the case of the 4f optical system, the control unit 20a can generate the reproduction reference light L3 obtained by inverting the transmitted light L2 by adjusting only the deflection angle of the rear surface side mirror 11. Thus, since it is not necessary to displace the back side mirror 11 in the Z direction, the control speed of the back side mirror 11 can be increased.

[Embodiment 3]
FIG. 8 is a schematic diagram showing the configuration of the hologram reproducing apparatus 102 according to the third embodiment of the present invention. The incident light generation unit 30b in FIG. 8 differs from the incident light generation unit 30 in FIG. 1 in that the deflection angle of the front-side mirror 5 is fixed. In the third embodiment, the incident angle of incident light is fixed to the Brewster angle. 8 is different from the reproduction reference light optical system 40 in FIG. 1 in that the reproduction reference light optical system 40a in FIG. 8 further includes an irradiation position adjusting lens 41 (41a, 41b). In other respects, hologram reproducing apparatus 101 in FIG. 8 is common to hologram reproducing apparatus 100 in the first embodiment in FIG. 1, and therefore, common portions are denoted by the same reference numerals and description thereof will not be repeated. In FIG. 8, the objective lens 14, the polarizing plate 15, the imaging element 16, and the aperture 19 that are the same as those in FIG. 1 are not shown.

  Referring to FIG. 8, the back side mirror 11, the irradiation position adjusting lens 41, and the hologram recording medium 6 of the reproduction reference light optical system 40a constitute a 4f optical system. That is, the back surface side mirror 11, the irradiation position adjustment lens 41b, the focal plane 42, the irradiation position adjustment lens 41a, and the hologram recording medium 6 are arranged in this order at intervals of the focal length of the irradiation position adjustment lenses 41a and 41b. . In the third embodiment, the control unit 20b changes the incident angle of the reproduction reference light on the back surface 6b by adjusting the deflection angle of the back surface side mirror 11. Thus, the control unit 20b sets the incident angle of the reproduction reference light to an angle corresponding to the target hologram recorded by the angle multiplex recording method.

  FIG. 9 is a flowchart showing the operation of the hologram reproducing apparatus 102 of FIG. The flowchart of FIG. 9 differs from the flowchart of FIG. 6 in that steps S3a and S5a are included instead of steps S3 and S5 of FIG.

  Referring to FIGS. 1, 8, and 9, in step S1, laser light source 1 generates a coherent laser beam. The generated laser light is expanded in diameter by the beam expander 3 and emitted as parallel light.

  In the next step S2, the polarizer 4 converts the laser light emitted from the beam expander 3 into P-polarized light.

  In the next step S3a, the front-side mirror 5 reflects the laser beam converted to P-polarized light and irradiates it as incident light toward the target hologram to be reproduced. The incident angle of incident light at this time is fixed at the Brewster angle. Thus, almost no reflected light is reflected from the front surface 6a of the hologram recording medium 6.

  The transmitted light that has passed through the hologram recording medium 6 passes through the irradiation position adjusting lens 41. In the next step S4, the quarter-wave plate 10 converts the P-polarized transmitted light that has passed through the irradiation position adjusting lens 41 into circularly-polarized light.

  In the next step S5a, the back side mirror 11 reflects the circularly polarized light at an angle set by the control unit 20b. At this time, the control unit 20b sets the deflection angle of the rear surface side mirror 11 according to the incident angle of the reference light when the target hologram is recorded so that the reproduction signal light is generated from the target hologram.

  The circularly polarized light reflected by the rear surface side mirror 11 is converted from circularly polarized light to S-polarized reproduction reference light by the quarter wavelength plate 10 in the next step S6. The S-polarized reproduction reference light passes through the irradiation position adjusting lens 41 and then enters the hologram recording medium 6 from the back surface 6b side. As the reproduction reference light is diffracted by the target hologram, S-polarized reproduction signal light is generated. The reproduction signal light is emitted from the front surface 6 a side of the hologram recording medium 6 and passes through the objective lens 14.

  In the next step S7, the polarizing plate 15 selectively transmits the S-polarized reproduction signal light transmitted through the objective lens 14. At this time, the P-polarized reflected light generated by reflecting the P-polarized incident light on the front surface 6 a of the hologram recording medium 6 is blocked by the polarizing plate 15.

  In the next step S8, the image sensor 16 receives the reproduction signal light L4 that has passed through the polarizing plate 15 and outputs a reproduction signal. Thus, the hologram reproduction procedure by the hologram reproduction apparatus 102 is completed.

  Thus, according to the hologram reproducing apparatus 102 of the third embodiment, the incident angle of the P-polarized incident light incident from the front surface 6a of the hologram recording medium 6 is fixed to the Brewster angle. Then, in order to reproduce the hologram recorded by angle multiplexing, the incident angle of the reproduction reference light is adjusted by changing the deflection angle of the back side mirror 11. By this method, the reflected light reflected by the front surface 6a of the hologram recording medium 6 can be made almost zero.

  If the polarization direction of the incident light or the incident angle of the incident light on the hologram recording medium 6 deviates from the setting, a reproduction signal is obtained by a polarizing plate provided immediately before the light receiving surface of the image sensor as in the first embodiment. Light is separated from reflected light. Therefore, according to hologram reproducing apparatus 102 of Embodiment 3, the influence of reflected light and scattered light can be avoided more reliably, and a reproduced signal with a good SN ratio can be obtained.

  The embodiment disclosed this time must be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is the schematic which shows the structure of the hologram reproduction apparatus 100 by Embodiment 1 of this invention. 4 is a graph showing the relationship between the incident angle of incident light and the reflectance at the surface of the substrate 7 of the hologram recording medium 6. It is a schematic diagram for demonstrating the reflected light and scattered light which are produced at the time of reproduction | regeneration of a hologram. It is a figure which shows typically the reproduction | regeneration signal 21 when SN ratio deteriorates by reflected light and scattered light entering. It is a figure which shows typically the reproduction | regeneration signal 24 at the time of selectively imaging only the reproduction | regeneration signal light L4 by providing the polarizing plate 15. FIG. 3 is a flowchart showing the operation of the hologram reproduction apparatus 100 of FIG. It is the schematic which shows the structure of the hologram reproduction apparatus 101 by Embodiment 2 of this invention. It is the schematic which shows the structure of the hologram reproduction apparatus 102 by Embodiment 3 of this invention. It is a flowchart which shows operation | movement of the hologram reproduction apparatus 102 of FIG. FIG. 6 is a schematic diagram showing a configuration of a conventional hologram recording / reproducing apparatus 200.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Laser light source, 3 Beam expander, 4 Polarizer, 5 Front side mirror, 6 Hologram recording medium, 6a Front surface, 6b Back surface, 7a Front side substrate, 7b Back surface side substrate, 8 Recording layer, 9 Hologram, 10 1/4 Wave plate, 11 Back side mirror, 14 Objective lens, 15 Polarizing plate, 16 Image sensor, 16a Light receiving surface, 19 Aperture, 20, 20a, 20b Control unit, 30, 30a, 30b Incident light generation unit, 40, 40a Optical optical system, 100, 101, 102 Hologram reproduction device, L1 incident light, L2 transmitted light, L3 reproduction reference light, L4 reproduction signal light.

Claims (9)

A hologram reproducing apparatus for reproducing a transmission type hologram recorded on a plate-shaped recording medium having first and second surfaces,
An incident light generation unit configured to generate coherent linearly polarized light and to enter the recording medium as incident light from the first surface;
Reproduction in which the incident light is incident on the recording medium from the second surface using transmitted light transmitted through the recording medium, and generates linearly-polarized reproduction reference light that is 90 ° different from the direction of polarization of the transmitted light. A reference beam optical system;
A light receiving unit that receives the reproduction signal light emitted from the first surface by diffracting the reproduction reference light by the hologram;
A hologram reproducing apparatus comprising: a polarizing plate provided on an optical path of the reproduction signal light between the first surface and the light receiving unit and selectively transmitting light in a polarization direction of the reproduction signal light.   The hologram reproducing apparatus according to claim 1, wherein the polarizing plate is provided in close contact with a light receiving surface of the light receiving unit that receives the reproduction signal light. The reproduction reference beam optical system includes:
A quarter-wave plate for converting the transmitted light into circularly polarized light;
A reflective portion that reflects the circularly polarized light,
The hologram reproducing apparatus according to claim 1, wherein the quarter-wave plate further converts the reflected circularly polarized light into the reproduction reference light.   The hologram reproducing apparatus according to claim 3, wherein the incident light is P-polarized light. The hologram reproducing apparatus further includes a control unit,
The control unit controls the incident light generation unit to change an incident angle of the incident light on the first surface according to each of a plurality of holograms recorded on the recording medium by an angle multiplexing method. And
The change range of the incident angle of the incident light includes a Brewster angle,
The hologram reproduction apparatus according to claim 4, wherein the control unit further controls a reflection direction of the reflection unit so that the reproduction reference light travels in a direction 180 ° opposite to a traveling direction of the transmitted light. The incident angle of the incident light on the first surface is fixed to a Brewster angle,
The hologram reproduction apparatus reflects the reflection unit so as to change an incident angle of the reproduction reference light on the second surface in accordance with each of a plurality of holograms recorded on the recording medium by an angle multiplexing method. The hologram reproducing apparatus according to claim 4, further comprising a control unit that controls a direction. A hologram reproducing method for reproducing a transmission type hologram recorded on a plate-shaped recording medium having first and second surfaces,
Generating coherent linearly polarized light to be incident on the recording medium as incident light from the first surface;
The incident light enters the recording medium from the second surface using the transmitted light transmitted through the recording medium, and generates linearly-polarized reproduction reference light that is 90 ° different from the polarization direction of the transmitted light. When,
Selectively transmitting the reproduction signal light emitted from the first surface by diffracting the reproduction reference light by the hologram using a polarizing plate;
Receiving the reproduction signal light transmitted through the polarizing plate. The step of entering as the incident light includes:
Generating a laser beam; and
Converting the generated laser light into P-polarized light with respect to the incident surface of the incident light;
Irradiating the first surface with the laser beam converted into P-polarized light at an incident angle corresponding to each of a plurality of holograms multiplex-recorded on the recording medium,
The step of generating the reproduction reference light includes:
Converting the transmitted light into circularly polarized light using a quarter-wave plate;
Reflecting the circularly polarized light in a reverse direction of 180 °;
The hologram reproducing method according to claim 7, further comprising: converting the reflected circularly polarized light into the reproduction reference light using the ¼ wavelength plate. The step of entering as the incident light includes:
Generating a laser beam; and
Converting the generated laser light into P-polarized light with respect to the incident surface of the incident light;
Irradiating the first surface with the Brewster angle with the laser light converted to P-polarized light,
The step of generating the reproduction reference light includes:
Converting the transmitted light into circularly polarized light using a quarter-wave plate;
Reflecting the circularly polarized light at a reflection angle corresponding to each of a plurality of holograms multiplex-recorded on the recording medium;
The hologram reproducing method according to claim 7, further comprising: converting the reflected circularly polarized light into the reproduction reference light using the ¼ wavelength plate.