JP2007101594A - Holographic recording apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a holographic recording apparatus having a simple configuration and suitable for high-density recording. <P>SOLUTION: The light from a light source 1 is modulated into recording beams in a light-and-dark pattern according to recording signals by a spatial optical modulator 2 and condensed by a lens 6 to irradiate a hologram recording medium 4, and the light transmitting the hologram recording medium is reflected by a reflector 5 placed in the back side opposite to the surface of the hologram recording medium to propagate toward the hologram recording medium. Thereby, the incident light and the reflected light interfere in the hologram medium to record interference fringes in the medium. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a holographic recording apparatus using a holographic technique.

  In current optical recording typified by CD and DVD, pits and the like are recorded in a plane on a recording medium for each bit. The recording density by this method is limited by the diffraction limit of light, that is, how much light can be reduced. On the other hand, in optical recording using the holographic technique, page data composed of a plurality of bits is recorded on a three-dimensional volume hologram layer. In addition, since each page data can exist while overlapping, multiple recording is possible. For this reason, in optical recording using a hologram, recording with dramatically high density and large capacity is possible compared with the conventional recording method for each 1 bit. In addition, the conventional method can transfer data only 1 bit at a time, whereas the method using a hologram can read and write data for one page consisting of a plurality of bits at a time, thereby greatly improving the data transfer speed. . For this reason, optical recording technology using holograms has been vigorously researched and developed for practical use in various places.

  In holographic recording, signal light having page data and reference light having the same wavelength are irradiated on the surface of the recording medium, and interference fringes formed from both lights are recorded as signal information on the recording medium. Since the interference fringes are used as recording information, the positional relationship between the signal light, the reference light, and the recording medium during recording is not allowed to deviate in the order of the wavelength of the light due to causes such as vibration. It was one of the major factors that hindered the practical application of

  In order to overcome the weakness against vibration, a method shown in the following Patent Document 1 has been proposed. This is a system in which the signal light and the reference light are arranged coaxially with the optical system so that the positional relationship does not shift even with respect to vibration.

  Furthermore, the inventor previously invented the device disclosed in Patent Document 2 as an improved device of the device disclosed in Patent Document 1 described above. This apparatus will be described with reference to FIG. 6. A light source 1, a spatial light modulator (SLM) 2 that modulates laser light emitted from the light source 1 with a recording signal, and light modulated by the spatial light modulator 2. To a holographic structure comprising: a diffraction grating 3 for generating a plurality of orders of diffracted light; and a holding means (not shown) for holding a hologram recording medium 4 for recording the diffracted light output from the diffraction grating 3. It is a recording device.

The operation is as follows. Light emitted from the light source 1 passes through the SLM 2, is modulated as recording light, passes through the diffraction grating 3, and reaches the hologram recording medium 4. The light that has passed through the diffraction grating 3 causes interference between diffracted lights (for example, 0th-order diffracted light and + 1st-order diffracted light), and this is used for hologram recording. by this,
(1) The optical system is simple and the number of parts constituting the apparatus can be greatly reduced.
(2) There is little loss of light generated from the light source.
(3) Strong against vibration during recording and reproduction.
(4) The design margin for scheduling in recording / reproducing system design, particularly recording can be increased.
(5) The device can be made smaller and lighter.
(6) The cost can be reduced.
There is a merit such as.
JP-A-11-311938 (abstract) JP 2005-128473 A (abstract, FIG. 3)

  However, in Patent Document 2, the diffraction grating 3 for causing the recording light to interfere is necessary. Therefore, in order to realize the diffraction grating 3 specifically, as described in Patent Document 2, the diffraction grating 3 is placed on the floating block. However, this requires a complicated manufacturing process. Further, in order to float the block by the gas flow, the surface of the hologram recording medium 4 must be strictly controlled. This is because if there is a defect on the surface of the hologram recording medium 4, a stable flying state cannot be maintained, and in the worst case, the block or the medium is damaged or destroyed.

  In Patent Document 2, there is a restriction in order to project the signal pattern formed by the SLM 2 onto the hologram recording medium 4. 7 and 8 are schematic views showing an embodiment of the holographic recording device disclosed in Patent Document 2. Convex lenses L1 and L2 for condensing the signal pattern formed by the SLM 2 and projecting it onto the hologram recording medium 4 are shown. It is explanatory drawing of the positional relationship with the hologram recording medium. The convex lens L2 may be arranged so that the diameter of the beam focused on the hologram recording medium 4 by the convex lenses L1 and L2 is large enough to maintain the state where the bright and dark portions before the focusing are distributed at the position of the hologram recording medium 4. is necessary. This is because if the beam diameter is made too small, the recording light beam is Fourier transformed, and the original light and dark state is destroyed.

The extent to which the beam diameter can be reduced depends on the relationship between the minimum width d of the light and dark pattern in the recording light immediately before entering the convex lens L2 and the distance a from the focal position of the convex lens L2 to a part of the recording layer of the hologram recording medium 4. Determined by. This situation is shown using FIG.
a> f · λ / d (1)
It must be arranged so that a part of the recording layer of the hologram recording medium 4 covers the position of a satisfying the above relationship with respect to the thickness direction of the recording layer. Here, f is the distance from the convex lens L2 to the focal point, and λ is the wavelength of the recording light. Rewriting equation (1),
d> λ · f / a (2)
This also includes a condition for reducing the influence of diffracting the recording light. For example, when f = 100 mm, λ = 500 nm, and d = 10 μm, a> 5 mm must be satisfied. In order to realize high-density recording, it is necessary to reduce d and a. However, if there are restrictions as described above, it is difficult to realize high-density recording.

  The present invention has been made in view of the above-described problems of the conventional example, and an object thereof is to provide a holographic recording apparatus having a simple configuration and suitable for high-density recording.

In order to achieve the above object, the present invention provides a light source,
A spatial light modulator that irradiates the light-transmitting hologram recording medium surface by modulating the light emitted from the light source so as to become a recording light beam of a light and dark pattern according to a recording signal;
A plurality of lenses arranged in an optical path between the spatial light modulator and the hologram recording medium so as to form an image on the hologram recording medium of the bright and dark pattern of the recording light beam modulated by the spatial light modulator When,
The recording light beam disposed on the back surface side of the front surface of the hologram recording medium and reflecting the recording light beam transmitted through the hologram recording medium toward the hologram recording medium side, and incident light of the recording light beam in the hologram recording medium A reflecting member that generates interference fringes due to reflected light and records the interference fringes as recording information on the hologram recording medium;
It is characterized by having.
With this configuration, even when signal light having a large amount of data per page is reduced in the hologram recording medium, the hologram recording medium is irradiated with a plurality of diffracted lights only on the part where the light corresponding to the recording signal has reached, The SLM pattern is recorded as interference fringes.

According to the present invention, since the reference light is not irradiated to the portion that is not irradiated with the recording light, there is no risk of the recording medium being exposed to the reference light, and the S / N during reproduction can be increased. In other words, when trying to obtain an S / N equivalent to that of the conventional holographic recording, the recording multiplexing can be approximately doubled by a simple calculation compared to the conventional holographic recording, and the recording capacity is also reduced. Can be doubled. In addition, recording medium characteristics and recording / reproducing optical systems that are not sensitive to reference light alone are not required, and a design margin for scheduling such as recording / reproducing system design, particularly recording power, multiplicity, and recording time can be increased. .
Furthermore, unlike conventional holographic recording, the light emitted from the same light source does not need to be separated into signal light and reference light by arranging optical components such as beam splitters and mirrors. The amount of light loss is small and the efficiency is good. There is no need for a new optical component such as an optical rotator shown in Patent Document 1. Thus, since many optical parts that have been necessary in the past are not required, the recording / reproducing apparatus can be simplified, simplified, and downsized.
Furthermore, it is not necessary to manufacture complicated parts such as a floating block having a diffraction grating. Also, there is no need to float the block with a gas flow, so even if there are defects on the surface of the recording medium, there is no risk of damaging or destroying the recording medium or other parts, and strict management of the surface of the recording medium is also necessary. No.
Furthermore, even if the recording light is reduced to a small position at the recording medium position, the SLM pattern is accurately imaged and the recording density can be improved.

That is,
(1) The optical system is simple and the number of parts constituting the apparatus can be greatly reduced.
(2) There is little loss of light generated from the light source.
(3) Strong against vibration during recording and reproduction.
(4) The design margin for scheduling in recording / reproducing system design, particularly recording can be increased.
(5) The device can be made smaller and lighter.
(6) The cost can be reduced.
In addition to the effect
(7) The storage capacity (recording density) can be increased.
(8) No complicated optical parts in the manufacturing process are required.
(9) Even if a defect exists in the recording medium, the recording medium and other parts are not damaged or destroyed, and strict management of the recording medium surface is not necessary.
It has the effect.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a basic configuration of a holographic recording apparatus according to the present invention. Light from a light source 1 is modulated by a spatial light modulator 2 so as to become a recording light beam of a bright and dark pattern according to a recording signal, and a lens. 6 condenses and irradiates the hologram recording medium 4, and further, the light transmitted through the hologram recording medium 4 by the reflection plate 5 disposed on the back side of the surface of the hologram recording medium 4 is directed to the hologram recording medium 4 side. reflect. Thereby, incident light and reflected light cause interference in the hologram recording medium 4, and interference fringes are recorded in the hologram recording medium 4 as recording information.

  Details of each part will be described below. The light source 1 generates light necessary for recording and reproduction, and is preferably a laser light source capable of generating light of a specific wavelength. As the type of laser, a semiconductor laser (LD: laser diode), a gas laser, or a pulsed laser represented by a YAG laser or a Ti sapphire laser can be used. Here, conventionally, since the light source used for hologram recording had to make the recording light and the reference light interfere with each other, a corresponding coherent length was required. On the other hand, the light source 1 used in this configuration performs hologram recording by causing the incident light and the reflected light from the reflecting plate 5 to interfere with each other, so that the coherent length is from the surface of the hologram recording medium 4 to the recording layer in the hologram recording medium 4. Since the distance up to 10 mm, specifically 10 mm, is sufficient, the selection range is wide.

  The spatial light modulator 2 is used to modulate light with a recording signal input as an electric signal. When digital information for one page to be recorded is input to the SLM 2 as an electrical signal, the light that has passed through the SLM 2 (reflected if the SLM 2 is a reflection type as shown in FIG. 3) corresponds to the input signal. The signal light is converted into a two-dimensional light / dark pattern. The SLM 2 is roughly classified into a transmission type using liquid crystal and ferroelectric liquid crystal, a reflection type represented by a liquid crystal on silicon (LCOS) and a MEMS type (DMD) using a micromirror, and a magneto-optic effect. There are MO type SLMs, and any of them can be used for this apparatus.

  The reflecting plate 5 may be made of any material that reflects the recording light. Examples of the reflecting plate 5 include a thin film made of an Al alloy or an Ag alloy, a so-called dielectric mirror formed by laminating dielectrics, and the like. . Further, as the configuration position of the reflection plate 5, it is necessary to reflect the light transmitted through the hologram recording medium 4 and pass again at least a part of the region through which the incident light modulated by the SLM 2 passes in the hologram recording medium 4. As a result, the incident light and the reflected light interfere in the hologram recording medium 4, and interference fringes are recorded as recording information in the hologram recording medium 4. The reflector 5 may be configured to be detachable from the hologram recording medium 4 or may be configured as a reflective film on the same substrate as the hologram recording medium 4.

  The lens 6 includes a plurality of lenses and at least two lenses, and is configured such that an image obtained by the SLM 2 is formed on the recording medium surface. An example of an optical system in the case where two convex lenses L1 and L2 are used as the lens 6 is shown in FIG. When the focal lengths of the convex lenses L1 and L2 are f1 and f2, respectively, the SLM2, the convex lenses L1 and L2, and the hologram recording medium 4 are arranged so as to maintain the relationship shown in FIG. 4 can be imaged. At this time, if the convex lenses L1 and L2 are selected so that f1> f2, the image on the surface of the hologram recording medium 4 can be made smaller than the image of the SLM2, which is advantageous for high recording density and large capacity. . The lens shown in FIG. 2 is the minimum two lenses that constitute the present invention. However, in order to correct various aberrations, more lenses can be combined.

  Next, a more specific configuration for realizing this configuration will be described. The configuration shown in FIG. 3 is an example. Here, it is assumed that a reflective liquid crystal element represented by LCOS is used as the SLM 2, and a beam expander 7 and a polarization beam splitter (PBS) 8 are inserted between the light source 1 and the SLM 2. The beam expander 7 plays the role of expanding the light emitted from the light source 1 to a necessary beam diameter and making it parallel light, and is usually composed of a plurality of lenses. If a spatial filter is formed by inserting a pinhole between lenses constituting the beam expander 7, light having a uniform wavefront can be obtained, which is convenient for hologram recording. PBS8 plays the role which isolate | separates the incident light and reflected light to SLM2. Of the light from the beam expander 7, the S-polarized light (polarized light in the direction perpendicular to the paper surface) component is reflected and reaches the SLM2. When the light is reflected by the SLM 2, the polarization is rotated by 90 ° due to the action of the liquid crystal and changes to the vertical direction of the paper, that is, the P-polarized light, so that the reflected light from the SLM 2 passes through the PBS 8 and reaches the lens 6.

  The above are the necessary elements for configuring this device, but if necessary, lenses and mirrors are inserted as necessary to adjust the optical path in the device, and a shutter is inserted to turn light on and off. May be. In addition, although the configuration necessary for recording has been described above, elements that are necessary for reproduction, for example, those having both recording and reproduction functions including an image sensor such as a CCD array and a C-MOS sensor may be provided.

  In addition, the hologram recording medium 4 used for recording / reproduction using the present apparatus is provided with at least a material that can be recorded as interference fringes when light of the same wavelength is irradiated from two directions to cause interference. Specific materials include materials such as a photopolymer system, a photorefractive crystal system, a chalcogenite glass system, and a photochromic system. As types of interference fringes, there are an amplitude modulation type composed of a light and dark distribution and a phase modulation type composed of a change in refractive index.

  FIG. 4 shows details of the recording unit in holographic recording. The output light from the SLM 2 is converted into a recording light beam in which a bright portion and a dark portion are distributed according to a recording signal (for example, a bright portion is “1” and a dark portion is “0”), and a plurality of lenses 6 (FIG. 3). To the hologram recording medium 4. Further, the recording light that has passed through the hologram recording medium 4 is reflected by the reflecting plate 5 and reaches the hologram recording medium 4 again. The hologram recording medium 4 is irradiated with interference light by incident light and reflected light only on the portion “1”, and is recorded on the hologram recording medium 4 as interference fringes. In FIG. 4, only one direction of light and dark (“1” and “0”) is shown for the sake of easy understanding, but it is actually a two-dimensional light and dark pattern. At this time, since no light is irradiated to the “0” portion, there is no risk that the recording medium is exposed to the reference light unlike the conventional holographic recording, and the S / N during reproduction as described above. And the multiplicity of recording can be increased, and the recording capacity can be increased.

  The mechanism for holding the hologram recording medium 4 has a mechanism for mounting the hologram recording medium 4 and moving the relative position to the recording light beam to a predetermined position to be recorded. The moving means includes a method of mounting the hologram recording medium 4 on the XY stage and moving the hologram recording medium 4 to a predetermined (x, y) coordinate, a method of rotating the hologram recording medium 4 by mounting it on a rotating part coaxial with the motor, A method of moving the beam of recording light to a predetermined location on the hologram recording medium 4 and a method of moving them in combination are used.

Next, an example of a procedure for reproducing the signal recorded by the above procedure will be described with reference to FIG.
Step S11: The relative position between the hologram recording medium 4 and the beam of reproduction light is moved to the same position as when recording. The moving means includes a method of mounting the hologram recording medium 4 on an XY stage and moving the hologram recording medium 4 to a predetermined (x, y) coordinate, a method of rotating the hologram recording medium 4 on a rotating portion coaxial with the motor, A method of moving the beam of recording light to a predetermined location on the hologram recording medium 4 and a method of moving them in combination are used.

Step S12: The hologram recording medium 4 is irradiated with the scattered light emitted from the light source 1 under the same conditions as when recording. However, at this time, the SLM 2 used in the recording process is not used and is not inserted. Further, it is preferable to remove the reflector 5 in order to obtain a clear reproduction pattern. The power of the irradiated light can be selected to a value convenient for reproduction. As shown in FIG. 5, since the interference fringes corresponding to the recording light are already formed on the hologram recording medium 4, the light that has passed through the hologram recording medium 4 in step S12 is only at the portion where the interference fringes are formed. Diffracted light is generated. That is, the diffracted light obtained here is light reproduced in the same pattern as the recording light in the recording process. By detecting this with an image sensor such as a CCD or C-MOS sensor, it is reproduced as the same electrical signal as the recording signal.
As described above, steps S11 and S12 are repeated for each page to reproduce the recorded signal.

It is a figure which shows the basic composition of the holographic recording device which concerns on this invention. FIG. 2 is an explanatory diagram showing a positional relationship among an SLM, a lens, and a medium in the holographic recording apparatus of FIG. It is a block diagram which shows one Embodiment of the holographic recording device which concerns on this invention. It is explanatory drawing which shows the recording process of the holographic recording medium based on this invention. It is explanatory drawing which shows the reproduction | regeneration process of the holographic recording medium based on this invention. It is a basic block diagram which shows the conventional holographic recording device. It is a block diagram which shows the specific structure of the holographic recording device of FIG. It is explanatory drawing which shows the positional relationship of the collector of FIG. 7, and a recording medium.

Explanation of symbols

1 Light Source 2 Spatial Light Modulator (SLM)
3 Diffraction grating 4 Hologram recording medium 5 Reflector 6 Lens 7 Beam expander 8 Polarizing beam splitter (PBS)
L1, L2 Convex lens

Claims (1)

A light source;
A spatial light modulator that irradiates the light-transmitting hologram recording medium surface by modulating the light emitted from the light source so as to become a recording light beam of a light and dark pattern according to a recording signal;
A plurality of lenses arranged in an optical path between the spatial light modulator and the hologram recording medium so as to form an image on the hologram recording medium of the bright and dark pattern of the recording light beam modulated by the spatial light modulator When,
The recording light beam disposed on the back surface side of the front surface of the hologram recording medium and reflecting the recording light beam transmitted through the hologram recording medium toward the hologram recording medium side, and incident light of the recording light beam in the hologram recording medium A reflecting member that generates interference fringes due to reflected light and records the interference fringes as recording information on the hologram recording medium;
A holographic recording device.

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