JP2002304109A - Hologram recording and reproducing method, and hologram recording and reproducing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hologram recording and reproducing method, and recording and reproducing device capable of recording interference patterns at a high density to a volume holographic memory. SOLUTION: This hologram recording and reproducing method and device include a recording step of approximately irradiating the principal surface of a recording medium 10 perpendicularly, with a recording reference light beam 12b inside the recording medium consisting of a photorefractive crystal of a uniaxial crystal having a parallel flat plate shape; intersecting a signal light beam 12a and the recording reference light beam with each other within the recording medium; forming regions of approximately perpendicularly extending refractive index gratings of a columnar shape, at the principal surface of the recording medium 10; and disposing the regions of the refractive index gratings adjacent to each other in parallel and a reproducing step of reflecting the recording reference light beam near the beam waist of the recording reference light beam irradiated approximately perpendicularly to the principal surface of the recording medium to return the beam approximately perpendicular to the principal surface of the recording medium and irradiating the refractive index gratings of the recording medium with a reproducing reference light beam which is coaxial with the recording reference light beam and propagates in an opposite direction to generate phase conjugate waves to the recording signal light beam.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a holographic recording medium made of a photorefractive material, a so-called holographic memory, and more particularly to a holographic recording / reproducing method using a holographic memory and an optical information recording / reproducing apparatus.

[0002]

2. Description of the Related Art A volume holographic recording system is known as a digital information recording system using the principle of hologram. A feature of this system is that an information signal is recorded on a recording medium as a change in refractive index.
For the recording medium, a photorefractive material such as lithium niobate single crystal is used.

One of the conventional hologram recording / reproducing methods is a method of recording / reproducing using Fourier transform. As shown in FIG. 1, in a conventional 4f-system hologram recording / reproducing apparatus, a laser beam 12 emitted from a laser light source 11
The beam splitter 13 splits the beam into a signal beam 12a and a recording reference beam 12b. The signal light 12a has a beam diameter expanded by the beam expander BX, and becomes a parallel light.
The light is applied to a spatial light converter SLM such as a panel of a transmission type TFT liquid crystal display (LCD). Spatial light converter SLM
Receives recording data converted by an encoder as an electric signal and forms a bright and dark dot pattern on a plane. When the signal light 12a passes through the spatial light converter SLM, it is optically modulated and includes a data signal component. The signal light 12 a containing the dot pattern signal component passes through the Fourier transform lens 16 separated by the focal length f, and the dot pattern signal component is Fourier-transformed and condensed on the recording medium 10. On the other hand, the recording reference beam 12b split by the beam splitter 13 is
9, the light path is guided into the recording medium 10 and crosses the optical path of the signal light 12a inside the recording medium 10 to form an optical interference pattern, and the entire optical interference pattern is recorded as a change in refractive index (refractive index grating). .

As described above, the diffracted light from the information data illuminated by the coherent parallel light is imaged by the Fourier transform lens, and the distribution on the focal plane, that is, the Fourier plane is converted to the coherent distribution. The interference fringes are recorded on a recording medium in the vicinity of the focal point by causing interference with the reference light. When the recording of the first page is completed, the rotating mirror is rotated by a predetermined amount, and the position thereof is moved in parallel by a predetermined amount to change the incident angle of the recording reference beam 12b with respect to the recording medium 10, and the second page is processed in the same procedure. Record. Angle multiplex recording is performed by sequentially recording in this manner.

On the other hand, at the time of reproduction, an inverse Fourier transform is performed to reproduce a dot pattern image. In information reproduction, as shown in FIG. 1, for example, the optical path of the signal light 12a is cut off by the spatial light converter SLM, and only the recording reference light 12b is irradiated onto the recording medium 10. At the time of reproduction, the position and angle of the mirror are controlled by changing the rotation and the linear movement of the mirror in combination so that the incident angle becomes the same as the recording reference light when the page to be reproduced is recorded. On the opposite side of the recording medium 10 irradiated with the recording reference light 12b, reproduced light that reproduces the recorded optical interference pattern appears. The reproduced light is guided to an inverse Fourier transform lens 16a, and a dot pattern signal can be reproduced by performing an inverse Fourier transform. further,
When this dot pattern signal is received by a photodetector 20 such as a charge-coupled device (CCD) at the focal length position, reconverted into an electrical digital data signal, and sent to a decoder, the original data is reproduced.

As described above, as shown in FIG. 2, conventionally, in order to record information at a high density in a certain volume in a recording medium, an angle multiplexing or a wavelength multiplexing is used in a volume of several mm square using a volume multiplexing. Multiple recording was performed. For this reason, in order to secure the angle selectivity and the wavelength selectivity, the coherence length between the signal light and the reference light is set to be long and wide. For this reason, the intensity per light unit used for recording is reduced, so that a recording medium having high sensitivity is required. Since high-density recording requires multiplex recording, multiplexing with a large erasing time constant is required. Generally, a recording medium having a large erasing time constant has a low recording sensitivity and limits the recording speed as a system.

Further, the conventional apparatus requires two high-performance Fourier transform lenses and an inverse Fourier transform lens, and further requires a high-precision paging control mechanism to control the reference light in recording and reproduction. There is a problem that it is disadvantageous for miniaturization of the device. Conventionally, since multiplex recording is performed at one location, a change in the refractive index of the recording medium due to multiplex recording is added, and the wavefront of the reference light after passing through the medium gradually changes. In order to perform phase conjugate reproduction as light, the number of multiplexing was limited. At the time of multiplex recording, it is necessary to provide a light-shielding means for the reflected light in order to eliminate the effects of the formation of the diffraction grating in the direction where the recording sensitivity is small.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a hologram recording / reproducing method and a hologram recording / reproducing apparatus for a hologram recording medium capable of performing higher density recording and miniaturization.

[0009]

According to the present invention, there is provided a hologram recording / reproducing method, wherein a coherent light of a first wavelength modulated according to information data is recorded in a recording medium comprising a uniaxial photorefractive crystal having a parallel plate shape. A hologram recording / reproducing method for forming a plurality of regions of a refractive index grating corresponding to a part of a three-dimensional optical interference pattern of a coherent signal light beam and a coherent reference light, wherein the recording reference light beam is converged; The recording medium is disposed near the beam waist and the upstream side thereof, and the recording reference light beam is irradiated substantially perpendicularly to the main surface of the recording medium, and the signal light beam and the recording reference light are radiated inside the recording medium. Crossing the beams to form a columnar refractive index region that extends substantially perpendicular to the main surface of the recording medium, and forming a region adjacent to the refractive index region adjacent to the recording step; The recording reference light beam is reflected in the vicinity of the beam waist of the recording reference light beam irradiated substantially perpendicular to the main surface of the recording medium, returned substantially perpendicular to the main surface of the recording medium, and shared with the recording reference light beam. Irradiating a reproduction reference light beam propagating in the opposite direction with respect to the axis to the refractive index grating of the recording medium to generate a phase conjugate wave with respect to the signal light beam.

In the hologram recording / reproducing method according to the present invention, in the recording step, the signal light beam and the recording reference light beam pass through a portion of the recording medium where they intersect and define a volume smaller than the volume of the portion. As described above, the recording medium is irradiated with a gate light beam having a second wavelength for expressing the recording sensitivity of the recording medium coaxially with the recording reference light beam.

In the hologram recording / reproducing method according to the present invention, the recording medium has an optical crystal axis substantially parallel or substantially perpendicular to a main surface thereof. In the hologram recording / reproducing method of the present invention, the irradiation is performed such that the cross-sectional area of the gate light beam or the recording reference light beam on the surface of the recording medium is smaller than the cross-sectional area of the signal light beam. It is characterized by doing.

In a hologram recording / reproducing method according to the present invention, each of the columnar refractive index grating regions is formed in a substantially columnar shape. In the hologram recording / reproducing method according to the present invention, the maximum inner diameter of the region of the refractive index grating is smaller than the distance between the peaks of the 0th-order and 1st-order diffracted light in the light intensity distribution of the signal light beam.

A hologram recording / reproducing apparatus according to the present invention is capable of transmitting a coherent signal light beam having a first wavelength modulated according to information data in a recording medium comprising a uniaxial crystal photorefractive crystal having a parallel plate shape. A hologram recording / reproducing apparatus for forming a plurality of regions of a refractive index grating corresponding to a part of a three-dimensional light interference pattern of coherent reference light, wherein a recording medium made of a photorefractive crystal having a parallel plate shape is freely mountable. A reference light means for converging and entering a coherent recording reference light beam of a first wavelength substantially perpendicular to the main surface of the recording medium; and a modulating means for modulating according to information data of one screen. The first coherent signal light beam having the first wavelength is incident on the recording medium, intersects the recording reference light beam inside the recording medium, and intersects the signal light beam and the recording reference light beam. A signal light means for generating a light interference pattern of the recording medium; and a converging beam light beam of the second wavelength for expressing the recording sensitivity of the recording medium concentrically with the recording reference light beam and substantially perpendicular to the main surface of the recording medium. When incident, the signal light beam and the recording reference light beam pass through a portion of the recording medium where they intersect and form a refractive index grating of a part of the light interference pattern that defines a volume smaller than the volume of the portion. And a plane mirror disposed near a beam waist of the recording reference light substantially perpendicularly incident on the main surface of the recording medium, wherein the plane mirror is coaxial with the recording reference light beam in a direction opposite to the recording reference light beam. Phase conjugate wave generating means for irradiating the refractive index grating of the recording medium with a reproduction reference light beam propagating to the recording medium to generate a phase conjugate wave with respect to the signal light beam; Characterized in that it has a separating means for separating from the optical path of the over arm, and a detection means for detecting information data imaged by the phase conjugate wave.

In the hologram recording / reproducing apparatus according to the present invention, the support means may form the recording medium such that the area of the refractive index grating has a columnar shape, and the areas of the refractive index grating are arranged adjacent to each other. It is characterized by having a moving mechanism for moving. In the hologram recording / reproducing apparatus according to the present invention, the recording medium has an optical crystal axis substantially parallel or substantially perpendicular to a main surface thereof.

In the hologram recording / reproducing apparatus of the present invention, the irradiation is performed such that the cross-sectional area of the gate light beam on the surface of the recording medium is smaller than the cross-sectional area of the signal light beam. Features.

[0016]

Embodiments of the present invention will be described below with reference to the accompanying drawings. One of the methods for reducing the size of the hologram memory system is a reproduction method using a phase conjugate wave. In order to realize the reproducing method using the phase conjugate wave, a reference light at the time of recording (hereinafter referred to as reproduction reference light) which is phase conjugate with respect to a reference light at the time of recording (hereinafter referred to as recording reference light) is used. That is, in the phase conjugate wave reproducing method, the method at the time of recording is the same as the conventional method, but by using the phase conjugate reproduction reference light having a symmetric property facing the reference light at the time of recording at the time of reproduction, the signal light is reproduced. The phase conjugate light is generated in the direction in which the signal light is incident, and the Fourier transform lens can also serve as the inverse Fourier transform lens.In addition, the phase of the wavefront is changed by passing the phase conjugate light through the recording medium or the lens. Even if it is disturbed, when it passes through the recording medium or lens again in reverse, the phase disorder is compensated and the property of phase conjugate light returns to the original state, making it a simple lens without requiring high performance of the lens can do. Therefore, this is a very effective recording / reproducing method for miniaturization.

As described above, in the phase conjugate reproduction method, it is necessary to make the recording reference light and the reproduction reference light symmetrical. Generally, plane waves which are symmetrically opposed to each other are used. It is realized by reflecting. In the present embodiment, by condensing the recording light (recording reference light and signal light) on a minute area, the light power density is increased even with a small amount of light to achieve high-speed recording. The reference light is vertically incident on the main surface of the recording medium, and a plane mirror is arranged at the beam waist thereof to reflect the reference light vertically. The signal light is incident on the reference light at an acute angle. The gate light is shaped into a beam coaxially with the reference light and by the same condensing method as the reference light, and is incident on the medium.

In the case where the light diameter has a certain size, even if the reference light is formed by restricting a part of the parallel light with a lens or a diaphragm, the influence of the diffraction phenomenon is small and the change rate of the light diameter is hardly changed. A columnar sector can be formed, and the reference light is vertically reflected by a plane mirror at an arbitrary position and opposed to each other, whereby the wavefronts of the reproduction reference light and the recording reference light can be made the same. However, in the case of recording in a minute area (about several tens of μm), a cylindrical minute sector cannot be formed due to a diffraction phenomenon due to a restriction by a stop (pinhole). Therefore, in the present embodiment, a substantially cylindrical sector is used for reference light close to a plane wave which is slowly focused by a lens having a long focal length, or a truncated conical recording sector is used for reference light of spherical wave. At this time, in order to obtain the reproduction reference light by using the reflection of the recording reference light, a plane mirror is arranged at the beam waist position of the recording reference light, and the recording reference light is reflected and opposed to the recording mirror so that it is similar to the recording reference light. The reproduction reference light can be obtained, and the wavefronts of both can be made the same.

As shown in FIG. 3, a light source 11 for generating signal light and reference light is provided with a first long wavelength, for example, a wavelength 8.
A 50 nm DBR (Distributed Bragg Reflector) laser is used as a near-infrared laser light source. A laser beam 12 emitted from a laser light source 11 is split by a beam splitter 13 into a signal beam 12a and a recording reference beam 12b. On the optical path of the signal light 12a, there are a shutter SHs, a mirror 14, a beam expander BX, and a spatial light converter SL.
M, a beam splitter 15, a Fourier transform lens 16, and a phase modulation plate 17 are arranged. On the other hand, a shutter SHr and a condenser lens Lr are provided between mirrors 18 and 19 on the optical path of the recording reference beam 12b split by the beam splitter 13. At the time of recording, the signal light 1
2a and the recording reference beam 12b follow different optical paths,
The light is irradiated to the intersection position P in the recording medium 10 having a parallel plate shape. That is, when the shutters SHs and SHr are opened,
The beam of the signal light 12a is obliquely incident on the main surface of the recording medium at a predetermined incident angle θ by the mirror 14, and the beam of the recording reference beam 12b is irradiated substantially perpendicular to the main surface of the recording medium by the mirror 19, and interferes in the recording medium 10. .

As the recording medium 10, for example, terbium (T
When lithium niobate having a stoichiometric composition containing 100 ppm of b) and 5 ppm of iron (Fe) is used, it has no sensitivity to near-infrared light as it is. Is expressed. Among the intersections of the signal light and the reference light, the recording sensitivity is developed only in the recording medium in the portion where the gate light passes, so when the gate light is condensed by the condenser lens and incident on the recording medium, for example, a columnar shape The hologram of the passing part is recorded. Furthermore, by narrowing down the recording reference light to the same beam diameter as the gate light, the density of the recording light can be increased, and high-speed recording can be performed even with a recording medium with low recording sensitivity or a light source with low output. As described above, a photorefractive material such as LiNbO _{3 to which} Tb is added is usually colorless and transparent. However, irradiation with ultraviolet light or the like causes visible light absorption to occur and the irradiated portion is colored,
At this time, when the colored portion is irradiated with visible light to the material, induced absorption (recording sensitivity) occurs in the near-infrared region. On the other hand, when the material is not irradiated with ultraviolet light, the recording sensitivity to near-infrared light is extremely reduced. Is used as gate light, and hologram recording is performed using near-infrared light as signal light and reference light. Therefore, this recording form is called a two-color hologram.

The light source 21 for generating the gate light has a second wavelength, for example, 325n of the second wavelength in the ultraviolet or short wavelength visible light band.
m He-Cd laser is used. Light source 2 for gate light generation
Reference numeral 1 denotes a light source that exhibits light-induced absorption of the recording medium 10 by the irradiation light, that is, has a sufficient power for coloring. When the shutter SHg is opened at the time of recording, the gate light 22 emitted from the gate light generation
The light is reflected by the half mirror 23 through the Hg and the condenser lens Lg, and is irradiated on the main surface of the recording medium 10 almost perpendicularly. Therefore, both the recording reference beam and the gate beam are irradiated coaxially. The condenser lens Lg converges the gate light beam 22 and irradiates the spot with a reduced beam diameter so as to form a columnar passage area at the intersection P inside the recording medium 10. Therefore, the recording medium 10 is arranged near the beam waist of the gate light 22.
The shutters SHs, SHr, and SHg are provided by the light beam 12.
a, 12b and 22 are provided for opening and closing the optical paths. The opening and closing of each of the shutters is driven via a driver by a signal sent by the controller 32, respectively.

As shown in FIG. 3, on the optical path of the recording reference beam 12b, a plane mirror 45 that is rotatable on the opposite side of the recording medium 10 and in close proximity to the recording medium 10 is provided. Plane mirror 45
Is used to generate a phase conjugate wave during reproduction. At the time of reproduction, the recording reference beam 12b is fixed so as to face the recording reference beam 12b that has passed through the recording medium 10, that is, the recording reference beam 12b is vertically incident. Therefore, by placing the plane mirror 45 at the position of the beam waist of the recording reference beam 12b, phase conjugate reproduction is performed from the recorded hologram. At the time of reproduction, when the recording reference light 12b is irradiated, the recording light passes through the recording medium 10, and the reference light is specularly reflected by the plane mirror 45 to become the reproduction reference light 12c. Phase conjugate light). On the other hand, at the time of recording, in order to prevent stray light, the plane mirror 45 is rotated under the control of the controller 32, and the recording reference beam 12b is fixed so as to be guided to a light absorber 50 such as a charcoal plate close to a perfect black body, for example. On the optical path of the signal light 12a, in order to reduce the stray light of the signal light, the light transmitted through the recording medium is condensed by a lens so as to have a predetermined beam diameter, and then the light absorber 50 or a similar type separately installed. To the light absorber.

On the optical path of the signal light 12a inclined with respect to the recording medium, the beam expander BX enlarges the beam diameter of the signal light 12a passing through the shutter SHs,
The signal light 12a is radiated as parallel rays so as to be perpendicularly incident on the spatial light converter SLM. The spatial light converter SLM receives a unit page sequence of electrical data corresponding to the two-dimensional plane page received from the encoder 25, and displays a light and dark dot matrix signal. The signal light 12a is optically modulated when passing through the spatial light converter SLM, and includes data as a dot matrix component. Furthermore, Fourier transform lens 1
Numeral 6 Fourier-transforms the dot matrix component of the signal light 12a transmitted through the beam splitter 15, and collects the light so as to be focused behind the position P of the recording medium 10. The beam splitter 15 supplies a phase conjugate wave, which will be described later, to the CCD 20, which is a light receiving device. Spatial light converter SLM
And the CCD 20 are arranged at the focal length of the Fourier transform lens 16. By arranging the phase modulation plate 17 between the spatial light modulator and the recording medium, it is possible to alleviate the focusing of the zero-order light to one point by the Fourier transform lens. Imbalances can be alleviated.

Further, an image pickup device, for example, a CCD 20 of a photodetector array, which branches off from the beam splitter 15, is arranged. The beam splitter 15 converts the phase conjugate wave to CCD
20. A decoder 26 is connected to the CCD 20. The decoder 26 is connected to the controller 32. In addition, the controller 32 attaches a mark corresponding to the type of the photorefractive crystal to the recording medium 10 in advance, and when the recording medium 10 is mounted on the movable stage 30 which is a support means for moving the same, an appropriate The marker can be automatically read by a sensor to control the movement of the recording medium.

Next, the procedure of the two-color hologram recording process will be described. First, as shown in FIG. 4, the plane mirror 45 is rotated to the stray light preventing state, and the recording reference light 12b and the gate light 22 are turned off.
Is fixed so that it does not reflect regularly. Next, the position of the n (n = 1 or 2) axis movable stage 60 holding the recording medium 10 is controlled by the controller 32, and the target recording medium 10 is moved to a predetermined position. Next, the recording signal is transmitted from the encoder 25 to the spatial light modulator SLM, and a predetermined pattern is displayed. Next, the shutter SHg is opened, the recorded portion is erased by the pre-irradiation of the gate light, and then the shutters SHs and SHr are opened together with the irradiation of the gate light to start recording. Next, recording is performed for a predetermined time controlled by the controller 32, that is, interference occurs. Next, the shutter S
Hg, SHr, SHs are closed. Next, the plane mirror 45 is rotated back to the regular reflection state and fixed. After the recording is completed, an optical interference pattern of the reference light and the signal light is formed in the recording medium 10, and information is recorded as a change in the refractive index by partially exciting the medium with the gate light. When the recording of the first page is completed, the recording medium 10 is moved by a predetermined amount to change the position of the recording reference beam 12b with respect to the recording medium 10, and the second page is recorded in the same procedure. Recording is performed by sequentially performing recording in this manner.

FIG. 5 shows an example of the incident state of the gate light, the recording reference light and the signal light on the recording medium. Each of the substantially cylindrical refractive index grating regions RG extends substantially perpendicular to the main surface of the hologram recording medium 10. The gate light or recording reference light and the gate light having a radius r on the main surface of the recording medium and the signal light having a radius rs on the main surface which are substantially perpendicularly incident are, for example, r
= 30 μm and rs = 100 μm so that rs> r. In order to record information in a substantially cylindrical shape, it is preferable that the recording medium 10 has a sufficient length with respect to the thickness T, for example, T / r> 2.

Further, consider the case where the refractive index change regions of the recording reference light and the signal light are limited by the gate light.
As shown in FIG. 6, the signal light that has been Fourier-transformed by the spatial light modulator in the holographic recording of the Fourier transform has 1 signal due to repetition of the pixels (pitch a) of the spatial light modulator.
The next-order diffracted light becomes the highest frequency component. When recording is performed in the recording medium 10 by causing the signal light and the reference light to interfere with each other, a spatial frequency spectrum distribution light intensity is generated on the Fourier plane.

Using the spatial frequency (1 / a) of the recording surface, the light wavelength (λ), and the focal length (f) of the Fourier transform lens, the distance (d1) between the zero-order and the first-order Fourier spectrum on the Fourier surface Can be expressed as d1 = (1 / a) · (λ) · (f). FIG. 6 shows the intensity distribution of the Fourier transform image. Assuming that the recording medium has a refractive index of 2 and a thickness of 3 mm, the optical system of the embodiment apparatus is a TFT liquid crystal of 1000 × 1.
When the pixel pitch is 10 μm, the signal light wavelength is 530 nm, and the focal length is 14 mm in the 000-light spatial light modulator, the Fourier spectrum interval (d1) corresponding to this is approximately 750 μm according to the above equation. Therefore, most of the information of the signal light exists in a range of about ± 750 μm from the optical axis. As shown in FIG. 6, the two-dimensional light appearing on the optical spatial modulator in the cross-shaped space formed by the first-order diffracted light and the zero-order light is shown.
This means that the dimensional data is distributed.

For this reason, most of the fundamental wave components important for recording are concentrated around the zero-order light, so that information on the harmonic components near the first-order diffracted light of the pixel becomes relatively insignificant. Therefore, in the present invention, the diameter (d2) of the gate light is set so that d1> d2 with respect to the Fourier spectrum interval (d1) even if the Fourier spectra of adjacent signal lights overlap each other during recording. Further, the areas to be recorded which are limited by the gate light are set so as to be adjacent to each other without overlapping, that is, the gate light pitch PG is set so that PG ≧ d2. By periodically spatially arranging in this manner, information can be multiplex-recorded by closest packing.

Thus, in the present invention, a parallel plate-shaped uniaxial photorefractive crystal having a plurality of refractive index grating regions corresponding to portions defined by the gate light in the interference pattern between the signal light and the recording reference light. In the hologram recording medium, each of the refractive index grating regions has a columnar shape, and the refractive index grating regions are adjacently arranged side by side. The beam of the signal light having an incident angle of substantially zero and the recording reference light having an incident angle θ shown in FIG. 3 generates interference fringes at an angle of θ / 2 with respect to the coaxial optical axis. In the photorefractive crystal, the recording sensitivity is high in the direction in which the direction of the generated diffraction grating is perpendicular to the optical crystal axis.
Is preferable, but if the incident angle θ of the signal light is reduced, the direction of the generated diffraction grating is close to the main surface, and the hologram recording medium is easy to process. The optical crystal axis may be provided substantially parallel to the main surface.

In FIG. 3, the optical crystal axis of the recording medium is substantially parallel to the direction of the main surface of the parallel flat plate. During recording, a diffraction grating is formed in the direction of the bisector of the angle between the recording reference light and the signal light. At this time, the recording sensitivity can be maximized when the direction of the diffraction grating is orthogonal to the optical crystal axis of the recording medium.
Sensitivity decreases trigonometrically as it deviates from orthogonal. In other words, the diffraction grating formed by the recording reference light and the signal light is close to the direction orthogonal to the optical crystal axis, so that a transmission hologram is easily recorded. However, the diffraction grating formed by the reference light and the signal light reflected by the reflecting mirror ( Reflection hologram)
Is nearly parallel to the optical axis of the recording medium, and the recording sensitivity is extremely small, making it difficult to record. At the time of reproduction, light reflected by the reflection mirror acts as reproduction reference light, and diffracted light (reproduction conjugate light) is obtained from the medium. Naturally, a normal hologram is also reproduced by the incident light, but since the diffraction direction is different, it does not affect the phase conjugate reproduction.

Next, the procedure of the two-color hologram reproduction process will be described. First, as shown in FIG. 7, the position of the n-axis movable stage 60 holding the recording medium 10 is controlled by the controller 32, and the target recording medium 10 is moved to a predetermined position. Next, only the shutter SHr is opened to start reproduction. Next, reproduction is performed for a predetermined time controlled by the controller 32. Next, the shutter SHr is closed. Thus, the reproduction is completed.

As shown in FIG. 5, the plane mirror 45 is arranged such that its reflection surface is located at the beam waist of the recording reference beam 12b and the normal of the reflection surface coincides with the incident optical path of the recording reference beam 12b. ing. In the reproduction process, when the recording reference light 12b passes through the recording medium 10 and is specularly reflected by the plane mirror 45 to become the reproduction reference light 12c and re-enters the recording medium 10, diffracted light from the recording medium 10 (phase conjugate light as reproduction light) ) Is obtained, and phase conjugate reproduction can be achieved from the recorded refractive index grating. Since the light becomes a spherical wave diverging through a plane wave from a spherical wave converging at the beam waist, if a plane mirror is arranged at the beam waist position in parallel with the wavefront of the plane wave, the convergent spherical wave and the divergent spherical wave are similar to each other. In addition, phase conjugate light having the opposite traveling direction can be obtained on the same axis.

At the time of reproduction, a normal hologram is also reproduced by the recording reference beam 12b, but does not affect the system because the direction is not related to the reproduction detection system. Since the plane mirror 45 is disposed so as to face the recording reference beam 12b, the light that the recording reference beam 12b has transmitted through the recording medium 10 and reflected by the plane mirror 45 is used as the reproduction reference beam 12c. Each time, the recording reference beam 12b and the reproduction reference beam 12c pass through the same optical path. In the reproducing method using the phase conjugate wave, the recording reference beam 12b and the reproduction reference beam 12c need to have a symmetric property, and a symmetrical plane wave or spherical wave is used for both, but the reproduction reference beam 12c is always used. The light propagates in the opposite direction coaxially with the recording reference light beam 12b and is incident on the recording medium 10 to generate diffracted light (phase conjugate wave) from the cylindrical refractive index grating. The Fourier transform lens 16 guides the diffracted light to the beam splitter 15, receives the dot pattern signal by the light receiving device of the CCD 20, reconverts the signal into an electrical digital data signal, and sends the signal to a decoder. Is played.

In still another modification, if a １ ／ wavelength plate is disposed between the recording medium 10 and the plane mirror 45, the recording reference beam 12b passes through the 媒体 wavelength plate twice after passing through the recording medium 10. Thus, the polarization direction of the reproduction reference light 12c incident on the recording medium 10 can be changed by 90 degrees, so that the signal light 12a from the spatial light converter SLM and the CCD 2
By using a polarizing beam splitter in an optical system that separates diffracted light to zero (phase conjugate light as reproduction light),
All signal light is on the recording medium 10 and all diffracted light is CCD
A separation optical system can be configured to guide the light to the side, and the light amount can be used effectively.

In the above embodiment, hologram recording is used with a transmission type diffraction grating, but the same effect can be obtained by using a reflection type diffraction grating. At this time, recording and reproduction are performed while the plane mirror 45 of FIG. 3 which has been rotated to prevent stray light during recording is fixed. At this time, the optical crystal axis of the recording medium is substantially perpendicular to the main surface direction of the flat plate. Again,
At the time of recording, a diffraction grating is formed in the bisecting angle direction of the recording reference light and the signal light. When this direction is perpendicular to the optical axis of the recording medium, the recording sensitivity is maximized. Decrease. That is, a phenomenon opposite to that of the above-described embodiment occurs in the reference light for recording and reproduction, and the diffraction grating formed by the reference light and the signal light reflected by the reflection mirror is close to being orthogonal to the optical axis of the recording medium. A reflection type hologram is formed with recording sensitivity. Diffraction grating (transmission type hologram) composed of incident reference light and signal light
Is nearly parallel to the optical axis of the recording medium, and the recording sensitivity is very small, making it difficult to record. At the time of reproduction, the incident reference light acts as reproduction reference light for phase conjugate reproduction, and diffracted light (reproduction conjugate light) from the medium is obtained. Similarly, a normal hologram is also reproduced by reflected light, but does not affect phase conjugate reproduction because the diffraction direction is different.

In this embodiment, since the two-color hologram is recorded with the gate light having a reduced diameter, the recording portion formed on the recording medium has a needle-like elongated shape, so that spatial multiplex recording can be performed efficiently. Similarly, since the density of the recording light intensity can be improved by narrowing down the reference light at the time of recording, high-speed recording can be performed even with a recording medium having low recording sensitivity or a light source having low output.

In the above embodiment, since the gate light irradiation area at the time of recording is activated and becomes the recording area, the gate light forming the minute sector is formed while forming the gate light by the same condensing method as the recording reference light. Although the two-color hologram having no reproduction deterioration has been described, even in the case of a single-color hologram in which gate light irradiation is not performed at the time of recording, at least a recording reference beam forms a minute sector by the above-described condensing method and phase conjugate reproduction at the time of reproduction. By doing so, it is possible to achieve a small-sized hologram memory system capable of high-speed recording even with a recording medium having low recording sensitivity or a low output light source.

[0039]

According to the present invention, at the same time as the signal light beam and the recording reference light beam cross each other inside the recording medium, the signal light beam and the recording reference light beam pass through the portion of the recording medium where they intersect, and Since the gate light beam for expressing the recording sensitivity of the recording medium is applied to the recording medium so as to define a volume smaller than the volume of the site,
The recording area has a needle-like elongated shape, and spatial multiplex recording can be performed efficiently. Further, according to the present invention, recording and reproduction are performed on a substantially cylindrical minute sector by using the reference light and the gate light near the plane wave near the beam waist of the light beam that is slowly focused by the lens having a long focal length. Since the plane mirror is arranged at the beam waist position of the recording reference light, and the recording reference light is reflected and opposed to constitute the reproduction reference light to perform the phase conjugate reproduction, even without using a large light source, Focusing the recording light (recording reference light and signal light) on a very small area, shortening the recording time by increasing the optical power density, and effectively utilizing the recordable capacity of the recording medium by overlapping the columnar recording areas, High-speed recording and high recording density can be achieved. Further, by reflecting the reference light at the beam waist, phase conjugate reproduction can be realized in which the wavefronts of the reference light for recording and reproduction are similar, so that the recording / reproduction optical system is simplified and the memory system can be downsized. .

In the present invention, since the recording is not multiplexed in the same sector, the change in the refractive index of the medium due to recording is not added, and even if the light transmitted through the recording medium is used as the reference light, there is little difference in the wavefront from the incident light, which is advantageous for signal reproduction. It is. In addition, the effects of the formation of the diffraction grating in the direction in which the recording sensitivity is low on the recording medium are eliminated, and the reproduction is not significantly affected without the means for blocking the reflected light.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a conventional hologram recording system.

FIG. 2 is a schematic sectional view showing a hologram recording medium.

FIG. 3 is a schematic configuration diagram illustrating a hologram recording / reproducing apparatus according to the present invention.

FIG. 4 is a schematic diagram illustrating a recording process in the recording / reproducing apparatus according to the present invention.

FIG. 5 is a schematic perspective view showing a hologram recording medium according to an embodiment of the present invention.

FIG. 6 is a schematic diagram illustrating a recording process in the hologram recording / reproducing apparatus according to the present invention.

FIG. 7 is a schematic diagram illustrating a reproduction process in the hologram recording / reproduction device according to the present invention.

[Explanation of symbols]

 Reference Signs List 10 recording medium 11 laser light source 12a signal light 12b recording reference light 12c reproduction reference light 13, 15 beam splitter 16 Fourier transform lens 17 phase modulation plate 14, 18, 19, 45 mirror (flat mirror) 23 half mirror (flat mirror) 20 CCD, Photodetector 21 Light source for gate light generation 22 Gate light 25 Encoder 26 Decoder 32 Controller 50 Light absorber 60 Movable stage BX Beam expander SLM Spatial light converter SHs, SHr, SHg Shutter

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Asami Takano 6-1-1, Fujimi, Tsurugashima-shi, Saitama Pioneer Corporation General Research Laboratory F-term (reference) 2K008 AA04 AA08 BB00 BB04 CC00 CC01 DD23 FF08 FF17 FF21 HH01 HH03 HH13 HH14 HH24 HH26 HH28 5D090 AA10 BB16 CC01 CC04 DD03 EE01 EE11 FF13 FF21 FF31 KK10 KK13 KK14

Claims (10)

[Claims] 1. A three-dimensional coherent signal light beam and a coherent reference light beam of a first wavelength modulated according to information data in a recording medium made of a uniaxial crystal photorefractive crystal having a parallel plate shape. A hologram recording / reproducing method for forming a plurality of regions of a refractive index grating corresponding to a part of a typical optical interference pattern, wherein the recording reference light beam is converged, and the recording medium is disposed near a beam waist and an upstream side thereof. Disposed, irradiating the recording reference light beam substantially perpendicularly to the main surface of the recording medium, intersecting the signal light beam and the recording reference light beam inside the recording medium, and substantially irradiating the main surface of the recording medium. A recording step of forming a columnar refractive index region extending vertically and arranging the refractive index regions adjacent to each other in a juxtaposed manner; and referring to the recording irradiated substantially perpendicular to the main surface of the recording medium. The recording reference light beam is reflected near the beam waist of the beam, returned substantially perpendicularly to the main surface of the recording medium, and a reproduction reference light beam propagating in the opposite direction coaxially with the recording reference light beam of the recording medium. A hologram recording / reproducing method, comprising: irradiating the refractive index grating to generate a phase conjugate wave with respect to the signal light beam. 2. In the recording step, the recording of the recording medium is performed such that the signal light beam and the recording reference light beam pass through a portion of the recording medium that intersects and define a volume smaller than the volume of the portion. 2. The hologram recording / reproducing method according to claim 1, wherein a gate light beam having a second wavelength for expressing sensitivity is irradiated on the recording medium coaxially with the recording reference light beam. 3. The hologram recording / reproducing method according to claim 1, wherein the recording medium has an optical crystal axis substantially parallel or substantially perpendicular to a main surface thereof. 4. The irradiation is performed such that the cross-sectional area of the gate light beam or the recording reference light beam on the surface of the recording medium is smaller than the cross-sectional area of the signal light beam. The hologram recording / reproducing method according to claim 2 or 3, wherein 5. The hologram recording / reproducing method according to claim 2, wherein each of the columnar refractive index grating regions is formed in a substantially cylindrical shape. 6. The apparatus according to claim 1, wherein a maximum inner diameter of the region of the refractive index grating is smaller than a distance between peaks of 0th-order and 1st-order diffracted light in the light intensity distribution of the signal light beam.
The hologram recording / reproducing method according to any one of the above items. 7. A three-dimensional coherent signal light beam and a coherent reference light beam of a first wavelength modulated according to information data in a recording medium made of a uniaxial photorefractive crystal having a parallel plate shape. A hologram recording / reproducing apparatus for forming a plurality of regions of a refractive index grating corresponding to a part of a typical light interference pattern, wherein the supporting means holds a recording medium made of a photorefractive crystal having a parallel plate shape in a freely attachable manner, Reference light means for converging and entering a coherent recording reference light beam of a first wavelength substantially perpendicular to the main surface of the recording medium; and coherence of a first wavelength modulated according to information data of one screen. Signal light beam incident on the recording medium, crosses the recording reference light beam therein, and generates an optical interference pattern of the signal light beam and the recording reference light beam. Light means, a gate light beam having a second wavelength for expressing the recording sensitivity of the recording medium converging and incident substantially coaxially with the recording reference light beam and substantially perpendicular to the main surface of the recording medium; And a gate light means for forming a part of a refractive index grating of the light interference pattern which passes through a portion of the recording medium where the recording reference light beam intersects and defines a volume smaller than the volume of the portion, and the recording Including a plane mirror disposed near the beam waist of the recording reference light that has been substantially perpendicularly incident on the main surface of the medium, the plane mirror allows the reproduction reference light beam to propagate coaxially and in the opposite direction to the recording reference light beam. Phase conjugate wave generating means for irradiating the refractive index grating of the recording medium to generate a phase conjugate wave with respect to the signal light beam; and a separating means for separating the phase conjugate wave from the optical path of the signal light beam. When the hologram recording and reproducing apparatus characterized by having a detecting means for detecting information data imaged by the phase conjugate wave. 8. The support means includes a moving mechanism for moving the recording medium such that the refractive index region has a columnar shape and the refractive index region is arranged adjacent to and adjacent to the recording medium. The hologram recording / reproducing apparatus according to claim 7, wherein: 9. The hologram recording / reproducing apparatus according to claim 7, wherein said recording medium has an optical crystal axis substantially parallel or substantially perpendicular to a main surface thereof. 10. The method according to claim 7, wherein the irradiation is performed such that the cross-sectional area of the gate light beam on the surface of the recording medium is smaller than the cross-sectional area of the signal light beam. Hologram recording / reproducing device.