JP2000284672A - Method and device for optical recording - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a sufficient interference effect and excellently reproduce a data image of an object beam when recording a Fourier-transformed image of the object beam holding data information as a hologram. SOLUTION: A data image of an object beam is Fourier-transformed, and zero-order to, for instance, 5th order light components of the Fourier-converted image are transmitted through a light transparent part 21 of a shading body 20 to irradiate an optical recording medium therewith. The light transparent part 21 has a constant light transmittance in all the area. Parallel light of spatially constant intensity is transmitted through an intensity modulation filter 17 having such a spatial transmittance distribution as the transmittance is maximal at the center part and decreased as it goes down to the periphery, and the transmitted light is transmitted through the light transparent part 21 of the shading body 20, or directly emitted reference light onto the area of the optical recording medium irradiated with the zero-order to 5th order light of the Fourier transformed image and not through the light transparent part. Then, the center part where the reference light is at the maximum in the intensity is made to be superimposed on the zero-order light in which the Fourier-transformed image is at the maximum in the intensity. At the reproduction, reading light having the same wave front as the reference light in recording is irradiated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method and an apparatus for recording data information as a hologram.

[0002]

2. Description of the Related Art As a next-generation computer file memory, a hologram memory having both a large capacity derived from a three-dimensional recording area and a high speed derived from a two-dimensional batch recording / reproducing method has been receiving attention.

[0003] In the hologram memory, a plurality of data pages can be recorded in a multiplexed manner in the same volume, and data can be collectively read for each page. Instead of an analog image, the binary digital data “0, 1” is converted into a digital image as “bright, dark” and recorded and reproduced as a hologram, so that digital data can be recorded and reproduced. Recently, specific optical systems of this digital hologram memory system, evaluation of SN ratio and bit error rate based on volume multiplex recording, or proposal of two-dimensional encoding have been proposed, and the influence of aberration of the optical system has been proposed. Research from a more optical point of view is also progressing.

FIG. 7 shows a document “D. Psaltis, M .;
Levene, A .; Pu, G .; Barbastathi
s and K.S. Curtis; Opt. Lett. 2
0 (1995) 782 ", which is an example of the volume multiplex recording method.

In the shift multiplex recording method disclosed in this document, a hologram recording medium 91 is formed in a disk shape, and an object beam 93 obtained through a spatial light modulator 92 is transmitted to a lens 9.
4 and the hologram recording medium 91
Simultaneously, the hologram recording medium 91 is irradiated with the reference light 96 of the spherical wave obtained through the objective lens 95, and a plurality of holograms are overwritten in the same region by the rotation of the hologram recording medium 91. For example, assuming that the beam diameter is 1.5 mmφ, another hologram can be recorded in almost the same area without crosstalk by moving the hologram recording medium 91 by several tens of μm. This utilizes the fact that the reference light 96 is a spherical wave, which is equivalent to the change in the angle of the reference light 96 due to the movement of the hologram recording medium 91.

[0006] The moving distance of the spherical reference wave shift multiplex recording, that is, the distance δ at which the holograms can be separated independently from each other, is shown as δspherical = δBragg + δNA ≒ (λzo / (Ltan θs)) + Λ / (2NA) (1) Here, λ is the wavelength of the object light, zo is the distance between the objective lens forming the spherical reference wave and the recording medium, L is the film thickness of the recording medium, θs is the intersection angle between the object light and the spherical reference wave, N
A is the numerical aperture of the objective lens.

From equation (1), as the film thickness L of the recording medium increases, the shift amount δ decreases, the degree of multiplexing can be increased, and the recording capacity can be increased. In order to more effectively increase the recording capacity in shift multiplex recording, the recording area may be reduced. By performing multiplex recording in a minute area, higher-density volume multiplex recording can be realized.

[0008] Therefore, in the hologram memory system, the object light is subjected to Fourier transform by a lens and is applied to a recording medium. When the image of the object light has a fine pitch (high spatial frequency), the spread 物体 of the object light on the recording medium surface at this time is expressed as follows: ζ = kλfωx (2) k is a proportionality constant, λ is the wavelength of the object light, f is the focal length of the Fourier transform lens, and ωx is the spatial frequency of the object light. Therefore, if a lens having a small focal length f is used as a Fourier transform lens, the recording area can be miniaturized.

Further, it has been considered that the recording area is miniaturized by arranging an aperture in front of the recording medium and extracting and recording only necessary spatial frequency components of the object light after Fourier transform.

A data image to be recorded as a hologram is
For example, an image as shown in FIG. By making the white part in the figure represent data "1" and the black part representing data "0", binary two-dimensional digital data can be recorded for each page. The size of one pixel of d × d corresponds to 1-bit data.

When the object light of such an image is Fourier-transformed by a lens, the Fourier-transformed image after the conversion is, as shown in FIG. 2, centered on the zero-order light F0 (ωx = 0, ωy = 0). In each of the x-axis direction and the y-axis direction, the primary light F1, the secondary light F2, the tertiary light F3... are symmetrically spread in the plus direction and the minus direction, and the light intensity is shown in FIG. In FIG. 3, a component having a higher intensity indicates a larger point, and as shown by a curve P1 in FIG. 3, the 0th-order light F0 is significantly stronger, and the higher the order, the weaker it becomes.

[0012]

A hologram is formed by causing an object beam and a reference beam to interfere in a recording medium. The more the intensity of the object beam and the reference beam is equal, the greater the interference effect, and Diffraction efficiency increases. Conversely, when the intensity of the object light and the intensity of the reference light are extremely different, the interference effect is small, and the diffraction efficiency at the time of reproduction is low.

Therefore, when a Fourier transform image having different intensities of the following components is recorded as a hologram as shown in FIGS. 2 and 3, a uniform and sufficient interference effect can be obtained for each of the following components. And the diffraction efficiency is not uniform and sufficient.

For example, when the intensity of the reference light is the same as that of the zero-order light F0, the interference effect with respect to the primary and subsequent components, especially the higher-order components such as the secondary light F2 and the tertiary light F3 is reduced. However, the diffraction efficiency of the higher-order components is reduced, and the Fourier transform image contains much contour information in the higher-order components. Therefore, the reproduced image is an image in which the reproduction of the higher-order components is insufficient and the outline is blurred. Become.

Conversely, for example, when the reference light has the same intensity as that of the third-order light F3, the interference effect on the low-order components such as the zero-order light F0 and the first-order light F1 is reduced, and the low-order components are reduced. For, the diffraction efficiency decreases, and the Fourier transform image contains much light amount information in the low-order components, so that the reproduced image is an image with insufficient reproduction of the low-order components and an insufficient light amount. .

Therefore, as shown by a straight line P2 in FIG. 3, the reference light is generally set to an intermediate intensity between the zero-order light F0 and the higher-order light. However, even in this case, a uniform and sufficient interference effect cannot be obtained for each of the following components, and a uniform and sufficient diffraction efficiency cannot be obtained.

Therefore, according to the present invention, when a Fourier transform image of an object light holding data information is recorded as a hologram, a uniform and sufficient interference effect can be obtained for each of the four-dimensional components of the Fourier transform image, and the object light can be obtained. The data image can be reproduced well.

[0018]

According to an optical recording method of the present invention, a Fourier transform image of an object light holding data information is provided.
The optical recording medium is simultaneously irradiated with a reference light having a spatial intensity distribution where the intensity is high where the intensity of the Fourier transform image is high and the intensity where the intensity is low where the intensity is low. Record on a recording medium.

[0019]

According to the optical recording method of the present invention, the reference light component that interferes with the low-order component of the Fourier transform image having a high intensity interferes with the high-order component of the Fourier transform image that has a low intensity. Since the intensity of the reference light component is weak, one of the Fourier transform image and the reference light having the weaker intensity does not become buried in the other with the stronger intensity. A uniform and sufficient interference effect is obtained, and the data image of the object light is reproduced well.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS [One Example of Optical Recording Method] In order to record data at a high density, the area of one pixel of a data image as shown in FIG. 1
It is desirable to pack more bit data in a page. Thus, high-speed recording and reproduction can be realized in addition to high-density recording.

However, since the spatial frequencies ωx and ωy in the x and y axis directions of the data image are proportional to 1 / d, if the area of one pixel is reduced in this manner, the spatial frequencies ωx and ωy
Becomes larger, and the Fourier transform image of the data image as the object light spreads on the optical recording medium according to the equation (2).

Therefore, a light-shielding member having a light-transmitting portion formed partially is disposed in front of the optical recording medium, and a Fourier-transformed image of a data image is irradiated on the optical recording medium through the light-transmitting portion. It is desirable to miniaturize the recording area on the optical recording medium.

Since the image to be recorded (object light) is not an analog image but a binary data image, it is not necessary to record the components having spatial frequencies ωx and ωy of infinite as a Fourier transform image. It is sufficient to record up to the fifth order or higher order component.

Therefore, in an example of the optical recording method of the present invention described below, only the components from the 0th order light to the 5th order light of the Fourier transform image are recorded as a hologram on the optical recording medium.

FIG. 4 shows an example of the optical recording method of the present invention. In this method, the light transmitting portion 2 as shown in FIG.
1 is arranged in front of the optical recording medium,
The components from the 0th-order light to the 5th-order light of the Fourier transform image as shown in FIG. 2 are transmitted through the light transmitting section 21 and irradiated on the optical recording medium. The light transmitting section 21 has a constant light transmittance over the entire area.

On the other hand, in a circle corresponding to the light transmitting portion 21 as shown in FIG. 2B, there is a spatial transmittance distribution in which the transmittance is maximum at the center and becomes smaller toward the periphery. The intensity modulation filter 17 is spatially arranged on the optical path of parallel light having a constant intensity. The parallel light having the constant intensity is transmitted through the intensity modulation filter 17, and the transmitted light is used as reference light for optical recording. The medium is irradiated with an area of the medium to which components from the 0th order light to the 5th order light of the Fourier transform image are irradiated.

In this case, the reference light transmitted through the intensity modulation filter 17 is transmitted through the light transmitting portion 21 of the light shield 20 to irradiate the optical recording medium similarly to the Fourier transform image, or is transmitted through the light transmitting portion 21. And directly irradiate the optical recording medium. However, in any case, on the optical recording medium, the central portion where the intensity of the reference light is the highest overlaps with the zero-order light where the intensity of the Fourier transform image is the highest.

According to this, the reference light radiated to the optical recording medium has a solid line P1 as shown by a broken line P3 in FIG.
Has a spatial intensity distribution similar to the spatial intensity distribution of the Fourier transform image, that is, the intensity is strong for the component with the strong intensity of the Fourier transform image, and is strong for the component with the weak intensity of the Fourier transform image. The strength becomes weak.

Therefore, the lower intensity of one of the Fourier transform image and the reference light does not become buried in the higher intensity of the other, so that each of the four components of the Fourier transform image is uniform and sufficient. The interference effect can be obtained, and the data image of the object light can be favorably reproduced.

[Example of Optical Recording Apparatus] FIG. 5 shows an example of an optical recording apparatus according to the present invention, which also serves as an optical reproducing apparatus.

The coherent light from the light source 6 is divided into two lights by a beam splitter 12, and during recording, the shutter 1
5 is opened, the light transmitted through the beam splitter 12 is converted into parallel light having a large diameter by the lenses 10a and 10b, and is incident on the spatial light modulator 4.

A binary two-dimensional digital data image as shown in FIG. 1 is displayed on the spatial light modulator 4 by a computer not shown in FIG. Accordingly, the light that has passed through the spatial light modulator 4 is intensity-modulated in accordance with the value of each pixel of the digital data image, and becomes the object light 1 having information of the digital data image.

The object light 1 is Fourier-transformed by the lens 7, and the components from the 0th-order light to the 5th-order light of the Fourier-transformed image 1f as shown in FIG.
The light is transmitted through the light transmitting portion 21 of the light shield 20 shown in FIG.

At the same time, after the light reflected by the beam splitter 12 is reflected by the mirrors 13 and 14, FIG.
The reference light 2 having the spatial intensity distribution corresponding to the spatial intensity distribution of the Fourier transform image 1f is transmitted through the intensity modulation filter 17 shown in FIG.
Is transmitted through the light transmitting portion 21 of the light shield 20 to irradiate an area of the optical recording medium 5 where components from the 0th order light to the 5th order light of the Fourier transform image 1f are irradiated.

As a result, in the optical recording medium 5, the components from the 0th order light to the 5th order light of the Fourier transform image 1f interfere with the reference light 2 having the spatial intensity distribution corresponding to the spatial intensity distribution. Then, a Fourier transform hologram is recorded in the optical recording medium 5. In this case, the optical recording medium 5 is moved in a predetermined shift direction by the driving means 40, and the hologram is shift-multiplex-recorded.

At the time of reproduction, the shutter 15 is closed, the object light 1 is blocked, and the same light as the reference light 2 at the time of recording, which has been transmitted through the intensity modulation filter 17, is used as readout light.
The light is transmitted through the light transmitting portion 21 to irradiate an area of the optical recording medium 5 where the hologram is recorded. The irradiated reading light 2 is diffracted by the hologram.

The diffracted light 3 is subjected to inverse Fourier transform by a lens 8 to form an image on a photodetector 9 such as a CCD.
Read data information.

[Verification by Experiment] The recording and reproduction of data information was actually attempted by the method and apparatus described above. If the optical recording medium 5 can record a hologram,
Although any material may be used, here, a polyester having cyanoazobenzene in a side chain represented by a chemical formula shown in FIG. 6 was used. This material is disclosed in Japanese Patent Application No. 10-3283.
As described in detail in No. 4, holograms can be recorded, reproduced, and erased by light-induced anisotropy caused by photoisomerization of side chain cyanoazobenzene.

As the light source 6, an oscillation line of an argon ion laser having a sensitivity of 515 nm for a polyester having cyanoazobenzene in a side chain was used as the optical recording medium 5.

The spatial light modulator 4 has a pixel size of 4
A liquid crystal panel 1.3 type for projectors having a size of 2 μm × 42 μm and 640 × 480 pixels was used. Using one pixel as one bit, a binarized pattern as shown in FIG. 1 was created by a computer and input to the spatial light modulator 4. Therefore,
The data image of the object light 1 has a spatial frequency component corresponding to a pitch of d = 42 μm. The focal length f of the lens 7 for performing the Fourier transform of the object light 1 is 55 m.
m.

As the intensity modulation filter 17, the same liquid crystal panel as that of the spatial light modulator 4 was used. In advance, CCD
The spatial intensity distribution of the Fourier transform image at a position on the optical recording medium 5 was measured by a camera, and spatial intensity distribution data was obtained.

First, a hologram was recorded by a conventional method for comparison. The intensities of the 0th-order light, the 2nd-order light, and the 5th-order light were detected from the previously obtained spatial intensity distribution data, and the hologram was recorded with reference light having the same intensity.

First, a hologram is recorded by irradiating a reference light having the same intensity as the 0th-order light, and thereafter, the object light is cut off, and the same readout light as the reference light is irradiated. Upon reproduction, a reproduced image with high brightness but with a blurred outline was obtained. Next, the hologram was recorded by irradiating the reference light having the same intensity as the fifth-order light, and then the object light was cut off, the same reading light as the reference light was irradiated, and the data image was reproduced from the obtained diffracted light. A reproduced image with a clear outline but low brightness was obtained. From these results, when the reference light has the same intensity as the 0th-order light, the high-order components are not sufficiently reproduced. Conversely, when the reference light has the same intensity as the 5th-order light, the low-order components are not sufficiently reproduced. Was not played back.

Further, after the hologram is recorded by irradiating the reference light having the same intensity as the secondary light, the object light is cut off and the same reading light as the reference light is irradiated, and the data image is reproduced from the obtained diffracted light. As a result, although the brightness was reduced, the blurring of the outline was slightly improved. In the conventional method, the intensity of the reference light is changed while confirming the reproduced image, and the intensity of the reference light that provides the most preferable reproduced image is selected.

Next, a hologram was recorded by the method of the present invention. First, intensity modulation data was created by a computer based on the previously obtained spatial intensity distribution data, and displayed on a liquid crystal panel on which the intensity modulation filter 17 was formed.

Next, the CCD is again placed on the optical recording medium 5.
The camera was arranged, the spatial intensity distribution of the reference light transmitted through the intensity modulation filter 17 was measured, and the intensity modulation data was adjusted so that the spatial intensity distribution of the reference light became the same as the spatial intensity distribution of the Fourier transform image. After the adjustment, the intensity modulation filter 17
The hologram was recorded by the reference light transmitted through the hologram. After that, the object light was cut off and the same reading light as the reference light was irradiated, and a data image was reproduced from the obtained diffracted light.
A reproduced image with high brightness and a clear outline was obtained, and the effect of the method of the present invention was confirmed.

[Other Examples] Instead of performing a Fourier transform on a data image of an object light with a lens, a computer obtains data of a Fourier transform image from the data information, thereby displaying the Fourier transform image on a spatial light modulator. An optical recording medium may be irradiated with a Fourier transform image.

[0048]

As described above, according to the present invention, a uniform and sufficient interference effect can be obtained for each order component of the Fourier transform image, and a data image of the object light can be reproduced well.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of an object beam.

FIG. 2 is a diagram illustrating an example of a Fourier transform image.

FIG. 3 is a diagram showing a light intensity distribution of a Fourier transform image.

FIG. 4 is a diagram showing an example of the optical recording method of the present invention.

FIG. 5 is a diagram showing an example of the optical recording device of the present invention.

FIG. 6 is a diagram showing a chemical formula of an example of a material of an optical recording medium.

FIG. 7 is a diagram for explaining a shift multiplex recording method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Object light 1f ... Fourier transform image 2 ... Reference light (reading light) 3 ... Diffracted light 4 ... Spatial light modulator 5 ... Optical recording medium 6 ... Light source 7 ... Fourier transform lens 8 ... Inverse Fourier transform lens 9 ... Photodetector 12 ... Beam splitter 15 ... Shutter 17 ... Intensity modulation filter 20 ... Light shield 21 ... Light transmitting part 40 ... Drive means

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2K008 AA04 BB04 EE01 FF07 FF21 HH26 5D090 AA01 BB16 BB18 CC01 DD03 DD05 FF42 FF50

Claims (9)

[Claims] 1. A Fourier transform image of an object light holding data information, and a reference light having a spatial intensity distribution where the intensity of the Fourier transform image is high where the intensity is high and where the intensity is low where the intensity is low. Simultaneously irradiate the optical recording medium,
An optical recording method for recording the Fourier transform image as a hologram on the optical recording medium. 2. The optical recording method according to claim 1, wherein the reference light is a parallel light having a spatially constant intensity and an intensity modulation filter having a spatial transmittance distribution according to the spatial intensity distribution of the Fourier transform image. An optical recording method characterized by being obtained by transmitting light. 3. The optical recording method according to claim 1, wherein the object light is an image whose intensity is binarized according to the data information. 4. The optical recording method according to claim 1, wherein the object light is subjected to Fourier transform by a lens to obtain the Fourier transformed image. 5. The optical recording method according to claim 1, further comprising: a light shielding member having a light transmitting part formed in front of the optical recording medium; An optical recording method comprising irradiating components up to a predetermined order to the optical recording medium through the light transmitting portion. 6. A light source that emits coherent light, a spatial light modulator that modulates light from the light source according to data information to obtain an object light holding the data information by its wavefront, An imaging optical system that performs Fourier transform and irradiates the Fourier transform image to an optical recording medium; a reference light optical system that obtains reference light from light from the light source and irradiates the optical recording medium; and the reference light. An intensity modulation filter that has a spatial intensity distribution in which the intensity is high where the intensity of the Fourier transform image is high, and the intensity is low where the intensity is low. 7. An optical recording apparatus according to claim 6, wherein said spatial light modulator generates, as said object light, an image whose intensity is binarized in accordance with said data information. apparatus. 8. A Fourier transform image of an object beam holding data information, and a reference beam having a spatial intensity distribution where the intensity of the Fourier transform image is high where the intensity is high and low where the intensity is low. Simultaneously irradiated, the optical recording medium on which the Fourier transformed image is recorded as a hologram,
Irradiating read light having the same wavefront as the reference light,
A light reproducing method for reproducing the hologram as diffracted light. 9. The optical reproducing method according to claim 8, wherein the data information is read by performing an inverse Fourier transform of the diffracted light.