JP3700752B2 - Optical recording method and optical recording apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for performing shift multiplex recording of data information as a hologram.
[0002]
[Prior 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 attracting attention. In the hologram memory, a plurality of data pages can be recorded by being multiplexed in the same volume, and the data can be read collectively for each page. Digital data can be recorded and reproduced by converting binary digital data “0, 1” into “bright, dark” as a digital image and recording and reproducing it as a hologram instead of an analog image. Recently, specific optical systems of this digital hologram memory system, SN ratio and bit error rate evaluation based on the volume multiplex recording system, or proposal for two-dimensional encoding have been made, such as the influence of aberrations of the optical system, etc. Research from a more optical viewpoint is also progressing.
[0003]
8, the volume multiplex recording system disclosed in the document “D. Psaltis, M. Levene, A. Pu, G. Barbastasis and K. Curtis; OPTICS LETTERS Vol. 20, No. 7 (1995) p782”. An example of the shift multiplex recording system is shown.
[0004]
In the shift multiplex recording method, a spherical wave is used as the reference light 32 that is applied to the hologram recording medium 35 simultaneously with the signal light 31, and the hologram recording medium 35 is formed into a disk shape, and a plurality of holograms are superimposed on the same region by the rotation of the disk 35. To write. For example, if the beam diameter is 1.5 mmφ, another hologram can be recorded in substantially the same region without causing crosstalk by simply moving the disk 35 by several tens of μm. This utilizes the fact that the reference beam 32 is a spherical wave and is equivalent to the change in the angle of the reference beam 32 due to the movement of the disk 35.
[0005]
The moving distance of this spherical reference wave shift multiplex recording, that is, the distance δ at which the respective holograms can be separated independently, as shown in the above document,
δspherical = δBragg + δNA
≈ (λzo / (Ltanθs)) + λ / (2NA) (1)
It is represented by Here, λ is the wavelength of the signal 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 crossing angle of the signal light and the spherical reference wave, and NA is the above This is the numerical aperture of the objective lens.
[0006]
From this equation (1), the greater the film thickness L of the recording medium, the smaller the shift amount δ, the more the multiplicity can be increased, and the recording capacity can be increased. Furthermore, in order to increase the recording capacity more effectively with shift multiplex recording, the recording area may be made smaller. By performing multiplex recording in a minute area, higher-density volume multiplex recording can be realized.
[0007]
For the above purpose, in the hologram memory system, the signal light is Fourier-transformed by a lens and irradiated onto a recording medium. When the image of the signal light has a fine pitch (high spatial frequency), the spread ζ of the signal light on the recording medium surface is
ζ = kλfωx (2)
It is represented by Here, k is a proportional constant, λ is the wavelength of the signal light, f is the focal length of the lens for Fourier transform, and ωx is the spatial frequency of the signal light. Therefore, if a lens having a small focal length f is used as a lens for Fourier transform, the recording area can be miniaturized.
[0008]
Furthermore, it is considered that the recording area is miniaturized by arranging an aperture in front of the recording medium. For example, in JP-A-55-41480, as shown in FIG. 9, an aperture board 39 having a circular aperture 38 whose light transmittance gradually increases toward the center is arranged in front of the recording medium. It is shown that the unnecessary spread of the signal light and the reference light is limited and the recording area is miniaturized.
[0009]
[Problems to be solved by the invention]
However, even with this method, if the image of the signal light is made finer to increase the density, the spatial frequency ωx increases, and the Fourier transform image spreads according to Equation (2). Accordingly, the aperture diameter is increased accordingly. In fact, it is not always possible to expect a sufficiently high density.
[0010]
Furthermore, in the shift multiplex recording method, the data written earlier deteriorates gradually due to the data overwritten later, and the shift amount is reduced and the recording density is increased as shown below. Then it becomes more prominent.
[0011]
A data page to be recorded as a hologram is, for example, an image as shown in FIG. By making the white portion in the figure represent data “1” and the black portion represent 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.
[0012]
In such a digital hologram memory, in order to improve the recording density or to give the hologram a shift invariant characteristic, as described above, a Fraunhofer diffraction image of a data image (signal light) by a lens. Record (Fourier transform image). This is called a Fourier transform hologram because it is proportional to the Fourier transform of the amplitude distribution of the data image as shown in FIG. FIG. 2 shows a Fourier transform image of the data image of FIG. This can be obtained from the above equation (2).
[0013]
In order to record digital data at a high density, it is desirable to reduce the area of one pixel of the data image, that is, to reduce the value of d and pack more bit data in one page. Thereby, in addition to high-density recording, high-speed recording / reproduction can be realized.
[0014]
However, if the area of one pixel is reduced, the Fourier transform image of the data image (signal light) spreads on the recording medium according to the equation (2). As described above, this is because the spatial frequency ωx proportional to 1 / d increases as the signal light image becomes finer.
[0015]
As a method for avoiding this, a method of reducing the wavelength λ of the signal light, a method of forming a Fourier transform image of the signal light using a lens having a short focal length f, and the like are conceivable. However, even if the Fourier transform image is made smaller by shortening the wavelength λ of the light source or shortening the focal length f of the lens, the Fourier transform image has an infinite spread in its focal plane in principle. It is not sufficient for miniaturization of the recording area.
[0016]
The spread in the x-axis direction of the Fourier transform image shown in FIG. 2 corresponds to the spatial frequency ωx in the x-axis direction of the data image shown in FIG. 1, and when viewed in the x-axis direction, the Fourier transform image is zero-order light. (Ωx = 0) Spreads symmetrically in the plus and minus directions around F0. The same applies to the y-axis direction. As described above, the spatial frequency has a positive value and a negative value. However, in order to record / reproduce the data image of the signal light, any one of the code components is sufficient.
[0017]
Further, since the image (signal light) to be recorded is not an analog image but a binary digital data image, it is not necessary to take out and record even infinite components of the spatial frequencies ωx and ωy, and one digital data image can be recorded. It is sufficient if a spatial frequency component having a period of about a pixel is recorded.
[0018]
Therefore, a light shield having a light transmitting portion 21 as shown in FIG. 3 is disposed in front of the optical recording medium, and the light transmitting portion 21 is transmitted to optically record the signal light and the reference light after Fourier transform. It is conceivable that the recording area is miniaturized by irradiating the medium, and the Fourier transform hologram is shift-multiplex recorded.
[0019]
In this case, the optical recording medium is moved to the left in the figure, the right direction in the figure is taken as the shift direction (advancing direction of shift multiplex recording), and the ωx axis of the Fourier transform image is made to coincide with the shift direction, 21 transmits, for example, the zero-order light F0 of the Fourier transform image and the first-order light F1 in all directions.
[0020]
Therefore, the Fourier transform image filled in black in the light transmitting portion 21 is data recorded (recorded) first, and the Fourier transform image thinly painted in gray is data recorded next (recorded), broken line without being painted. The Fourier transform image showing only the 0th-order light F0 is data recorded (recorded) next.
[0021]
As is clear from this, when shift multiplex recording is performed with the ωx axis of the Fourier transform image coincident with the shift direction, the components of the Fourier transform image before and after are likely to overlap on the optical recording medium, while the Fourier transform image The unevenness between the area where the component is written and the area where the component is not written tends to occur.
[0022]
When the Fourier transform images before and after locally overlap in this way, the previously written data gradually deteriorates due to the data overwritten later, and the SN ratio of the previously written data decreases, In addition, as the shift amount δ is decreased and the multiplicity is increased, the deterioration of the data or the decrease of the SN ratio becomes more significant, and the data may be lost in some cases.
[0023]
Therefore, even if the recording area is miniaturized by the light transmitting portion 21, the shift amount δ must be made larger than the illustrated amount to reduce the multiplicity, and as a result, high-density shift multiplex recording can be realized. I can't.
[0024]
Accordingly, the present invention is capable of reducing the deterioration of data due to shift multiplex recording and improving the recording density in a method of performing shift multiplex recording as data information as a hologram.
[0025]
[Means for Solving the Problems]
The optical recording method of the present invention comprises:
An optical recording method in which a Fourier transform image of signal light of a two-dimensional digital data image is shift-multiplexed recorded on an optical recording medium as a hologram,
A plurality of straight lines connecting the 0th-order light component and each primary light component of the Fourier transform image are formed so that the component of the later Fourier transform image on the optical recording medium fills the gap between the components of the previous Fourier transform image. The Fourier transform image is shift-multiplex-recorded on the optical recording medium, each tilted with respect to the traveling direction of shift-multiplex recording.
[0026]
[Action]
In the optical recording method of the present invention based on the above method, the component of the Fourier transform image of the subsequent data is written on the optical recording medium so as to fill the gap between the components of the Fourier transform image of the previously written data. As a result, the local overlap of the front and rear Fourier transform images is reduced. Therefore, the deterioration of the previously written data due to the later overwritten data is reduced, and even if the shift amount is reduced and the multiplicity is increased, the data can be read with a high SN ratio.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
[Embodiment of Optical Recording Method]
FIG. 4 shows an example of the optical recording method of the present invention.
[0028]
The optical recording medium is moved in the left direction in the figure, and the right direction in the figure is defined as a shift direction (advancing direction of shift multiplex recording). In the present invention, as shown in FIG. The axis (or ωy axis) is tilted at a predetermined angle θ with respect to the shift direction. When the angle θ is set to 45 °, the axis between the ωx axis and the ωy axis of the Fourier transform image coincides with the shift direction, so that the angle θ is set in the range of 0 ° <θ <45 ° as described later.
[0029]
Further, in this example, a light shielding body in which a circular light transmission portion 21 is formed is disposed in front of the optical recording medium, and nine components including the zero-order light F0 and the first-order light F1 in all directions of the Fourier transform image. Is transmitted through the light transmitting portion 21 and recorded on the optical recording medium.
[0030]
Therefore, the Fourier transform image filled in black in the light transmitting portion 21 is the first recorded (recorded) data, and the Fourier transform image thinly painted in gray is the next recorded (recorded) data, solid line without filling. The Fourier transform image showing only the 0th-order light F0 is data recorded (recorded) next.
[0031]
The positions and intervals of orthographic projection points a1, a2,..., A9 with respect to the shift direction at the center of the nine components (dots) recorded on the optical recording medium of the Fourier transform image vary depending on the angle θ. As shown in the figure, the angle θ is set so that the orthogonal projection points a1 to a9 do not overlap each other.
[0032]
More specifically, the angle θ is set so that the smallest distance among the distances a1 to a2, a2 to a3,... A8 to a9 between two adjacent points of the orthogonal projection points a1 to a9 is the largest. Set. This minimizes the overlap of adjacent Fourier transform image components in the case of shift multiplex recording.
[0033]
In this way, by tilting the ωx axis of the Fourier transform image by a predetermined angle θ with respect to the shift direction, as shown in the figure, the gap between the components of the Fourier transform image of the previously written data is filled. As described above, the component of the Fourier transform image of the subsequent data is written, and the local overlap of the front and rear Fourier transform images is reduced. Therefore, the deterioration of the previously written data due to the later overwritten data is reduced, and the data can be read with a high SN ratio even if the shift amount δ is reduced and the multiplicity is increased. .
[0034]
Note that the size of each dot (component) in the Fourier transform image varies depending on the writing energy. Therefore, when the writing energy is small and the individual dots are small, the angle θ can be freely set within a range in which the orthogonal projection points a1 to a9 with respect to the shift direction of the center of each dot do not overlap each other.
[0035]
FIG. 5 shows another example of the optical recording method of the present invention. In this example as well, the ωx axis (or ωy axis) of the Fourier transform image is tilted at a predetermined angle θ with respect to the shift direction on the optical recording medium. Of the four quadrants defined by the ωx axis and the ωy axis of the Fourier transform image, and 10 components up to the third order in one quadrant including the ωx axis and the ωy axis. Then, the light is transmitted through the light transmitting portion 21 and recorded on the optical recording medium.
[0036]
Therefore, the Fourier transform image filled in black is the first recorded (recorded) data, and the Fourier transform image thinly painted in gray is the next recorded (recorded) data. Is a data to be recorded (recorded) after that.
[0037]
Also in this case, more specifically, the orthogonal projection points a1, a2,..., A10 with respect to the shift direction at the center of the ten components (dots) recorded on the optical recording medium of the Fourier transform image do not overlap each other. Sets the angle θ so that the minimum distance among the distances a1 to a2, a2 to a3,... A9 to a10 between two adjacent points of the orthogonal projection points a1 to a10 is the largest.
[0038]
When recording components in all directions of a Fourier transform image as shown in the example of FIG. 4, if recording is performed up to higher order components such as secondary light and tertiary light, the center shift of each recorded component is performed. It is difficult to prevent orthogonal projection points with respect to directions from overlapping each other, and the recording area increases.
[0039]
On the other hand, when only the components in one quadrant including the components on the ωx axis and the ωy axis of the Fourier transform image are recorded as in the example of FIG. Even when recording up to higher order components, it is possible to prevent orthogonal projection points in the shift direction of the center of each component to be recorded from overlapping each other, and the recording area does not increase.
[0040]
[Embodiment of optical recording apparatus and optical reproduction method]
FIG. 6 shows an embodiment of the optical recording apparatus of the present invention, which is a case where the optical recording apparatus is also used. The above-described light shielding body 20 is disposed in front of the optical recording medium 5.
[0041]
The coherent light from the light source 6 is divided into two lights by the beam splitter 12, the shutter 15 is opened at the time of recording, and the light that has passed through the beam splitter 12 is converted into parallel light having a wide aperture by the lenses 10a and 10b. The light is incident on the modulator 4.
[0042]
A binary two-dimensional digital data image as shown in FIG. 1 is displayed on the spatial light modulator 4 by a computer omitted in the figure. As a result, the light that has passed through the spatial light modulator 4 is intensity-modulated according to the value of each pixel of the digital data image, and becomes the signal light 1 having information of the digital data image.
[0043]
The signal light 1 is Fourier transformed by the lens 7, transmitted through the light transmitting portion 21 of the light shielding body 20, and irradiated onto the optical recording medium 5. At the same time, the light reflected by the beam splitter 12 is reflected by the mirrors 13 and 14 as the reference light 2 and transmitted through the light transmitting portion 21 of the light shield 20, and the signal light 1 after the Fourier transform of the optical recording medium 5 is generated. Irradiate the irradiated area.
[0044]
As a result, the Fourier transform signal light 1 and the reference light 2 interfere in the optical recording medium 5, and 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 to perform shift multiplex recording of the hologram.
[0045]
At the time of reading, the shutter 15 is closed to block the signal light 1, and the same light as the reference light 2 at the time of recording is used as the reading light and transmitted through the light transmitting portion 21 of the light shielding body 20 to record the hologram of the optical recording medium 5. The irradiated area is irradiated. The irradiated readout light 2 is diffracted by the hologram.
[0046]
The diffracted light 3 is inverse Fourier transformed by a lens 8 and imaged on a photodetector 9 such as a CCD to read data information of the signal light 1.
[0047]
〔Example〕
Actual recording / reproduction was attempted by the method described above. The optical recording medium 5 is not particularly limited as long as it can record a hologram, but here, a polyester represented by the chemical formula shown in FIG. 7 and having cyanoazobenzene in the side chain is used. As described in detail in Japanese Patent Application No. 10-32834, this material has photoinduced anisotropy (photoinduced birefringence, photoinduced dichroism) due to photoisomerization of cyanoazobenzene in the side chain. It is possible to record, reproduce and erase holograms. Actually, a thin film of 50 μm thick polyester having cyanoazobenzene in the side chain was used.
[0048]
For recording the data information, the optical recording apparatus shown in FIG. 6 was used. As the light source 6, an oscillation line 515 nm of an argon ion laser sensitive to polyester having cyanoazobenzene in the side chain as the optical recording medium 5 was used.
[0049]
As the spatial light modulator 4, a projector liquid crystal panel 1.3 type having a size of one pixel of 42 μm × 42 μm and 640 × 480 pixels was used. The chessboard pattern shown in FIG. 1 was created by a computer with one pixel as one bit and input to the spatial light modulator 4. Therefore, the data image of the signal light 1 has a spatial frequency component corresponding to a pitch of d = 42 μm.
[0050]
A lens 7 having a focal length f of 55 mm was used as the lens 7 for Fourier transforming the signal light 1. As the light shielding body 20, a light transmitting portion 21 having a circular shape and a diameter of 2 mm was used as shown in FIG.
[0051]
Shift multiplexing is performed by moving the optical recording medium 5 in one direction by the driving means 40, and tilting the ωx axis of the Fourier transform image by 22.5 ° with respect to the shift direction, so that the Fourier transform image and the light transmitting portion 21 are moved. The positional relationship was as shown in FIG. For comparison, recording was also performed in which the ωx axis of the Fourier transform image coincided with the shift direction, and the Fourier transform image and the light transmission portion 21 were in the positional relationship as shown in FIG.
[0052]
An attempt was made to read data from the hologram recorded as described above by using the optical reproducing apparatus shown in FIG. As the light source 6, the same argon ion laser oscillation line 515 nm as that used during recording was used. The shutter 15 was closed to block the signal light 1 and the optical recording medium 5 was irradiated with the same light as the reference light 2 during recording as readout light.
[0053]
As a result, in the case of shift multiplex recording with the ωx axis of the Fourier transform image coincided with the shift direction, the same chessboard pattern as the signal light 1 was reproduced as the diffracted light 3 until the shift amount was 100 μm. When the amount was smaller than 100 μm, the chessboard pattern was not reproduced. On the other hand, when multiplex recording was performed by tilting the ωx axis of the Fourier transform image by 22.5 ° with respect to the shift direction, a clear chessboard pattern was reproduced even when the shift amount was reduced to 70 μm. . This result also shows that recording with higher density is possible when the axis of the Fourier transform image is tilted with respect to the shift direction.
[0054]
【The invention's effect】
As described above, according to the optical recording method and the optical recording apparatus of the present invention, it is possible to reduce the deterioration of data due to shift multiplex recording and to improve the recording density.
[Brief description of the drawings]
FIG. 1 is a diagram showing an example of signal light recorded by an optical recording method of the present invention.
2 is a diagram illustrating a Fourier transform image of the signal light in FIG. 1. FIG.
FIG. 3 is a diagram showing a possible example of a shift multiplex recording method.
FIG. 4 is a diagram showing an example of an optical recording method of the present invention.
FIG. 5 is a diagram showing another example of the optical recording method of the present invention.
FIG. 6 is a diagram showing an embodiment of an optical recording method and an optical recording apparatus of the present invention.
FIG. 7 is a view showing a chemical formula of an example of a material of an optical recording medium used in the optical recording method of the present invention.
FIG. 8 is a diagram for explaining a shift multiplex recording method.
FIG. 9 is a diagram for explaining a conventional optical recording method.
[Explanation of symbols]
1 ... Signal light 2 ... Reference light (reading light)
DESCRIPTION OF SYMBOLS 3 ... Diffracted light 4 ... Spatial light modulator 5 ... Optical recording medium 6 ... Light source 9 ... Photo detector 12 ... Beam splitter 15 ... Shutter 20 ... Light-shielding body 21 ... Light transmission part 40 ... Driving means

Claims (8)

An optical recording method in which a Fourier transform image of signal light of a two-dimensional digital data image is shift-multiplexed recorded on an optical recording medium as a hologram,
A plurality of straight lines connecting the 0th-order light component and each primary light component of the Fourier transform image are formed so that the component of the later Fourier transform image on the optical recording medium fills the gap between the components of the previous Fourier transform image. An optical recording method, wherein the Fourier transform image is shifted and multiplexed recorded on the optical recording medium, each tilted with respect to the traveling direction of shift multiplexed recording. The optical recording method according to claim 1, wherein
The plurality of straight lines so that the minimum distance among the two adjacent points of the orthogonal projection point with respect to the traveling direction of the shift multiplex recording of the zero-order light component and each primary light component is the largest. Is tilted with respect to the traveling direction of the shift multiplex recording. The optical recording method according to claim 1, wherein
A light-shielding body having a light transmission part formed in part is disposed in front of the optical recording medium, and the Fourier transform image is transmitted through the light transmission part to irradiate the optical recording medium. An optical recording method comprising: recording a hologram having a predetermined shape and size. The optical recording method according to claim 1, wherein
An optical recording method, wherein only one quadrant component including a component on an axis among four quadrants partitioned by mutually orthogonal axes of the Fourier transform image is recorded on the optical recording medium. A light source that emits coherent light;
A spatial light modulator that modulates light from the light source according to two-dimensional digital data information to obtain signal light of a two-dimensional digital data image;
An imaging optical system that Fourier-transforms the signal light and irradiates the Fourier-transformed image onto 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;
A plurality of straight lines connecting the 0th-order light component and each primary light component of the Fourier transform image are formed so that the component of the later Fourier transform image on the optical recording medium fills the gap between the components of the previous Fourier transform image. Driving means for moving the optical recording medium so that the Fourier-transformed image is shifted to the optical recording medium as a hologram by tilting with respect to the traveling direction of the shift multiplex recording, respectively.
An optical recording apparatus comprising: The optical recording apparatus according to claim 5.
The plurality of straight lines so that the minimum distance among the two adjacent points of the orthogonal projection point with respect to the traveling direction of the shift multiplex recording of the zero-order light component and each primary light component is the largest. Is inclined with respect to the traveling direction of the shift multiplex recording. The optical recording apparatus according to claim 5.
A light-transmitting portion formed in part and disposed in front of the optical recording medium, comprising a light-shielding body that transmits the light-transmitting portion and irradiates the optical recording medium with the Fourier transform image and the reference light. An optical recording apparatus. The optical recording apparatus according to claim 5.
An optical recording apparatus, wherein only one quadrant component including a component on an axis among four quadrants defined by mutually orthogonal axes in the Fourier transform image is recorded on the optical recording medium.

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