JP4062982B2 - Optical memory device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical memory device using a holographic recording medium.
[0002]
[Prior art]
As a recording medium of a computer device such as a personal computer, a floppy disk, a hard disk (magnetic disk) and the like have been mainly used. Recently, in addition to these recording media, various recording media such as CD-ROM, MOD (Magneto-Optical Disk), DVD (Digital Versatile Disk), etc. have been used, and research and development of other recording media is also active. Has been done.
[0003]
In addition, the cost reduction of hard disks has spurred the reduction of memory costs, and the current memory cost is said to be about 10-20 yen per 1 megabyte of storage capacity. DVDs are expected to cost several yen per megabyte.
[0004]
Furthermore, volume holographic memory, which has been viewed as a promising next-generation memory element following DVD in recent years, has improved various characteristics such as future CPU speedup, data capacity increase, access speed increase, etc. A price reduction of about 5 to 10 yuan per megabyte can be realized. By the way, various materials such as a lithium niobate single crystal, a barium titanate single crystal, and a strontium barium titanate single crystal have been proposed as materials for the volume holographic memory. What is said to be large is said to be a lithium niobate (LiNbO ₃ ) single crystal (hereinafter abbreviated as LN single crystal). This is because high reproduction efficiency can be expected, recording and erasing can be repeated, multiple recording can be performed, and high resolution can be expected.
[0005]
At the same time, volume holographic memories using organic compounds such as azobenzene and diarylethene have been proposed.
[0006]
The outline of the holographic memory system will be described below with reference to FIG. FIG. 3 is an outline of a conventional optical memory device and an explanatory diagram of its recording principle.
[0007]
In FIG. 3, an encoder 125 converts digital data to be recorded in the holographic memory 101 as a bright and dark dot pattern image on a plane and rearranges the data into, for example, a vertical 480 bit × horizontal 640 bit data array. Is generated. This data is sent to a spatial light modulator (SLM) 115 such as a transmissive TFT liquid crystal display (LCD) panel.
[0008]
The spatial light modulator 115 has a modulation processing unit of vertical 480 pixels × horizontal 640 pixels corresponding to a unit page, and turns on and off the spatial light according to the unit page sequence data from the encoder 125. Optical modulation is performed on the signal, and the modulated signal light is guided to the Fourier transform lens 116. More specifically, the spatial light modulator 115 passes the signal beam in response to the logical value “1” of the unit page series data, which is an electrical signal, and blocks the signal beam in response to the logical value “0”. Electro-optical conversion according to the contents of each bit in the unit page data is achieved, and modulated signal light is generated as signal light of the unit page series.
[0009]
The signal light is incident on the volume holographic memory 101 through the Fourier transform lens 116. In addition to the signal light, reference light is incident on the volume holographic memory 101 with an incident angle β from a predetermined reference line orthogonal to the optical axis of the signal light beam. The signal light and the reference light interfere in the holographic memory 101, and the interference fringes are stored in the volume holographic memory 101 as a refractive index grating, that is, a hologram, thereby recording data.
[0010]
Also, three-dimensional data recording can be performed by changing the incident angle β and making the reference light incident to perform angle-multiplex recording of a plurality of two-dimensional plane data. When the recorded data is reproduced from the volume holographic memory 101, only the reference light is supplied to the volume holographic memory 101 at the same incident angle β as that at the time of recording toward the center of the region where the signal light beam and the reference light beam intersect. Make it incident. That is, unlike recording, no signal light is incident. As a result, the diffracted light from the interference fringes recorded in the volume holographic memory 101 is guided to the CCD (Charge Coupled Device) 120 of the photodetector through the Fourier transform lens 119.
[0011]
The CCD 120 converts the intensity of incident light into the strength of an electric signal and outputs an analog electric signal having a level corresponding to the luminance of the incident light to a decoder 126 (not shown).
[0012]
The decoder 126 compares the analog signal with a predetermined amplitude value (slice level), and reproduces the corresponding “1” and “0” data.
[0013]
Since the holographic memory 101 performs recording in a two-dimensional plane data series as described above, angle multiplex recording can be performed by changing the incident angle β of the reference light. That is, by changing the incident angle β of the reference light, a plurality of two-dimensional planes as recording units can be defined in the holographic memory, and as a result, three-dimensional recording is possible.
[0014]
In the rewritable volume holographic memory 101 using the above-described photorefractive effect, an LN single crystal to which Fe is added is usually used as a recording material, and an Nd: YAG laser is used as recording light with an SHG (2nd harmonic generator). ), And only the wavelength 532 nm, which is the second harmonic, is used by the wavelength selection filter after passing through the SHG.
[0015]
In this conventional recording system (referred to as a conventional monochrome recording method), corresponding to the interference fringes formed from signal light and reference light is recorded light, the bright interference fringes region, quasi of Fe ²⁺ Electrons are excited to the conductor from the position, and finally passed through the photorefractive process to be trapped at the Fe ³⁺ level, completing the storage. In the volume holographic memory for recording information in this way, it goes without saying that the characteristics of the spatial light modulator 115 for converting the data to be recorded into optical signals as described above greatly affect the recording characteristics. That is, the high-speed responsiveness and spatial resolution (high saturation) of the spatial light modulator 115 directly become high-speed and large-capacity recording performance as an optical memory device.
[0016]
However, conventional nematic liquid crystals are often used for conventional spatial light modulators, and can display 10 to 100 ms as responsiveness and several tens of lp / mm (lp / mm: 1 mm wide) as spatial resolution. The number of vertical lines expressed in light and dark, the number of so-called line pairs). Therefore, the high speed and large capacity recording performance as an optical memory device are limited.
[0017]
[Problems to be solved by the invention]
The present invention completely improves the above-mentioned conventional defects, improves the high speed of hologram recording, increases the amount of data that can be recorded in one recording process, and can improve the recording performance. The purpose is to provide.
[0018]
[Means for Solving the Problems]
The present invention relates to an optical memory device that spatially modulates a light beam according to unit page sequence data from an encoder by a spatial light modulator and records an interference pattern of a plurality of light beams including the light beam. An optical memory device characterized in that a dielectric liquid crystal is used for a spatial light modulator, and multi-value recording is performed on the interference pattern by the gradation characteristics of the antiferroelectric liquid crystal, and has high-speed reactivity and gradation. This improves the writing speed and the writable capacity.
[0020]
[Means for Solving the Problems]
According to the first aspect of the present invention, an optical memory that spatially modulates a light beam according to unit page sequence data from an encoder by a spatial light modulator and records an interference pattern of a plurality of light beams including the light beam. An optical memory device characterized in that an antiferroelectric liquid crystal is used for a spatial light modulator and multi-value recording is performed on the interference pattern by the gradation characteristics of the antiferroelectric liquid crystal. By improving the reactivity and gradation, the writing speed and writable capacity are improved.
[0026]
Embodiments of the present invention will be described below with reference to the drawings.
[0027]
(Embodiment 1)
FIG. 1 is a block diagram showing an optical memory device according to Embodiment 1 of the present invention.
[0028]
In the first embodiment, the case where the oscillation wavelength of the laser 15 is low in sensitivity to the volume holographic memory 10 and the oscillation wavelength of the laser 15 is converted to ½ by the SHG 16 will be described. The light oscillated by the laser 15 is divided into a signal light beam traveling straight by a beam splitter 18a and a reference light beam deflected upward, and each is guided to an optical path for the signal light beam optical system and the reference light beam optical system. It is burned. The spectral ratio of the beam splitter 18a is set to an appropriate value so that the recording light quantity is not excessive or insufficient in consideration of the conversion efficiency of the SHG 16, the reflectance / transmittance of other optical components, the sensitivity of the CCD 22, and the like. It can be set in advance or can be varied within a certain range in response to a change in the amount of light in the recording portion in the volume holographic memory 10 due to the rotational multiplexing or the like.
[0029]
The signal light beam converted in wavelength by the SHG 16 and passed through the beam splitter 18 enters the volume holographic memory 10 through the shutter 6a, the light beam expander 14, the spatial light modulator 12, and the Fourier transform lens 13. The light guided to the optical path of the reference light beam optical system is further guided to the half mirror 19 by the mirror 23 and guided to the reference light beam optical system which is substantially coaxial with the reference light obtained by separating the light after SHG conversion. In this embodiment, the writing operation is performed with the shutters 6a and 6b open, and the reading operation is performed by closing the shutters 6a and 6b and using the unconverted light of the laser 15 as reference light as described above.
[0030]
First, the write operation and the functions and operations of each unit will be described. The time for irradiating the volume holographic memory 10 of the light beam to the volume holographic memory 10 is controlled by a shutter 6a controlled by a controller (not shown), and the signal light beam is expanded to parallel light of a predetermined diameter.
[0031]
Here, the spatial light modulator 12 is a two-dimensional planar liquid crystal display (LCD) made up of fine pixels divided into a finite number as described above, and many of them transmit a light beam. Placed in the correct position.
[0032]
This modulation operation will be further described. In the spatial light modulator 12, by applying a voltage to each pixel of the liquid crystal in the spatial light modulator 12 according to the recording page data as described above, the molecular arrangement of the liquid crystal moves so as to be twisted. As a result, the plane of polarization of the transmitted light can be rotated by an arbitrary angle, and light can be transmitted or non-transmitted for each pixel or intermediate by the function of a polarizing filter placed at a position adjacent to the liquid crystal in the device. With respect to the rotation of the polarization plane, the light is transmitted with an intermediate transmittance. That is, the light passing there is partially intensity modulated. As a result, the beam whose diameter has been expanded by the beam expander 14 can be spatially modulated according to the signal in accordance with digital recording data supplied from an encoder (not shown). In addition to the transmission type, the spatial light modulator 12 includes a reflection type having a reflection surface on the side opposite to the beam incident surface with a liquid crystal layer interposed therebetween. In the above example, the light is transmitted or not transmitted through the polarizing filter. However, when the rotation of the polarization plane is used as the modulation element itself, the phase of the light can be modulated instead of the polarization plane of the liquid crystal. It is also possible to use the phase as a modulation element using parallel alignment liquid crystals.
[0033]
The light beam thus modulated is Fourier-transformed by the Fourier transform lens 13, condensed in the volume holographic memory 10, and formed as a Fourier transform image in the volume holographic memory 10. For example, the cylindrical holographic memory 10 is arranged so that the Fourier plane by the Fourier transform lens 13 is parallel to the rotational symmetry axis of the volume holographic memory 10.
[0034]
The photorefractive crystal volume holographic memory 10 can use a uniaxial crystal cylinder such as LiNbO ₃ having its optical crystal axis parallel to its rotational symmetry axis.
[0035]
On the other hand, in the reference light beam optical system, the reference light beam is reflected by the half mirror 19 and the mirror 20, is incident on the volume holographic memory 10, and interferes with the signal light beam from the lens 13 at a position inside the medium. Create interference fringes of dimensions. Here, the optical system such as the mirror 20 and the Fourier transform lens 13 is arranged so that the reference light and the signal light interfere not in the Fourier plane but in front of or behind the Fourier plane.
[0036]
The signal light beam and the reference light beam are arranged in a plane having a normal line perpendicular to the rotational symmetry axis of the volume holographic memory 10. As described above, when recording data, the signal light and the reference light are simultaneously irradiated onto a predetermined portion in the volume holographic memory 10 to record the interference pattern as a refractive index grating having a changed refractive index. The hologram formation time is controlled by interlocking an unillustrated automatic shutter or shutter 6a and shutter 6b of the laser light source device. When the Fourier plane is present in the volume holographic memory, the intensity of the signal light is maximum on the Fourier plane, so that the 0th-order light of the signal light on the Fourier plane having this high light intensity and the reference light interfere with each other. The photorefractive effect is saturated and nonlinear distortion of the recorded image tends to occur. The optical system of the system can be arranged so that the reference beam and the signal beam interfere with each other before or behind the Fourier plane, so that the problem of nonlinear distortion can be further avoided.
[0037]
The cylindrical volume holographic memory 10 is arranged on a means for rotating at a predetermined pitch in the direction of the optical crystal axis and rotating at a predetermined pitch around the rotational symmetry axis, that is, on a vertical movement and rotational movement mechanism.
[0038]
The vertical movement and rotation movement mechanism includes a drive unit 21 and a vertical movement mechanism 21b that is connected to the drive unit 21 and includes a rotary table 21a. The drive unit 21 is controlled to rotate and move up and down the table 21a by a controller (not shown).
[0039]
The volume holographic memory 10 is arranged on the table 21 a so that the crystal optical axis thereof coincides with the rotation axis of the drive unit 21.
[0040]
The volume holographic memory 10 is moved in the direction of the arrow A in FIG. 1 by the rotation of the driving unit 21 and simultaneously the volume holographic memory 10 is rotated in the direction of the arrow B in FIG. Due to the vertical movement of the volume holographic memory 10 in the direction of arrow A, the recording position in the volume holographic memory 10 of interference fringes produced by the reference light and the signal light is shifted in the direction of arrow A, and spatial multiplexing recording is performed. Realized.
[0041]
Further, when the volume holographic memory 10 is rotated in the direction of arrow B together with the table 21a, the recording surface of the interference pattern is rotated, and angle multiplex recording and spatial multiplex recording are realized.
[0042]
Further, at the time of reading, a detection position adjusting means 27 for moving the position of the detecting means according to a signal corresponding to a positioning image from the detecting means similar to the table 21a is further added to the CCD 22 of the light detecting means. It is. The detection position adjusting means 27 also causes the light receiving surface of the CCD 22 to have two x and y perpendicular to the optical axis of the optical path included in the z direction of the optical path of the signal light beam optical path and the meridional plane yz plane and sagittal plane xz plane, respectively. In addition to being translated in the direction, it is rotated around the optical axis of the optical path and around the two x and y directions, respectively.
[0043]
A controller (not shown) adjusts the position of the CCD 22 by moving the detection position adjusting means with a step motor or the like in accordance with a signal corresponding to the positioning image from the CCD 22 of the light detecting means. The detection position adjusting means 27 is not necessary when the apparatus manufacturing error is small, but is provided to improve the recording / reproducing accuracy.
[0044]
Here, the different wavelengths of the recording light and the reproduction light in the first embodiment will be described. It is known that the selective reflection of light of a specific wavelength at a specific angle by a Lippmann hologram or the like is caused by interference of scattered light from many layers.
[0045]
FIG. 2 is an explanatory diagram of the wavelength of the recording light and the wavelength of the reproduction light according to Embodiment 1 of the present invention. As shown in FIG. 2, the surface on which interference fringes are recorded as a refractive index grating in the volume holographic memory 10 has a plurality of layers having refractive index differences, and scatters incident light having a wavelength λ. Of the light scattered by the interference fringes recorded in a number of equally spaced layers, all the light in the direction specularly reflected by the surface of the layer is intensified because the phases are equal. Further, considering the scattered wave from the next plane, AB and AD in FIG. 2 become equiphase planes, and the optical path length from B through C to D is an integral multiple of the wavelength. When the distance is satisfied, the interval is d, and 2d · sin θ = mλ (where m is an integer and θ indicates an incident angle to each layer).
Light having a wavelength satisfying the relationship is strongly reflected. Since there is interference of scattered light from the plurality of layers, only light having a wavelength used for recording is reflected at a specific angle even when reproducing with light having a wide wavelength band. Further, no conjugate image is generated. That is, reproduction is possible with light having an integral multiple of the wavelength at the time of recording or a wavelength of an integer.
[0046]
In the first embodiment, as described above, a unit capable of generating laser light having two oscillation wavelengths is provided, and the sensitivity of the volume holographic memory 10 to the first light and the first light is doubled. The second light having the wavelength of 1 and capable of interfering with the first light is obtained. Recording is performed using the first light having a wavelength with good sensitivity, and the written light can be read without being destroyed by using the second light having a wavelength with low sensitivity as the reference light.
[0047]
As an actual operation at the time of reading, as described above, the reading operation is performed by closing the shutter 6a and the shutter 6b and using the unconverted light of the laser 15 as the reference light as described above. Can be read without destroying the written record.
[0048]
If the CCD 22 is configured to have at least sensitivity at an unconverted oscillation wavelength of the laser 15, the spectral ratio for the reference light at the beam splitter 18a can be lowered, and a further effect can be obtained. Needless to say, you can.
[0049]
Note that the two shutters can be replaced with one shutter by placing the shutter in the optical path between the wavelength selection filter 17 and the beam splitter 18.
[0050]
In the first embodiment, the light whose wavelength is converted by the SHG 16 is used as the recording light. However, depending on the material characteristics of the volume holographic memory 10 to be used and the oscillation wavelength of the laser to be used, the relationship between the wavelengths having good recording sensitivity. However, the unconverted second light may be used for recording, and the converted first light may be used for reading.
[0051]
In the hologram memory device that performs writing and reading with such a configuration, when it is intended to realize further high-density recording and high-speed recording, the performance of the spatial light modulator 12 is greatly involved.
[0052]
If the resolution of the spatial light modulator 12 is fine, the number of modulated information bits in the recording beam increases, and the amount of information that can be recorded in one place can be increased. Needless to say, if the modulation response time is short, the number of recording patterns that can be recorded per unit time increases and the recording speed increases. A conventional spatial light modulator uses a nematic liquid crystal, which is a paraelectric liquid crystal, and the response time in this case is about 0.15 s. On the other hand, the ferroelectric liquid crystal has a ferroelectric function using a smectic liquid crystal in which molecules are aligned in a layered state, takes a bistable state, and has a memory property. Also, the transition between the bistable states is as fast as several tens to several hundreds μs depending on the voltage. Although it was difficult to create a stable orientation state, shock resistance and the like were problems, element technology to solve it was developed, and spatial light modulators using it have been realized. In the first embodiment, this ferroelectric liquid crystal is used for the spatial light modulator 12 of the optical memory device.
[0053]
Further, if the surface-stabilized ferroelectric liquid crystal is used as the spatial light modulator 12 among the ferroelectric liquid crystals, the resolution can be improved. As described above, the conventional spatial light modulator uses a nematic liquid crystal which is a paraelectric liquid crystal, and the resolution in this case is about 20 lp / mm. On the other hand, it has been found that a resolution of about 300 to 400 lp / mm can be obtained by using a surface-stabilized ferroelectric liquid crystal. In other words, if this surface-stabilized ferroelectric liquid crystal is used for the spatial light modulator of the optical memory device, the pixel size can be reduced to about 1/5 of the conventional one, and the number of pixels in the recording page data is increased. Therefore, the amount of data that can be recorded by one write operation increases, and as a result, the recording capacity and recording speed of the optical memory can be further increased.
[0054]
Further, in the first embodiment, high speed can be realized by using a ferroelectric liquid crystal as the spatial light modulator 12, but since a bistable state is taken, a value therebetween cannot be taken. In other words, it can be said that it is good at high-speed binary driving. However, in the future, when further higher speed is required for information recording, multi-level gradation using multi-level gradation is required instead of binary. When the multi-value is increased, this requirement can be met by using an antiferroelectric liquid crystal as the spatial light modulator 12 instead of the ferroelectric liquid crystal. The antiferroelectric liquid crystal uses a phase transition from the antiferroelectric phase caused by an electric field to the ferroelectric phase, and in the antiferroelectric phase state where there is no electric field, the optical axis is in the normal direction of the layer. In the ferroelectric phase state where an electric field is applied, the optical axis is tilted and birefringence becomes “white (bright)”. In this way, “bright / dark” is controlled by applying an electric field, Is spatially modulated. For this liquid crystal, a method for obtaining gradation characteristics has been proposed, and the reproducibility is also excellent. As a result, the representation of one pixel in the conventional recorded page data changes from “white, black” → “0, 1” to the gradation “white, gray 1, gray 2,..., Black” → multi-value “0, 1, 2”. ,..., N ", the amount of data that can be recorded in one pixel increases, so the amount of data that can be recorded in one write operation increases, and as a result, the recording capacity and recording speed of the optical memory further increase. can do.
[0055]
【The invention's effect】
As described above, according to the present invention, by using a spatial light modulator using a ferroelectric liquid crystal, a surface-stabilized ferroelectric liquid crystal, or an anti-ferroelectric liquid crystal, high recording density and high-speed recording performance can be achieved. An excellent optical memory device can be provided.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an optical memory device according to a first embodiment of the present invention. FIG. 2 is an explanatory diagram of the wavelength of recording light and the wavelength of reproduced light according to the first embodiment of the present invention. Outline of the device and explanation of its recording principle [Explanation of symbols]
6a, 6b Shutter 10 Volume holographic memory 12, 115 Spatial light modulator 13, 116, 119 Fourier transform lens 14 Light beam expander 15 Laser 16 SHG
17 Wavelength selection filter 18 Beam splitter 19 Half mirror 20, 23 Mirror 21 Driving unit 21a Rotary table 21b Vertical movement mechanism 22, 120 CCD
27 Detection position adjusting means 125 Encoder 126 Decoder

Claims (1)

In an optical memory device in which a light beam is spatially modulated by a spatial light modulator in accordance with unit page sequence data from an encoder and an interference pattern of a plurality of light beams including the light beam is recorded, an antiferroelectric liquid crystal An optical memory device characterized by being used for a spatial light modulator and performing multi-value recording on the interference pattern by the gradation characteristics of the antiferroelectric liquid crystal .

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