JP2000081829A - Optical recording method, optical recording device, optical reproducing method, optical reproducing device and optical recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make recordable and reproducible data information in a high density and at a high speed and also at a high SN ratio without necessitating difficult controls and to make realizable a low cost of a device. SOLUTION: At the time of recording, the light passed through a polarization rotating element 26 is cast on an optical recording medium 10 as a signal light 1 by making the light passed through a beam splitter 24 by opening a shutter 25 to be incident on the polarization rotating element 26 and by modulating the polarizing angle of the light passing through the polarization rotating element 26 in accordance with multilevel information. Simultaneously, the light reflected in the beam splitter 24 is cast on an area which is irradiated with the signal light 1 of the optical recording medium 10, as a reference light 2. Thus, the polarizing angle of the signal light 1 is recorded on the medium 10 as a hologram. At the time of reprodiction, the same light as the reference light 2 at the time of recording is read out by interrupting the signal light 1 while closing the shutter 25 and it is irradiated on the medium 10 as a light. A read-out light 2 is diffracted by the hologram recorded in the medium 10 and the diffracted light 3 becomes a light in which the polarizing angle of the signal light 1 is preserved. The diffracted light 3 is made to be incident on a polarizing plate 52 and the intensity of the diffracted light 4 passed through the polarizing plate 52 is detected with a photodetector 53.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method and an apparatus for recording data information on an optical recording medium and reproducing the data information from the optical recording medium.

[0002]

2. Description of the Related Art Rewritable optical disks such as a phase change type and a magneto-optical type are already widely used. These optical disks have a higher recording density than ordinary magnetic disks, but in order to further increase the recording density, it is necessary to reduce the beam spot diameter and shorten the distance between adjacent tracks or adjacent bits. There is.

A DVD that has been put into practical use by the development of such a technique is DVD. Read-only DVD-RO
M is 4.7 GByte on one side on a 12 cm diameter disk
e can be recorded. In addition,
An erasable DVD-RAM uses a phase change method.
High-density recording of 5.2 GByte is possible on both sides of a disk having a diameter of 12 cm.

As described above, the density of optical disks has been increasing year by year. On the other hand, since the above-mentioned optical disks record data in a plane, the recording density is limited to the diffraction limit of light. 5Gbit / i called physical limit
Approaching nch ² . Therefore, in order to further increase the capacity, three-dimensional (volume type) recording including the depth direction is required.

Therefore, as a next-generation computer file memory, large capacity derived from a three-dimensional recording area and 2
A hologram memory that has high speed derived from the one-dimensional batch recording / reproducing method has attracted 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 out collectively 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 the SN ratio and bit error rate based on the volume multiplexing recording method, or proposal of two-dimensional encoding have been proposed. Research from a more optical point of view is also progressing.

FIG. 5 shows a document “D. Psaltis, M .;
Levene, A .; Pu, G .; Barbastathi
s and K.S. Curtis; OPTICS LET
TERS Vol. 20, no. 7 (1995) p78
2 shows a shift multiplex recording method as an example of the volume multiplex recording method.

In the shift multiplex recording method, a spherical wave is used as the reference light 32 for irradiating the hologram recording medium 35 at the same time as the signal light 31, and the hologram recording medium 35 is formed in a disk shape. Overwrite the hologram. 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 disk 35 by several tens of μm. This is based on the fact that the reference light 32 is a spherical wave, which is equivalent to a change in the angle of the reference light 32 due to the movement of the disk 35.

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, as shown in the above-mentioned document, δspherical = δBragg + δNA ≒ (λzo / Ltanθs) + λ / 2 (NA) (1) 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 intersection angle between the signal 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 multiplicity can be increased, and the recording capacity can be increased. Further, in order to increase the recording capacity more effectively by the shift multiplex recording method, the recording area may be miniaturized. By performing multiplex recording in a minute area, higher-density volume multiplex recording can be realized.

[0010] For the above purpose, in the hologram memory system, the signal light is Fourier-transformed by a lens and irradiated on a recording medium. As a result, when the image of the signal light has a fine pitch (high spatial frequency), the signal light is subjected to franfofer diffraction on the surface of the recording medium, and the spread 回 折 of the diffraction image is represented by ζ = kλfωx (2) You. Here, k is a proportional constant, λ is the wavelength of the signal light, f is the focal length of the Fourier transform lens, and ωx is the spatial frequency of the signal light.

To record digital data at high density,
It is desired to reduce the area of one pixel of the data image and pack more bit data in one page. Thus, high-speed recording and reproduction can be realized in addition to high-density recording.

However, when the area of one pixel is reduced, the Fourier transform image of the data image of the signal light spreads on the optical recording medium according to the equation (2). This is because the spatial frequency ωx increases as the data image of the signal light becomes finer.

As a method of avoiding this, a method of reducing the wavelength λ of the signal light or a method of forming a Fourier transform image of the signal light by using a lens having a short focal length f can be considered. However, there is a limit to shortening the wavelength λ of the light source or shortening the focal length f of the lens. Further, in principle, the Fourier transform image has an infinite spread at the focal plane, and thus it is difficult at present to miniaturize the recording area.

Therefore, it has been considered that the recording area is miniaturized by disposing an aperture in front of the recording medium. For example, JP-A-55-41480 discloses that
As shown in FIG. 6, by disposing an aperture board 39 having a circular aperture 38 whose light transmittance gradually increases toward the center in front of the recording medium, unnecessary spread of the signal light and the reference light can be prevented. Restrict,
It is shown that the recording area is miniaturized.

[0015]

However, even with this method, if the Fourier transform image becomes large due to the high density, the aperture diameter must be increased, and in practice, it is expected that a sufficient high density is expected. Can not.

Further, in the digital hologram memory system, a two-dimensional spatial light modulator is used for data input during recording, and a two-dimensional photodetector array is used for data detection during reproduction. There is a problem of becoming high.

As a method of solving this problem, there is a device of multi-valued which is also considered in other memory technologies. For example, Japanese Patent Application Laid-Open No. 2-180116 discloses a method of arranging a shutter array in an optical path of a signal light to form a signal light whose intensity (density) is modulated according to multi-valued information, and recording this as a hologram. It is shown.

However, in the hologram, when the intensity of the recording light changes, the exposure condition for recording changes, and the diffraction efficiency at the time of reproduction changes. Therefore, the above method makes it difficult to control the exposure at the time of recording. There are drawbacks. Further, if the recording intensity is reduced by modulating the intensity, there is a problem that it is difficult to reproduce a signal with a low intensity at a high SN ratio due to noise in a recording medium or fluctuation in signal intensity due to multiplexing.

Therefore, the present invention provides a method of recording data information on an optical recording medium and reproducing the data information from the optical recording medium.
It is possible to record and reproduce data information at a high density and at a high speed with a high SN ratio without requiring difficult control, and to realize a reduction in the cost of the apparatus.

[0020]

According to the optical recording method of the present invention, a signal light for holding multi-valued information is formed by a polarization rotator by a difference in polarization angle, and the signal light and the reference light are simultaneously recorded on an optical recording medium. , The polarization angle of the signal light is recorded as a hologram in the optical recording medium.

According to the optical reproducing method of the present invention, the signal light for holding the multi-valued information due to the difference in the polarization angle is irradiated with the reading light onto the optical recording medium recorded as the hologram by the reference light, and the signal light is read from the hologram. The multi-valued information is read out according to the polarization angle of the diffracted light.

[0022]

A material exhibiting light-induced birefringence (light-induced anisotropy, light-induced dichroism) is sensitive to the polarization state of light incident thereon and records the polarization angle (polarization direction) of the incident light. be able to. For example, a polymer having a group that photoisomerizes in a side chain or a polymer liquid crystal, or a polymer in which a molecule that photoisomerizes is dispersed, is irradiated with linearly polarized light, photoisomerization is induced, and linearly polarized light is irradiated. Anisotropy of the refractive index occurs depending on the direction, and the polarization direction can be recorded and stored. At this time, by simultaneously irradiating the reference light, the polarization angle of the signal light can be recorded as a hologram.

Focusing on this point, as described above, in the optical recording method of the present invention, a signal light for holding multi-valued information is formed by a polarization rotator by a difference in polarization angle, and the signal light is referred to. By simultaneously irradiating the optical recording medium with light, the polarization angle of the signal light is recorded as a hologram in the optical recording medium, and in the optical reproduction method of the present invention, the multi-value information is held by the difference in the polarization angle as described above. The optical recording medium in which the signal light is recorded as a hologram is irradiated with readout light to read out multi-valued information based on the polarization angle of the diffracted light from the hologram.

Therefore, according to the present invention, multivalued information can be recorded on an optical recording medium and reproduced from the optical recording medium.

Moreover, since the intensity of the signal light is not modulated, but the polarization angle of the signal light is modulated in accordance with the multi-valued information,
Regardless of the gradation (value) of the multi-valued information, the intensity of the signal light can be made constant and strong, and the multi-valued information can be recorded at a high SN ratio, and the intensity can be modulated. Does not require any difficult control. Further, the intensity of the hologram diffracted light at the time of reproduction is constant irrespective of the gradation of the multi-valued information, so that the multi-valued information can be reproduced at a high SN ratio regardless of the gradation of the multi-valued information.

Further, by condensing the signal light by a condensing optical system used in a CD player or the like, the recording area can be miniaturized to the order of μm, and angle multiplex recording and shift multiplex recording are advantageous as holograms. Since it can be combined with a multiplex recording method such as recording, high-density recording becomes possible. Also, since the information is multi-valued, high-speed transfer is possible.

Further, at the time of recording, the polarization angle of the signal light is modulated in accordance with the multi-value information by the polarization rotation element which can rotate the polarization angle of the transmitted light. do not need. During reproduction, the hologram diffracted light is transmitted through a polarizing plate, etc., and the difference in the polarization angle of the diffracted light is converted into a difference in intensity, and the intensity is detected by a photodetector to reproduce multi-valued information. Therefore, a two-dimensional element such as a two-dimensional photodetector array is not required. Therefore, cost reduction of the device can be realized.

[0028]

FIG. 1 shows an example of an optical recording medium used in an optical recording method according to the present invention, in which a polarization sensitive layer 12 is formed on one surface of a transparent substrate 11 such as a glass substrate. The signal light and reference light at the time of recording and the reading light at the time of reproduction irradiate the polarization sensitive layer 12 side.

The polarization-sensitive layer 12 may be made of any material as long as it exhibits photoinduced birefringence and can record polarization information as a hologram. Having a polymer or polymer liquid crystal,
Alternatively, a polymer in which photoisomerizable molecules are dispersed can be used. Further, as the photoisomerizable group or molecule, for example, those containing an azobenzene skeleton are preferable.

As a preferred example of the polarization sensitive layer 12, a polyester having cyanoazobenzene in a side chain represented by a chemical formula shown in FIG. 2 can be used. As described in detail in Japanese Patent Application No. 10-32834, this material can record, reproduce, and erase holograms having polarization information by photoinduced isomerization due to photoisomerization of side chain cyanoazobenzene. It is. Further, the recorded hologram is maintained without relaxation for several years or more under room-temperature natural light.

In order to record a hologram volumetrically (three-dimensionally), the thickness of the polarization sensitive layer 12 needs to be at least about 10 μm. As the thickness is increased, the storage capacity can be increased. Note that the entire optical recording medium 10 may be formed as a polarization-sensitive layer exhibiting photoinduced birefringence.

FIG. 3 shows an embodiment of an optical recording method or apparatus and an optical reproducing method or apparatus according to the present invention.

As the light source 21 of the recording / reproducing head 20,
A material that emits sensitive coherent light is used for the polarization sensitive layer of the optical recording medium 10. For example, as a polarization sensitive layer,
When a polyester having cyanoazobenzene in the side chain shown in FIG. 2 is used, a wavelength 515 sensitive to this is used.
nm argon laser is used.

The light from the light source 21 is transmitted to the spatial filter 2.
After passing through 2 to remove the disturbance of the wavefront, the light is converted into parallel light by a lens 23 and further split into two light beams by a beam splitter 24.

At the time of recording, the shutter 25 is opened, the light transmitted through the beam splitter 24 is made incident on the polarization rotation element 26, and the polarization rotation element 26 is turned on in accordance with multi-value information transferred from a control circuit (not shown). By controlling, the polarization angle of the light transmitted through the polarization rotation element 26 is modulated. A liquid crystal valve, a Pockels element, a Faraday element, or the like can be used as the polarization rotation element 26 that can rotate the polarization angle of transmitted light as described above.

The light whose polarization angle has been modulated according to the multi-valued information is condensed by the lens 27 as the signal light 1 by passing through the polarization rotator 26 in this manner, and the light of the optical recording medium 10 is polarized. Irradiate the sensitive layer.

At the same time, the light reflected by the beam splitter 24 is reflected as a reference light 2 by a mirror 28a, condensed by a lens 29, reflected by a mirror 28b, and irradiated with the signal light 1 of the optical recording medium 10. Irradiate the area to be illuminated.
Thus, the polarization angle of the signal light 1 is recorded as a hologram in the polarization sensitive layer of the optical recording medium 10.

At the time of reproduction, the signal light 1 is blocked by closing the shutter 25, and the same light as the reference light 2 at the time of recording is applied as read light to the area of the optical recording medium 10 where the hologram is recorded. The irradiated read light 2 is applied to the optical recording medium 10.
Is diffracted by the hologram recorded in the hologram, and the diffracted light 3 preserves the polarization angle of the signal light 1 recorded as a hologram.

The diffracted light 3 is converted into parallel light by a lens 51 and is incident on a polarizing plate 52. The intensity of the diffracted light 4 transmitted through the polarizing plate 52 is detected by a photodetector 53.

The polarization angle of the diffracted light 3 before passing through the polarizing plate 52 depends on the polarization angle of the signal light 1 recorded as a hologram. The intensity changes according to the polarization angle of the recorded signal light 1. Therefore, by detecting the intensity of the diffracted light 4 transmitted through the polarizing plate 52 with the photodetector 53, the recorded multi-valued information can be reproduced.

With the above-described method or apparatus, recording and reproduction of multi-valued information were actually attempted. As the optical recording medium 10, a polarization-sensitive layer formed of polyester having cyanoazobenzene in a side chain was used, and as the light source 21, the above-described argon laser having a wavelength of 515 nm was used.

FIG. 4 shows the results. This was measured by changing the polarization angle of the signal light 1 from 0 degree to 90 degrees at 10-degree intervals by the polarization rotation element 26, and setting the polarizing plate 52 to transmit light having a polarization angle of 90 degrees. The result.

In the theoretical recording / reproducing characteristics in this setting, the relationship between the input angle (the polarization angle of the signal light 1) and the detection intensity of the photodetector 53 becomes a cosine function.
It was almost the same, and it was confirmed that the multi-valued information could be recorded and reproduced by the difference in the polarization angle. This is because, as described above, the signal light 1 can be recorded and reproduced with a constant high intensity regardless of the gradation of the multi-valued information.

The characteristic shown in FIG. 4 has poor linearity. Therefore, while the linear recording / reproducing characteristics are obtained, the polarization angle of the transmitted light is changed by at least about 1 degree in the polarization rotator 26, and the photodetector 53 also has an intensity of about 1% on the strong side where the SN ratio increases. When recording and reproduction were performed in a state where a sufficient noise margin was set for the polarization rotation element 26 and the photodetector 53 so as to detect the change, recording and reproduction of 64 gradations were performed.
This is a result of setting a considerably large noise margin, and it is considered that recording and reproduction of a larger number of gradations can be realized by optimization.

In the embodiment shown in FIG. 3, the optical recording medium 10 is formed into a disk shape, and the optical recording medium 10 is rotated by a motor 40, thereby recording a plurality of holograms at different locations in the circumferential direction of the optical recording medium 10. can do.
In this case, if a spherical wave is used as the reference light 2, shift multiplex recording can be performed. Also, the head moving mechanism 60
By moving the entire recording / reproducing head 20 in the radial direction of the optical recording medium 10, the hologram can be recorded in the optical recording medium 10 so as to form concentric recording tracks.

The embodiment shown in FIG. 3 is a case in which recording and reproduction can be performed by one device, but it is also possible to use a recording-only or reproduction-only device. In an apparatus dedicated to recording, the lens 51, the polarizing plate 52, and the photodetector 53 are unnecessary, and by excluding them, it is possible to reduce the size and weight of the recording head and reduce the cost of the recording apparatus. In a playback-only device, the shutter 25, the polarization rotation element 26 and the lens 27, and the beam splitter 24 depending on the configuration are unnecessary, and by eliminating these components, the size and weight of the playback head and the cost of the playback device can be reduced. Can be realized.

[0047]

As described above, according to the present invention, data information can be recorded / reproduced at a high density and a high speed with a high SN ratio without requiring difficult control. According to the present invention, higher density and higher speed can be realized by at least about one digit as compared with the case of recording and reproducing binary information. Furthermore, shift multiplex recording,
By combining with multiplex recording methods such as angle multiplex recording and wavelength multiplex recording, a memory system with a larger capacity can be realized.

Further, according to the present invention, since a two-dimensional element such as a two-dimensional spatial light modulator or a photodetector array is not required, the cost of the apparatus can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of an optical recording medium used for an optical recording method of the present invention.

FIG. 2 is a diagram showing a chemical formula of an example of a material of a polarization sensitive layer of an optical recording medium.

FIG. 3 is a diagram showing an embodiment of an optical recording method or apparatus and an optical reproducing method or apparatus of the present invention.

FIG. 4 is a diagram showing a result when actually recording and reproducing multi-valued information.

FIG. 5 is a diagram for explaining a shift multiplex recording method.

FIG. 6 is a diagram for explaining a conventional optical recording method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Signal light 2 ... Reference light, readout light 3,4 ... Diffraction light 10 ... Optical recording medium 12 ... Polarization sensitive layer 20 ... Recording / reproducing head 21 ... Light source 24 ... Beam splitter 25 ... Shutter 26 ... Polarization rotating element 52 ... Polarization Plate 53: Photodetector

Claims (10)

[Claims] A signal light for holding multi-value information is formed by a polarization rotator according to a difference in polarization angle, and the signal light and a reference light are simultaneously irradiated on an optical recording medium, so that the optical recording medium has An optical recording method for recording the polarization angle of the signal light as a hologram. 2. The optical recording method according to claim 1, wherein the optical recording medium is formed in a disk shape, the optical recording medium is rotated, and a recording head including the polarization rotating element is moved in a radial direction of the optical recording medium. Optical recording method to be moved. 3. A light source that emits coherent light, and a polarization rotator that modulates a polarization angle of light from the light source according to the multi-valued information to form signal light that holds multi-valued information according to a difference in polarization angle. An imaging optical system that condenses the signal light and irradiates the optical recording medium, and a reference light optical system that forms reference light that irradiates the optical recording medium from light from the light source. Recording device. 4. An optical recording apparatus according to claim 3, wherein said optical recording medium is disk-shaped, said optical recording apparatus comprises: a medium driving mechanism for rotating said optical recording medium; An optical recording apparatus comprising: a head moving mechanism that moves a recording head including an image optical system and a reference light optical system in a radial direction of the optical recording medium. 5. An optical recording medium in which signal light for holding multi-valued information due to a difference in polarization angle is recorded as a hologram by reference light. 6. An optical recording medium recorded as a hologram with reference light is irradiated with readout light by a signal light holding multi-value information due to a difference in polarization angle, and a polarization angle of diffracted light from the hologram is read. An optical reproducing method for reading the multi-valued information by the method. 7. The light reproducing method according to claim 6, wherein the multi-value information is read by detecting a polarization angle of the diffracted light from the hologram as a light intensity. 8. The optical reproducing method according to claim 6, wherein said optical recording medium is disk-shaped, said optical recording medium is rotated, and a reproducing head including said read-out light optical system is optically recorded by said optical recording medium. An optical reproduction method for moving a medium in the radial direction. 9. A light source for emitting coherent light, and a signal light for holding multi-valued information based on a difference in polarization angle from the light from the light source, for reading by irradiating an optical recording medium recorded as a hologram with reference light. An optical reproduction apparatus comprising: a readout optical system that forms light; a polarizing plate that detects a polarization angle of diffracted light from the hologram as light intensity; and a photodetector. 10. The optical reproducing apparatus according to claim 9, wherein the optical recording medium has a disk shape, the optical reproducing apparatus comprises: a medium driving mechanism for rotating the optical recording medium; A head moving mechanism for moving a reproducing head including a polarizing plate and a photodetector in a radial direction of the optical recording medium.