JPH11232691A - Optical record medium, optical recording reproduction method and optical recording reproduction device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical record medium, an optical recording reproduction method and an optical recording reproduction device, by which high-density recording is carried out by multivaluing. SOLUTION: When double refraction is induced by the recording light of (s) polarized light at a polarizing angle 0 deg. at the time of the recording of information, the azimuth of a fine wave plate (such as a 1/4 wave plate) in a recording region is recorded in the 0 deg. direction. When double refraction is induced by recording light at a polarizing angle 45 deg., the azimuth of the above-mentioned fine wave plate is recorded in the 45 deg. direction. Binary recording is conducted by utilizing the difference of the recorded azimuths. '0' can be set when the polarizing angle of reflected light from the recording region is (s) polarized light (bright) and '1' at the time of (p) polarized light (dark). Accordingly, '1' is recorded by conducting recording, by which reflected light reaches (p) polarized light, and '0' is recorded by performing recording, by which reflected light reaches (s) polarized light. Reflected light is detected and reproduced at the time of reproduction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical recording medium, an optical recording / reproducing method, and an optical recording / reproducing apparatus capable of recording data at high density and transmitting data at high speed.

[0002]

2. Description of the Related Art Rewritable optical disk devices of the phase change type, the magneto-optical type and the like are already widely used. These optical disk devices have a recording density one order of magnitude higher than that of general magnetic disk devices, but are not yet sufficient for digital recording of image information. In order to increase the recording density, it is necessary to reduce the beam spot diameter and shorten the distance between adjacent tracks or adjacent pits.

A DVD-ROM is already being put into practical use due to the development of such technology. DVD-
ROM is 4.7 GB on one side of a 12 cm disk
To accommodate the data. Writable / erasable DVD-
The RAM is capable of high-density recording of 5.2 Gbytes on both sides of a disk having a diameter of 12 cm by a phase change method. This can write and read information of a capacity equivalent to 7 times or more of a read-only CD-ROM, and a capacity of 3600 or more floppy disks. As described above, the density of optical discs is increasing year by year. However, on the other hand, the above-mentioned optical disc is limited to the diffraction limit of light in order to record data in a plane, and the recording density is approaching 5 Gbit / inch ² which is called the physical limit of high-density recording.

In order to realize higher density and higher speed, a technique for recording multi-value data in one recording pit has been proposed. For example, Japanese Patent Laid-Open No.
Japanese Patent Application Laid-Open No. 6-64294 discloses a technique for recording multi-valued information by increasing the size of a recording pit stepwise. Japanese Patent Application Laid-Open No. Hei 1-204225 discloses a technique for recording multi-valued information by changing only the length of a recording pit without changing the width of the recording pit.

[0005]

In the above-described technique of increasing the size of recording pits in a stepwise manner, the size of the minimum recording pit is different from that of the conventional binary type in consideration of application to a rewritable optical memory. Absent. The recording pit containing the maximum multi-valued information increases to a multiple of the minimum recording pit size when trying to increase the number of multi-values, so this technology can perform multi-value recording, but the recording area used is large. There is a problem that the effect due to the multi-value is reduced by the amount.

On the other hand, even in the technique of changing only the length of the recording pit, the minimum recording pit size is not different from that of the conventional binary type. It becomes several times larger than the minimum recording pit size in the length direction. This technique is more efficient than the former technique in that the size in the width direction is not increased, but the effect of the multi-value is reduced due to the increase in the recording pit area.

As described above, in the conventional technique, since the minimum recording pit size cannot be reduced, there is a problem that the effect of sufficient multi-value cannot be obtained.

Accordingly, an object of the present invention is to provide an optical recording medium, an optical recording / reproducing method, and an optical recording / reproducing apparatus which enable high-density recording by multi-value recording.

[0009]

The object of the present invention is to provide an optical recording layer in which an optical recording material having photo-induced birefringence functions as a half-wave plate and the orientation and length of the half-wave plate. Is recorded by multi-level modulation and recording. Similarly, the azimuth and length of the quarter-wave plate are multilevel-modulated and recorded on the optical recording layer configured so that the optical recording material having photoinduced birefringence functions as a quarter-wave plate. Is achieved by

Therefore, the optical recording medium according to the present invention is manufactured so as to include at least one optical recording layer composed of an optical recording material having photoinduced birefringence, and the optical recording layer is irradiated with light to be one layer. It is configured to function as a half-wave plate or a quarter-wave plate.

In the optical recording method according to the present invention, a medium is irradiated with linearly polarized recording light to induce birefringence, and a half-wave plate or a quarter-wave plate is formed in the direction of the plane of polarization. Binary recording is possible using the azimuth (polarization angle) of the polarization plane of the recording light. Since this binary recording can be rewritten by overwriting existing data, by adjusting the rewriting timing, a minimum recording pit can be produced with an area smaller than the laser spot diameter used for recording. The length of the recording pit can be changed by extending the area to be irradiated with the laser beam in the length direction of the recording pit. As described above, in the present invention, the length of the recording pit is modulated by the multivalued information to be recorded, and the multilevel recording is performed.

In the optical reproducing method of the present invention, a half-wave plate or a quarter-wave plate recorded in a specific direction by recording light is irradiated with reproduction light having an arbitrary polarization angle, and the transmitted light is transmitted. Alternatively, the recorded content is reproduced by detecting a difference in light intensity due to a change in the polarization angle of the reflected light. At this time, the temporal change of the light intensity to be reproduced corresponds to the length of the multi-value recorded recording pit, so that the multi-value recording can be reproduced.

An optical recording apparatus according to the present invention comprises a light source for generating recording light, a spatial light modulator for changing a polarization angle of the recording light based on multi-value data, and an optical recording device for recording light from the spatial light modulator. An imaging optical system for irradiating the medium. Here, the spatial light modulator uses a polarization rotation element that rotates the polarization of light from a light source. Thus, the axis of the half-wave plate or the quarter-wave plate in the optical recording medium is binary-recorded, and the width and time for the binary recording are multi-level modulated and recorded.

An optical reproducing apparatus according to the present invention is a reproducing optical system for irradiating a reproducing light to an optical recording medium on which the azimuth and length of a half-wave plate or a quarter-wave plate are multi-level modulated and recorded. And an analyzing means for detecting a polarization angle of transmitted light or reflected light from the optical recording medium. Thereby, the data information recorded by the multi-level modulation is reproduced.

By doing so, according to the present invention, it is possible to obtain an optical recording medium, an optical recording / reproducing method and an optical recording / reproducing apparatus which enable high-density recording by multi-valued recording.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The principle of binary recording of a polarization angle will be described below. According to the recording / reproducing method of the present invention, the reflected light in the recording area with respect to the linearly polarized reproduction light becomes linearly polarized light, and the light has two types of polarization angles depending on the recording state. Therefore, the recording area was made to function as a quarter-wave plate. The function will be described with reference to FIG. In the 波長 wavelength plate, the optical path difference Δ between the fast axis and the slow axis is set to the following (Equation 1).

[0017]

Δ = (m + ／) * λ (Expression 1)

Here, m is an integer and λ is a wavelength. Therefore, when the light passes through this quarter-wave plate, the following changes occur. [1] If the incident light is linearly polarized light, it becomes circularly polarized light. [2] If the incident light is circularly polarized light, it becomes linearly polarized light.

First, as shown in FIG. 1, the angle .theta.
When linearly polarized light A having a polarization angle of 0 ° <θ <90 ° is applied to a 波長 wavelength plate and passed therethrough, the transmitted light B
Becomes counterclockwise circularly polarized light. Thereafter, when the transmitted light B is reflected by the reflection plate, the reflected light C becomes clockwise circularly polarized light. Next, when the reflected light C enters the quarter-wave plate from the opposite side, the transmitted light D becomes linearly polarized light having a polarization angle of an angle θ on the opposite side to the first incidence with respect to the fast axis. . From this, the first incident light A and the reflected light D finally obtained by reflection are at an angle of 2θ about the fast axis.

The combination of a reflector and a quarter-wave plate used for recording / reproducing in the reflection type has been described above. However, in the case of performing recording and reproduction in a transmission type, the same effect can be obtained by making the recording area function as a half-wave plate. This will be described with reference to FIG.
In the 波長 wavelength plate, the optical path difference Δ between the fast axis and the slow axis is set to the following (Equation 2).

[0021]

Δ = (m + ／) * λ (Expression 2)

Here, m is an integer and λ is a wavelength. FIG.
And the angle θ with respect to the fast axis (where 0 degree <θ <90
When the linearly-polarized light A having a polarization angle of (degree) is irradiated on a half-wave plate and passed therethrough, the transmitted light B is a linearly-polarized light having a polarization angle of θ on the opposite side to the fast axis. Becomes Therefore, by utilizing the function of the half-wave plate, it is possible to obtain transmitted light having an angle of 2θ with the incident linearly polarized light by controlling the angle θ between the fast axis and the incident linearly polarized light.

As described above, regardless of whether a combination of a reflector and a 板 wavelength plate or a 波長 wavelength plate is used,
By controlling the polarization angle θ of the incident linearly polarized light, the polarization angle of the transmitted or reflected linearly polarized light can be made 2θ with respect to the incident light.

Next, an optical recording material exhibiting the effects of the half-wave plate and the quarter-wave plate described above will be described.

As such a recording material, any material may be used as long as it exhibits light-induced birefringence and the birefringence is recorded and held. Here, the light-induced birefringence means that anisotropic medium is irradiated with light to cause anisotropy of refractive index (birefringence). As an example of a material exhibiting such photoinduced birefringence, there is a polymer compound or a polymer liquid crystal having a photoisomerizable group in a side chain, or a polymer material in which molecules to be photoisomerized are dispersed. This material is macroscopically isotropic, but irradiation with linearly polarized light induces photoisomerization, which results in anisotropy in the refractive index. As the group or molecule to be photoisomerized, a group or molecule having a large birefringence due to isomerization is desired. For example, a group or molecule containing an azobenzene skeleton is preferable. As a polymer compound or a polymer liquid crystal material having a photoisomerizable group or molecule, the induced anisotropy of the photoisomerizable group is transmitted to the polymer compound or the polymer liquid crystal, and as a result, the polymer compound or the polymer It is desired that a large birefringence occurs in the entire molecular liquid crystal and the birefringence is recorded. For example, a polymer compound or a polymer liquid crystal, which is at least one monomer polymer selected from the polyester group, can be used. In particular,
Polymethyl methacrylate and polyvinyl alcohol are preferred. Hereinafter, azobenzene will be described as an example of the photoisomerizable group.

Azobenzene is transformed into trans-
3 shows photoisomerization of cis. In the case of the trans form, the molecular structure is as shown in FIG. 3A, while in the case of the cis form, the molecular structure is as shown in FIG. 3B.

Azobenzene alone shows anisotropy,
When randomly dispersed in the recording material as shown in FIG. 4A, the recording material as a whole exhibits isotropic properties. Also, in the recording material, there are many trans-types before the photoexcitation. On the other hand, the trans-form changes to the cis-form by photoexcitation, and many cis-forms are present in the material. In particular, when the material is irradiated with linearly polarized pump light having a certain polarization direction, only azobenzene, which is in the same direction as the polarization direction, absorbs light and changes to the cis-type as shown in FIG. In this case, the birefringence of azobenzene itself caused by isomerization of azobenzene and the birefringence of a polymer compound or polymer liquid crystal induced by isomerization of azobenzene are combined to change the polarization direction of pump light in the optical recording medium. Birefringence occurs as an axis. By utilizing the birefringence, the polymer film can function as the above-described wavelength plate.

First, consider the case where it is used as a 波長 wavelength plate. Let d be the thickness of the polymer film and Δn be the photoinduced refractive index change.
Then, the optical path difference that occurs when light having the wavelength λ passes through the polymer film is Δn · d. So this is just λ
At / 4, the polymer film functions as a quarter-wave plate. That is, birefringence may be induced so as to satisfy the following condition (Equation 3).

[0029]

[Expression 3] Δn · d = λ / 4 (Expression 3)

Similarly, when used as a half-wave plate, the optical path difference Δn · d should just be λ / 2.
That is, birefringence may be induced so as to satisfy the following condition (Equation 4).

[0031]

Δn · d = λ / 2 (Equation 4)

Next, the recording characteristics of the optical recording material used in the present invention will be described with reference to the drawings. As the optical recording material, a polyester having cyanoazobenzene in the side chain shown in FIG. 3C is used. FIGS. 5A to 5D are diagrams each showing a structure of an optical recording medium 11 using the present material. FIGS. 5A and 5B show an embodiment of an optical recording medium used in a system for recording and reproducing by transmitting light. The optical recording medium 11 shown in FIG. 5A is manufactured by forming an optical recording layer 12 using the above-described optical recording material on one surface on a transparent substrate 13 such as a glass substrate. As another embodiment, the optical recording medium 11 may be formed of only the optical recording layer 12 as shown in FIG. FIGS. 5C and 5D show an embodiment of an optical recording medium used in a method of recording and reproducing by reflecting light. FIG.
In the optical recording medium 11 shown in (c), a reflective film 14 is first formed on one surface of a substrate 13 such as glass or resin by aluminum evaporation or the like, and the above-described optical recording layer 12 is formed on one surface. It is made by doing. As another embodiment, as shown in FIG.
May be formed as a two-layer structure of the reflective layer 14 and the optical recording layer 12. In any of the embodiments, the shape of the optical recording medium may be a sheet or a disk.

Here, for the optical recording medium 11 having the shape shown in FIG. 5A, the recording characteristics of light-induced birefringence were measured by the optical system shown in FIG. The thickness of the optical recording layer 12 is 2
μm was used.

As shown in FIG. 6, the light source 63 of the pump light 65 used to induce anisotropy (birefringence) in the optical recording medium 11 is sensitive to polyester having cyanoazobenzene in the side chain. A 515 nm argon ion laser was used. The polarization of the light emitted from the light source 63 is s-polarized light (perpendicular to the plane of the paper), and the laser light passes through a half-wave plate 62,
Irradiate the optical recording medium 11. The optical recording medium 1 is changed by changing the polarization angle of the pump light 65 with the half-wave plate 62.
The direction of the anisotropy induced by 1 changes. The direction of the induced anisotropy is measured by the probe light 66 generated by the light source 61 different from the pump light. As the light source 61, a 633 nm helium neon laser having a wavelength that does not affect the anisotropy induced in the optical recording medium 11 was used. This laser light was transmitted through the polarizer 64 to obtain s-polarized light (perpendicular to the paper). This light is applied to the optical recording medium 11, and the transmitted light is guided to the analyzer 67. If birefringence is induced in the optical recording medium 11 by the irradiation of the pump light 65, the polarization direction of the probe light 66 should rotate by transmitting through the optical recording medium 11. Then, by rotating the analyzer 67, the polarization direction of the probe light 66 transmitted through the optical recording medium 11 is checked.

When optically inducing the birefringence, the probe light 66 is not irradiated, and only the pump light 65 is applied to the optical recording medium 11.
Irradiation. Recording was performed with the light intensity of the pump light 65 set to 1 W / cm ² . After that, the irradiation of the pump light 65 was stopped, and the light-induced birefringence induced only by the probe light 66 was measured.

FIGS. 7A to 7D show changes in the polarization angle of the pump light 65 to induce birefringence in the optical recording medium 11 and allow the probe light 66 to pass therethrough to measure the birefringence recording characteristics. It is a figure which shows the result. The horizontal axis indicates the polarization rotation angle of the analyzer 67, and 90 degrees coincides with the s-polarized light. The vertical axis is the analyzer 67
Is the transmitted light intensity. The characteristics of the transmitted probe light 66 when the pump light 65 is not irradiated are as shown in FIG. As can be seen from the figure, since the analyzer transmission intensity increases at a polarization angle of 90 degrees, the probe light 66 is s-polarized light. Before irradiation with the pump light 65, the probe light 6 is isotropic.
6 does not change the polarization angle when transmitted.
Next, the pump light 65 is converted into s-polarized light by the half-wave plate 62,
FIG. 7 shows the result of recording the light-induced birefringence on the optical recording medium 11.
(B). It can be seen from the figure that the analyzer transmission intensity increases at 80 degrees and 260 degrees. From this, the pump light 6
5 indicates that birefringence is recorded on the optical recording medium 11. Next, the pump light 65 is changed by the half-wave plate 62 to s.
FIG. 7C shows the result of recording the light-induced birefringence on the optical recording medium 11 by changing the polarization angle by 30 degrees from the polarization. It can be seen from the figure that the analyzer transmission intensity increases at the polarization angles of 40 degrees and 220 degrees. This indicates that the azimuth of the birefringence recorded on the optical recording medium 11 was shifted by 40 degrees by changing the polarization angle of the pump light 65. Further, the pump light 65 is s
FIG. 7D shows the result of recording the light-induced birefringence on the optical recording medium 11 by changing the polarization angle by 60 degrees from the polarization. It can be seen from the figure that the analyzer transmission intensity increases at a polarization angle of 130 degrees. This indicates that the azimuth of the birefringence recorded on the optical recording medium 11 is further shifted by 90 degrees by changing the polarization angle of the pump light 65.

From the above results, in the polyester having cyanoazobenzene in the side chain, it is possible to record by inducing birefringence, and by rotating the polarization angle of the pump light, the birefringence It turns out that the bearing can be rotated. It was also confirmed that the recorded light-induced birefringence was maintained at room temperature for a sufficiently long period. Further, the above-mentioned respective recordings in which the polarization angle of the pump light 65 is changed are performed in the same area.
In particular, no process such as erasing the previous record was performed. From this, it was also confirmed that the next data can be overwritten on the optical recording medium 11 without erasing the previous recording.

The optically induced refractive index change of the optical recording material used this time was measured by a measuring system shown in FIG. FIG. 9 shows the measurement results.
Shown in Also in this measurement, the optical recording medium 11 having the shape shown in FIG. The thickness of the optical recording layer 12 was 2 μm.

As shown in FIG. 8, the light source 83 of the pump light 85 used to induce anisotropy in the optical recording material is a 515 nm argon ion laser sensitive to polyester having cyanoazobenzene in the side chain. Using. Light source 83
The polarization of the light emitted from the optical recording medium 11 is s-polarized light (perpendicular to the plane of the paper), and the laser light passes through a half-wave plate 82 and irradiates the optical recording medium 11. This half-wave plate 82 allows pump light 8
The polarization angle of No. 5 was changed from the s-polarized light by 45 degrees. The birefringent azimuth induced by the pump light 85 is measured by a probe light 86 generated by a light source 81 different from the pump light. As the light source 81, a 633 nm helium neon laser having a wavelength that does not affect the anisotropy induced in the optical recording medium 11 was used. This laser light was transmitted through a polarizer 84 to be s-polarized light (perpendicular to the paper). This light is applied to the optical recording medium 11, and the transmitted light is guided to a polarizing beam splitter (PBS) 87. The polarization beam splitter 87 separates the s-polarized light component and the p-polarized light component of the transmitted probe light 86. By measuring the components with the optical power meters I1 and I2, the polarization direction of the probe light 86 transmitted through the optical recording medium 11 can be checked. Then, the refractive index change Δn is obtained from the measured polarization angle.

In the measurement, the initialized optical recording medium 11 is irradiated with pump light 85 to induce birefringence. The light intensity of the pump light 85 was 1 W / cm ² . At the same time, the probe light 86 was irradiated, and the polarization directions of the probe light 86 transmitted through the optical recording medium 11 were measured by the optical power meters I1 and I2.

FIG. 9 shows the result of the refractive index change Δn converted from the measured polarization direction of the probe light 86. Here, it was assumed that the light-induced dichroism Δα was negligible. The horizontal axis represents the irradiation amount (product of intensity and time) of the pump light, and the saturation point of the refractive index change Δn is indicated by 1.0. The vertical axis shows the magnitude of the refractive index change Δn obtained by the measurement. From this figure, it is understood that the refractive index change Δn due to the birefringence induced by the pump light 85 increases with the irradiation amount of the pump light and saturates. When the change of the saturated refractive index Δns is obtained from this figure, it is about 0.033 in the present material.

As described above, in order to use this optical recording material as a half-wave plate or a quarter-wave plate,
What is necessary is to satisfy (Equation 3) or (Equation 4). Further, in order to manufacture a half-wave plate or a quarter-wave plate having stable characteristics, it is desirable that the thickness of the optical recording material and the refractive index change Δn are constant from (Equation 3) or (Equation 4). I understand. Therefore, in the optical recording medium of the present embodiment, as the refractive index change Δn, a saturated refractive index change Δns which becomes a stable value at a certain light irradiation amount or more is used. Thus, the value of the change in the saturated refractive index Δns and (Equation 3)
Alternatively, the thickness of the 波長 wavelength plate or １ ／ wavelength plate is determined from (Equation 4). If the optical recording material is manufactured with the thickness, a constant refractive index change Δns can be induced if the exposure amount is equal to or more than the light amount that gives the saturated refractive index change Δns. It can be performed. As for the recording speed, the light intensity of the laser currently available is about 10 ⁶ to 10 ⁷ times larger than that used for the measurement. Considering that point, the recording speed is approximately ms
Speed up to the order of is possible.

As described above, in the recording material of the present invention, birefringence is induced by light irradiation and a half-wave plate or
The wave plate can be recorded, and the direction of the wave plate can be set as the polarization angle direction of the light used for recording.

Next, a method for binary recording and reproduction of information utilizing the above-described characteristics will be described. Here, a 波長 wavelength plate will be described as an example. Binary recording utilizes the difference in the polarization angle of the recording laser beam. In the present invention, the recording light has two polarization angles of 0 degree and 45 degrees. Here, the polarization angle of 0 degree is s
Polarized light. FIGS. 10A and 10B schematically show recording on a 記録 wavelength plate using this recording light. That is, if birefringence is induced by the recording light having a polarization angle of 0 degree, the orientation of the minute quarter-wave plate in the recording area becomes as shown in FIG.
As shown in (a), it is recorded in the 0 degree direction. If birefringence is induced by recording light having a polarization angle of 45 degrees, the orientation of the minute quarter-wave plate in the recording area is recorded in the direction of 45 degrees as shown in FIG. Binary recording can be performed by associating the difference in recorded orientation with binary information to be recorded.

Reproduction of the recorded information is performed as follows. The reproduction laser light is s-polarized. The reproduction light is applied to a minute quarter-wave plate recorded in the azimuth of either 0 degree or 45 degrees. First, consider the case of FIG. 10A in which the azimuth of the quarter-wave plate is recorded at 0 degrees. At this time, since the polarization angle of the reproduction light coincides with the azimuth of the quarter-wave plate, the polarization angle of the reflected light does not change and is zero.
And s-polarized light. On the other hand, in the case of FIG. 10B in which the azimuth of the quarter-wave plate is recorded at 45 degrees, the angle is 90 times which is twice 45 degrees from the principle of the quarter-wave plate.
The light is reflected with a degree of polarization angle. That is, the reflected light becomes p-polarized light and is reflected. After that, the light is passed through a polarizer that allows only the s-polarized light to pass, and the light intensity is detected by the photodetector. Accordingly, when the polarization angle of the reflected light is 0 degree (s-polarized light), the reflected light passes through the polarizer, and is thus detected as “bright”. Conversely, the polarization angle of the reflected light is 9
In the case of 0 degree (p-polarized light), the reflected light cannot be passed through the polarizer and is detected as “dark”. Therefore, when the azimuth of the 波長 wavelength plate is recorded at 0 ° as shown in FIG. 10A, the detector detects “bright”, and conversely, as shown in FIG. If the direction of the wave plate is recorded at 45 degrees, it is detected as "dark". By detecting the “bright” and “dark”, the recorded binary information can be reproduced.

As described above, in the recording / reproducing method of the present invention,
The azimuth of the quarter-wave plate can be recorded at 0 or 45 degrees in correspondence with binary information, and by irradiating the reproduction light, the azimuth can be reproduced in the form of a difference in light intensity. Can be.

Next, the principle of performing multi-valued recording and reproduction using the recording and reproduction method described above and the overwriting characteristics described above will be described. The basic principle of multi-level modulation is that the length of a recording pit is modulated in correspondence with multi-level data. This will be described with reference to FIGS.
FIG. 3A shows the timing of writing data by a recording pulse, and FIG. 3B shows the magnitude of a recording laser beam. The recording pit is represented by the detected brightness. When the recording data is “1”, it is “dark”,
When it is “0”, it is “bright”.

First, although not shown, all the 0 gradations are written with "0" data, and the portions detected as dark are eliminated. Next, for the first gradation, as shown in FIG.
Write “dark” with data, and write 1 /
"Bright" is written with "0" data at the position shifted by three. In this case, since the recording medium and the method of the present invention have an overwriting characteristic, the overlapping portion of the “dark” portion and the “bright” portion is “bright”. Thus, the length of the recording pit at the center can be reduced to 1/3 of the recording laser beam size.
Next, as shown in FIG. 11D, the second gradation is “1” at a position shifted by ず れ of the recording laser beam size from the initial position.
Write “dark” with the data, and
At a position shifted by three, "bright" is written with "0" data. Also in this case, the portion where the "dark" portion and the "bright" portion overlap is "bright". Therefore, as a result, the length of the recording pit at the center can be set to 2/3 of the recording laser beam size. Thereafter, when the writing position is sequentially shifted by “３” of the recording laser beam size in “bright”, the length of the recording pit at the center portion becomes as shown in FIGS. The light size can be increased to 3/3, 4/3.

As described above, according to the present invention, the length of the recording pit can be controlled from a region smaller than the size of the recording laser beam to a larger region. The same applies to not only the length of the recording pit but also the area of the recorded "dark" area. Therefore, if the length and area of the recording pit are modulated by the multi-level data, the multi-level modulation recording becomes possible. As still another method, a "light" area and a "dark" area can be written in a recording pit of a predetermined length, and these can be detected and reproduced by pattern recognition.

In the example of FIG. 11, for the sake of explanation, the shift amount of the recording pit between gradations is set to 1/3 of the size of the recording laser beam.
However, this can be further refined.
Thus, the length and area of the recording pit can be smoothly changed continuously. For example, assuming that the first gradation is 1/10 of the recording laser beam size and the subsequent shift amount per gradation is 9/1000 of the recording laser beam size, the relationship between the number of gradations and the recording pit at that time is as follows. As shown in FIG. In FIG. 12, the horizontal axis is the number of gradations,
The vertical axis represents the length of the recording pit when the recording laser beam size is 1. As can be seen from this figure, in the present invention, the length of the recording pit is set to 1/1 of the recording laser beam size.
It is possible to smoothly change the size from zero to twice the size of the recording laser beam. In addition, when viewed within the recording laser beam size, the present invention can perform multi-level recording / reproduction with about 100 gradations.

FIG. 13 is a view showing an embodiment of an optical recording / reproducing apparatus according to the present invention. In this apparatus, a reflective optical recording medium 1406 having a structure as shown in FIG.
The disk was used as shown in FIG. In this apparatus, the optical recording / reproducing of the reflection type is performed.
The thickness d of the optical recording layer 6 above is a value that functions as a 波長 wavelength plate as described above. The thickness d is expressed as the following (Equation 5).

[0052]

D = λ / 4 / Δn (Equation 5)

As the optical recording material, the above-mentioned polyester having cyanoazobenzene in the side chain was used.
For this, an argon ion laser sensitive to this material was used. Substituting Δn = 0.03 and the argon laser wavelength of 0.515 μm obtained by the measurement into (Equation 5), the thickness d becomes 3.9 μm. Therefore, in the present invention, the optical recording medium 1406 was manufactured so that the thickness of the optical recording layer was 3.9 μm. The laser beam used was one whose polarization was s-polarized light. Here, the s-polarized light has a certain direction parallel to the disk surface of the optical recording medium 1406.

First, an information recording method and apparatus will be described with reference to FIG. The recording laser light from the light source 1401 is collimated by the collimator lens 1402 and enters the polarization rotation element 1403. Polarization rotating element 1
Reference numeral 403 denotes a λ / 2 plate, a liquid crystal valve, a Pockels element, a Faraday element, or the like. A liquid crystal valve will be described as an example of the polarization rotation element 1403. The liquid crystal functions as a half-wave plate, and the polarization direction of the incident light and the axis of the half-wave plate are parallel to each other when no voltage is applied. Since the incident light is s-polarized, the transmitted light is s-polarized. On the other hand, when voltage is applied,
The axis of the half-wave plate is rotated by 22.5 degrees, and the polarization of the incident light is rotated by 45 degrees. Rotation of the optical recording medium 1
406 is performed in a plane direction parallel to the disk surface. Thus, the light that has passed through the polarization rotation element 1403 is
The polarization angle changes in a binary state of 0 degree or 45 degrees depending on the presence or absence of the voltage supplied to 403. Here, the polarization angle of the s-polarized light was set to 0 degree. The recording light having passed through the polarization rotator 1403 passes through the beam splitter 1404 and enters the objective lens 1405. This objective lens 1405
Then, the recording light is condensed and applied to the optical recording medium 1406. Thereby, as described above, the photo-induced birefringence can be recorded on the optical recording medium 1406.

As described above, in this apparatus, the polarization rotation element 14
03, the polarization angle θ of the recording light is 0
Or 45 degrees, the azimuth of the recorded quarter-wave plate can be set to 0 degrees or 4 degrees as shown in FIGS. 10 (a) and 10 (b).
Can be 5 degrees. Therefore, the continuous binary information of “0” and “1” corresponding to the multi-value information is converted to the polarization rotator 1
By making it correspond to the voltage applied to 403, it is possible to perform the recording in which the length of the recording pit is multi-level modulated as described above.

Next, a method and an apparatus for reproducing recorded information will be described with reference to FIG. The reproduction laser light from the light source 1401 is collimated by the collimator lens 1402 and enters the polarization rotation element 1403. However, the light intensity at this time is greatly reduced as compared with the time of recording so as not to destroy the recorded light-induced anisotropy. This reproduction light is made incident on the polarization rotation element 1403. At this time, no voltage is applied to the polarization rotator 1403 so that the transmitted light becomes s-polarized light. Further, the reproduction light having passed through the polarization rotation element 1403 passes through the beam splitter 1404 and enters the objective lens 1405. The objective lens 1405 condenses the reproduction light and irradiates it to the optical recording medium 1406. Thereafter, the reproduction light is incident on the recorded minute quarter-wave plate, and thereafter reflected by the reflection film in the recording medium, again transmitted through the minute quarter-wave plate to become reflected light, and becomes reflected light to become the optical recording medium 1406. Come out of. At this time, the reproduction light is s-polarized light as shown in FIG. 10, and the orientation of the recorded minute quarter-wave plate is 0 degree or 4 degrees.
Either 5 degrees. Therefore, as described above, the reflected light is reflected as s-polarized light or p-polarized light. Light is reflected by beam splitter 1404 and passes through polarizer 1408 to detector 1409. The polarizer 1408 is configured to pass only s-polarized light, and the detector 1409 detects "bright" when the polarization angle of the reflected light is 0 degree (s-polarized light), and conversely. When the polarization angle of the reflected light is 90 degrees (p-polarized light),
"Dark" is detected.

At this time, the detector 1409 detects "bright" of the reflected light.
And "dark" are detected, but what is multi-valued recorded is the length of the recording pit. Therefore, the "bright" output from the detector
And a circuit for measuring the time length of the "dark" signal is provided, and the multilevel information is reproduced by demodulating the multilevel modulated information from the length of each signal. For this detector, for example, a CCD or a photodetector can be used. The spatial resolution of the detector is set to be equal to or less than the minimum recording pit length.

The optical recording medium 1406 of this embodiment has a disk shape as shown in the figure. By rotating a motor 1407, a plurality of data can be recorded at different locations in the circumferential direction of the recording medium 1406. it can. As shown in FIG. 13, the entire recording / reproducing head 1410 is
06 in the radial direction of the optical recording medium 1
Concentric recording tracks can be formed at 406 for recording.

The embodiment of the present invention described with reference to FIG. 13 is configured so that recording and reproduction can be performed simultaneously. However, it is possible to make only recording or reproduction only by changing a part of the configuration. It is possible. First, since it is not necessary to relate the detection part of the reflected light in order to make it exclusively for recording, FIG.
3, the beam splitter 1404, the polarizer 140
8, a configuration in which the detector 1409 is removed. This makes it possible to reduce the size and weight of the recording head. Further, a device that is less expensive than a recording and reproducing device can be manufactured. In addition, in order to make it exclusively for reproduction, the polarization rotator 1 necessary for recording from FIG.
403 is removed. This makes it possible to reduce the size and weight of the reproducing head. Further, a device that is less expensive than a recording and reproducing device can be manufactured.

FIG. 14 is a diagram showing another embodiment of the optical recording / reproducing method according to the present invention. In the recording method described above, the length of the recording pit is modulated by the number of consecutive recording pulses. However, it is also possible to perform multilevel modulation recording of the length and area of the recording pit by the pulse width. In this embodiment, the recording laser beam is continuously irradiated, and the polarization rotating element is driven with a pulse width. The recording medium is in the form of a disk and constantly rotates, and the recording area is moving at a constant speed. At this time, as shown in FIG. 14A, the recording in which the reproduction reflected light becomes p-polarized light is performed with the pulse width Tw, and thereafter, the recording in which the reproduction reflected light becomes s-polarized light is performed. Thus, since the recording area is moving at a constant speed, a recording pit having a length Lw corresponding to the pulse width Tw can be produced as shown in FIG. By modulating the pulse width Tw, the recording pit length Lw is also modulated correspondingly, so that multi-level recording / reproduction is possible.

FIG. 15 is a diagram showing another embodiment of the reproducing method according to the present invention. At the time of reproduction, intensity fluctuation of reproduction light,
The S / N ratio may be reduced due to external light, noise generated in a recording medium, an optical system, or the like. Therefore, in this embodiment, these noises are canceled by using a differential detection system, and the S / N is reduced. / N is intended to be improved. Here, a case where the length of the recording pit as shown in FIG. 15 is detected will be described. Light reflected from recording pit 50
Is s-polarized light or p-polarized light. The signal is first split by a polarization beam splitter 51 into an s-polarized component and a p-polarized component. Thereafter, the respective components are detected by detectors 52 and 53.
To detect. In the present invention, since the s-polarized light and the p-polarized light are reproduced in a binary manner, the detected signal is an inverted signal. When this signal is differentially detected by the differential detector 54 (taking the difference between the two signal components), the noise component is canceled, and the remaining signal is only the signal component excluding the noise. This can reduce noise and S / N
High reproduction is possible.

FIGS. 16A and 16B are views showing another embodiment of the reproducing method according to the present invention. In the above embodiments, the length of the recording pit is detected, but in the present embodiment, the area of the recording pit is detected. In the present embodiment, the size of the reproduction laser beam is made approximately equal to the size of the recording pit. The optical system and the detector of the apparatus shown in FIG. 13 are set so that the light intensity of the entire reflected light pattern can be detected. The polarizer is designed to pass only s-polarized light. As a result, the light intensity detected by the detector varies according to the pulse width Tw, as shown in FIGS.
It changes depending on the size of the recording pit (pit width W) in the size of the reproduction laser beam. That is, when the size of the recording pit (pit width W) increases, the s-polarized light pattern portion incident on the detector decreases, and the detected light intensity decreases. By using this,
The size (pit width W) of the recorded recording pit can be detected. In this method, the signal can be demodulated without counting the time change of the detected signal.

FIG. 17 is a view showing another embodiment of the optical recording / reproducing apparatus according to the present invention. In this apparatus, a transmission type optical recording medium 1806 having a structure as shown in FIG. 5A was used as a disk shape as shown in FIG. In this device,
Since transmission optical recording and reproduction are performed, the thickness d of the optical recording material on the optical recording medium is set to a value that functions as a half-wave plate as described above. The thickness d is expressed by the following (Equation 6).

[0064]

D = λ / 2 / Δn (Equation 6)

As the optical recording material, the same material as that of the reflection type was used, and the light source 1801 was an argon ion laser. Therefore, when Δn = 0.033 obtained by measurement and the wavelength of argon laser of 0.515 μm are substituted, the thickness d
Is 7.8 μm. Thus, the optical recording medium 18 of the present embodiment
No. 06 was prepared such that the thickness of the optical recording layer was 7.8 μm. The laser beam used was one whose polarization was s-polarized light. Here, the s-polarized light has a certain direction parallel to the disk surface of the optical recording medium 1806.

First, an information recording method will be described with reference to FIG. The elements used in this device are the same as those of the reflection type. The recording laser light from the light source 1801 is collimated by the collimator lens 1802 and enters the polarization rotation element 1803. The polarization rotation element 1803 changes the polarization angle of the passing light to 0 degree or 45 degrees according to the voltage supplied to the polarization rotation element 1803. Here, the polarization angle of the s-polarized light is set to 0 degree, and the rotation is performed in a plane direction parallel to the disk surface of the optical recording medium 1806. This polarization rotation element 1803
The recording light that has passed through the optical disk enters an objective lens 1805, where the recording light is condensed and collected by the optical recording medium 18.
06. Thus, the photo-induced anisotropy can be recorded on the optical recording medium 1806. At this time, by controlling the voltage applied to the polarization rotation element, FIGS.
As shown in (1), the orientation of the minute half-wave plate manufactured by the birefringence induced by the recording light can be set to 0 degree or 45 degrees. This enables multi-level modulation recording as described above.

Next, a method for reproducing recorded information will be described with reference to FIG. The reproduction laser light from the light source 1801 is converted into parallel light by the lens 1802 and enters the polarization rotation element 1803. However, the light intensity at this time is greatly reduced as compared with the time of recording so as not to destroy the light-induced birefringence recorded. The reproduced light is made incident on the polarization rotation element 1803. At this time, the polarization rotation element 1
At 803, the transmitted light is set to be s-polarized light. Further, the reproduction light that has passed through the polarization rotation element 1803 enters the objective lens 1805, where the reproduction light is condensed and
06. After this, the reproducing light is
The light passes through the half-wave plate and emerges from the lower surface of the optical recording medium 1306. At this time, the reproduction light is s-polarized light as shown in FIG. 10, and the azimuth of the recorded minute half-wave plate is 0.
The transmitted light is s-polarized light or p-polarized light based on the principle of a half-wave plate. The transmitted light then passes through polarizer 1808 and is sent to detector 1809. By performing the same detection as that of the reflection type by the polarizer 1808 and the detector 1809, multi-value reproduction can be performed.

In the transmission-type recording / reproducing apparatus described with reference to FIG. 17, as in the reflection-type recording / reproducing apparatus shown in FIG.
Recording and reproduction can be performed in the circumferential direction and the radial direction 806. Further, in the present embodiment, recording and reproduction are performed, but it is also possible to perform recording and reproduction only by partially changing the configuration. First, since it is not necessary to relate the detection part of the transmitted light in order to make it dedicated to recording, the polarizer 1808 and the detector 1109 are shown in FIG.
Is removed. In addition, in order to perform reproduction only, the configuration is such that the polarization rotator 1103 necessary for recording is removed from FIG. With such a dedicated machine, the size and weight of the head can be reduced. In addition, a device that is less expensive than a recording / reproducing device can be manufactured.

As described above, in the recording medium of the present invention, the polarization angle of the recording light can be recorded in a binary manner and reproduced at twice the polarization angle. As for the S / N, the change in the refractive index of the recording material is saturated with the light intensity to a constant value, so that the S / N decrease due to the material and the S / N due to the change in the light intensity are caused.
The drop can also be prevented. Further, as for rewriting, since overwriting is possible, an erasing process is not required, and high-speed rewriting can be realized. Further, by making the shape of the optical recording medium into a disk shape, the storage capacity and the data transfer rate can be improved.

Further, according to the optical recording / reproducing method and the optical recording / reproducing apparatus of the present invention, the binary recording is performed by the difference in the polarization angle of the recording light, and at the same time, the length and area of the binary-recorded area are reduced. You can change and record. By modulating the length and area of the recording area with multi-level data, multi-level recording and reproduction can be performed. Further, by performing differential detection, a high S / N can be obtained in reproduction. This multi-level recording enables high-density and high-speed recording and reproduction.

[0071]

According to the present invention, it is possible to obtain an optical recording medium, an optical recording / reproducing method, and an optical recording / reproducing apparatus capable of performing high-density recording by multi-value recording.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a １ ／ wavelength plate.

FIG. 2 is a diagram illustrating a half-wave plate.

3 (a) to 3 (c) are diagrams showing chemical formulas of a trans structure and a cis structure of azobenzene, respectively, and a chemical structure of a polyester having cyanoazobenzene in a side chain.

FIGS. 4A and 4B are diagrams illustrating light-induced birefringence, respectively.

FIGS. 5A to 5D are diagrams each showing a cross-sectional structure of the optical recording medium of the present invention.

FIG. 6 is a diagram for explaining an optical system used for measuring recording characteristics.

FIGS. 7A to 7D are diagrams showing recording characteristics of the optical recording medium of the present invention.

FIG. 8 is a diagram for explaining an optical system used for measuring a change in refractive index.

FIG. 9 is a diagram showing a saturation characteristic of a change in refractive index.

FIGS. 10A and 10B are diagrams illustrating an optical recording / reproducing method according to the present invention.

FIGS. 11A to 11F are diagrams illustrating the principle of the optical multilevel recording / reproducing method of the present invention.

FIG. 12 is a diagram illustrating the gradation characteristics of the optical multilevel recording / reproducing method of the present invention.

FIG. 13 is a diagram illustrating an embodiment of the optical recording / reproducing apparatus of the present invention.

14A and 14B are diagrams illustrating another embodiment of the optical recording / reproducing method of the present invention.

FIG. 15 is a diagram illustrating another reproduction detection method of the present invention.

16A and 16B are diagrams illustrating another optical recording / reproducing method according to the present invention.

FIG. 17 is a diagram illustrating another embodiment of the optical recording / reproducing apparatus of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Optical recording medium 12 Optical recording layer 13 Substrate 14 Reflection film 1401 Light source 1402 Lens 1403 Polarization rotation element 1404 Beam splitter 1405 Objective lens 1406 Optical recording medium 1407 Motor 1408 Polarizer 1409 Detector 1410 Recording / reproducing head

Claims (25)

[Claims] 1. An optical recording material having photo-induced birefringence is 1
An optical recording medium comprising: an optical recording layer configured to function as a half-wave plate, wherein the azimuth and length of the half-wave plate are recorded in the optical recording layer by multi-level modulation. 2. An optical recording material having photo-induced birefringence is 1
An optical recording layer configured to function as a 波長 wavelength plate,
A light reflection layer formed on one surface of the recording layer,
An optical recording medium, wherein the azimuth and length of a quarter-wave plate are recorded in the optical recording layer by multi-level modulation. 3. The optical recording medium according to claim 1, wherein the optical recording layer contains a polymer compound having a photoisomerizable group in a side chain or a polymer liquid crystal. 4. The optical recording medium according to claim 1, wherein the optical recording layer is made of a polymerized material in which molecules to be photoisomerized are dispersed. 5. The optical recording medium according to claim 1, wherein said optical recording medium has a disk shape. 6. An optical recording medium, wherein the polarization angle of the linearly polarized recording light is changed in a binary manner based on the multi-value data, and the recording light is irradiated on the optical recording medium.
An optical recording method, characterized in that a half-wave plate having an azimuth and a length corresponding to the multi-value data is modulated and recorded on the recording medium. 7. The optical recording medium, wherein the polarization angle of the linearly polarized recording light is changed in a binary manner based on the multi-valued data, and the recording light is irradiated on the optical recording medium.
An optical recording method, wherein a ４ wavelength plate having an azimuth and a length corresponding to the multi-value data is modulated and recorded on the recording medium. 8. The optical recording medium is formed in a disk shape,
8. The optical recording method according to claim 6, wherein the modulation recording is performed by rotating the optical recording medium and moving a recording position in a radial direction. 9. The optical recording method according to claim 6, wherein overwriting is performed without erasing the previous recording state at the time of the second or subsequent recording on the optical recording medium. 10. A light source for generating recording light, a spatial light modulator for changing a polarization angle of the recording light based on multi-value data, and a light source for irradiating the optical recording medium with light from the spatial light modulator. An optical recording device comprising an image optical system. 11. A medium driving mechanism for rotating and driving the optical recording medium, a head moving mechanism for moving an optical recording head including the light source, the spatial light modulator and the imaging optical system in a radial direction of the optical recording medium. 11. The apparatus according to claim 10, further comprising:
The optical recording device as described in the above. 12. The optical recording apparatus according to claim 10, wherein said optical recording medium is built-in. 13. The optical recording apparatus according to claim 10, wherein the spatial light modulator includes a polarization rotating element that rotates a polarization angle of the recording light. 14. An optical recording medium in which the azimuth and length of a half-wave plate have been modulated by multi-level modulation and irradiated with reproduction light, and the state of the polarization angle of light transmitted through the optical recording medium is detected. By
An optical reproducing method characterized by reproducing recorded contents. 15. An optical recording medium in which the azimuth and length of a quarter-wave plate are multilevel-modulated and recorded is irradiated with reproduction light, and the state of the polarization angle of reflected light from the optical recording medium is detected. By
An optical reproducing method characterized by reproducing recorded contents. 16. The recording device according to claim 14, wherein the transmitted light or the reflected light is separated into two polarization components orthogonal to each other, and the recorded content is reproduced based on the separated two polarization components. Light regeneration method. 17. An optical recording medium in which the azimuth and length of a half-wave plate are multilevel-modulated and recorded is irradiated with reproduction light, and the intensity of transmitted light from the optical recording medium is detected. An optical reproducing method characterized by reproducing recorded contents. 18. An optical recording medium in which the azimuth and length of a quarter-wave plate are multilevel-modulated and recorded is irradiated with reproduction light, and the intensity of reflected light from the optical recording medium is detected. An optical reproducing method characterized by reproducing recorded contents. 19. The recorded content is reproduced by forming the optical recording medium in a disk shape and rotating the optical recording medium and moving a reading position in a radial direction. The light reproducing method according to any one of the above. 20. A reproduction light optical system for irradiating reproduction light to an optical recording medium on which the azimuth and length of a half-wave plate are multi-level modulated and recorded, and a polarization angle of light transmitted from the optical recording medium An optical reproducing device, comprising: an analyzing means for detecting the light. 21. A reproduction light optical system for irradiating a reproduction light to an optical recording medium on which an azimuth and a length of a quarter-wave plate are multilevel-modulated and recorded, and a polarization angle of light reflected from the optical recording medium An optical reproducing device, comprising: an analyzing means for detecting the light. 22. A reproduction light optical system for irradiating reproduction light to an optical recording medium on which the azimuth and length of a half-wave plate are multi-level modulated and recorded, and transmitting light from the optical recording medium orthogonal to each other. An optical system that separates the two polarized components into two, a detecting unit that detects the two separated polarized components, and a signal processing circuit that obtains a reproduction signal from a signal from the detecting unit. Optical regeneration device. 23. A reproducing light optical system for irradiating a reproducing light to an optical recording medium on which an azimuth and a length of a quarter-wave plate are multilevel-modulated and recorded, and a reflected light from the optical recording medium orthogonal to each other. An optical system that separates the two polarized components into two, a detecting unit that detects the two separated polarized components, and a signal processing circuit that obtains a reproduction signal from a signal from the detecting unit. Optical regeneration device. 24. A medium driving mechanism for rotating and driving the optical recording medium, and a head moving mechanism for moving an optical reproducing head including the reproducing optical system and the light detecting means in a radial direction of the optical recording medium. 24. The method according to claim 20, wherein:
The optical reproducing device according to any one of the above. 25. The optical reproducing apparatus according to claim 20, wherein said optical recording medium is built-in.