JP3059802B2 - Reproduction method of optical recording medium - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for reproducing an optical recording medium capable of multiplex recording.

[0002]

2. Description of the Related Art In recent years, research on using a photochromic material for a recording layer of an optical recording medium has been actively conducted.

When this photochromic material is irradiated with light of a predetermined wavelength, a photochemical reaction occurs to change the molecular structure,
Optical characteristics such as absorbance for light of a specific wavelength change.

[0004] When irradiated with light of another predetermined wavelength, it has a property that its molecular structure returns to its original state.

Since the photoreaction of the photochromic material has wavelength selectivity, an optical recording medium containing a plurality of types of photochromic materials having different light absorption wavelength ranges is irradiated with light in a wavelength range corresponding to each material. A method of performing multiplex recording of information (wavelength multiplex recording) is disclosed in
No. 203450 (G03C1 / 733).

For example, two types of photochromic materials a and b
Has a spectrum having an absorption maximum at wavelengths λa and λb as shown in FIG. 8, and these selectively cause a reaction by light irradiation at wavelengths λa and λb, and their respective absorbances change (that is, the absorption maximum disappears). In this case, two-wavelength multiplex recording can be performed using this property, and reproduction can be performed by detecting a change in absorbance.

However, the magazine "Chemistry and Industry" No. 42
As pointed out on page 2028 of Vol. 11 (1989), the wavelength selectivity of the photochromic reaction between the materials a and b is not perfect. For example, as shown in FIG. Have a small absorption with respect to the material a.
When reproduction is performed with the light of a and λb, there is a problem that crosstalk (mixing of information) occurs between information recorded on each material.

A method of performing polarization multiplexing using another characteristic of the photochromic material is disclosed in Japanese Patent Application Laid-Open No. 2-260127 (G11B 7/00).

In such a method, when an optical recording medium in which a photochromic material is uniformly dispersed is irradiated with linearly polarized light for recording, for example, a recording light beam is emitted at that wavelength.
Using the property that the change in absorbance is large for linearly polarized light having a plane of polarization parallel to the system, while the change in absorbance is small for linearly polarized light having a plane of orthogonal polarization, linearly polarized lights that are orthogonal to each other are used. In this manner, double recording is realized by independently recording information.

However, even in such a method, the photochromic material molecule has incomplete reaction selectivity and absorption selectivity with respect to polarized light, so that a polarization multiplex crosstalk similar to the above-described multiplex crosstalk by wavelength is generated.

This is because the reaction is highly likely to occur when the transition moment of the photochromic molecule is parallel to the plane of polarization of the light, but the photochemical reaction also occurs when these are not parallel.

As disclosed in Japanese Patent Application No. 3-64798 for the purpose of reducing these crosstalks, the present inventors have applied the present invention to reproducing light from an optical recording medium into multiplex-recorded information. After correspondingly separating the light and individually detecting the separated light with a photodetector, each signal is amplified or attenuated based on the ratio of each crosstalk in the detection signal, and the crosstalk component is subtracted by a subtraction process. A method of removing is proposed.

However, in such a method, the only crosstalk that can be removed by subtraction processing is only the crosstalk by linear superposition.

However, in actual wavelength multiplexing recording or the like, the crosstalk becomes linear only under very limited conditions, and there is a nonlinear crosstalk component which cannot be reduced only by the subtraction in the above-described example. In many cases, reduction of crosstalk cannot be realized.

[0015]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, the present invention is capable of reducing crosstalk generated when reproducing information multiplex-recorded on an optical recording medium irrespective of a linear component or a non-linear component. It is an object to reduce it.

[0016]

According to the present invention, an optical recording medium on which information is multiplex-recorded is irradiated with reproduction light, and the reproduction light reflected by the medium is converted into different waves corresponding to the multiplex-recorded information.
Split into long light or light of different polarization components.
The first recording signal and the second recording signal are detected independently of each other.
And performing a linear operation process between these signals and a non-linear operation process for integrating the first recording signal and the second recording signal to obtain the first and second recording signals. It is characterized in that a linear crosstalk component and a non-linear crosstalk component are removed from a signal.

In this case, the linear operation processing and the non-linear operation processing include linear operation such as subtraction of a signal obtained by amplifying and attenuating a detection signal of each reproduction light corresponding to linear crosstalk reduction;
This means subtraction or the like corresponding to the product of the respective detection signals for reducing the nonlinear crosstalk or the product of the respective detection signals after the linear crosstalk reduction.

[0018]

The principle of the present invention will be described with reference to FIG.
In the case of the two-wavelength multiplex recording described with reference to FIG.
Can take four values.

In FIG. 1, Rd is unrecorded, Rc is recorded by irradiating only light of λb, Rb is irradiating only light of λa, and Ra is irradiated by both light of λa and λb. And the respective reflectivities when recording.

Here, the difference ΔR1 between Rb and Rd, and Ra and Rc
Is not equal to the difference ΔR2, and the difference δR between Ra and Rb is
And the difference δR * between Rc and Rd is not equal.

At this time, the reflectance at the wavelength λa is the same as the wavelength λa in the portion where the recording signal light of the wavelength λb is not irradiated.
Rb to R according to the recording signal light (the angular frequency is ω1)
This corresponds to the case where d is changed by the amplitude ΔR1 / 2, while the portion irradiated with the recording signal light having the wavelength λb also corresponds to the case where the amplitude is changed by the amplitude ΔR2 / 2.

Therefore, the reflectance change ΔRch1 (∝
The reproduction output) converts the signal of angular frequency ω1 into angular frequency ω2 (wavelength λ
The recording signal is a signal obtained by superimposing a signal having an angular frequency ω2 on a signal which is amplitude-modulated by the signal b).

That is, ΔRch1 can be represented by the following equation 1.

[0024]

(Equation 1)

The reflectance change ΔRch2 at the wavelength λb is obtained in the same manner. Then, by rearranging the expressions of ΔRch1 and ΔRch2, the following standardized reproduction output of Expression 2 is obtained.

[0026]

(Equation 2)

In the above equation 2, η 'and ξ' are similarly defined for the reflectance change Rch2 at λb. Ξ, ξ ′, η, η ′ in Equations 1 and 2 are parameters representing the magnitude of the crosstalk.

From the equations (1) and (2), the crosstalk for multiplex recording includes a linear crosstalk similar to the conventional crosstalk between tracks, and a non-linear crosstalk corresponding to the product of signals to be multiplex-recorded. It can be seen that there is a term

Therefore, such linear and nonlinear cross
An operation as shown in the following Expression 3 is performed in order to reduce the peak component.

[0030]

(Equation 3)

The normal parameters 通常, ξ ', η,
Considering that these squares are sufficiently smaller than 1 since η 'is smaller than 1, the term proportional to cos ω1t of the first item corresponding to the main signal of the equation (3) is of the order of 1. In contrast to the magnitude, the terms corresponding to the other crosstalks are the second or third order of the parameters 前 記, ξ ′, η, η ′.
Next order.

Therefore, it can be seen from the comparison of the equation (2) and the equation (3) that the crosstalk component is reduced.

[0033]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for reproducing an optical recording medium according to the present invention, in which two-wavelength multiplex recording is performed on a medium containing two kinds of diarylethene photochromic materials, will be described in detail with reference to the drawings.

As the first material of the optical recording medium, 2,3-
Bis (2-methylbenzo [b] thiophen-3-yl)
FIG. 2 shows the structural formula of maleic anhydride (hereinafter abbreviated as symmetric benzothiophene type) in the ring-opened state and the absorbance characteristics of the ring-opened state (solid line) and the ring-closed state (dashed line).

This material is, for example, a wavelength λ of an Ar laser light source.
= 458 nm changes from the ring opening state to the ring closing state, and the absorption spectrum characteristic changes from the solid line to the broken line in FIG.

When light of λ = 514.5 nm is irradiated, a reverse reaction (return from the ring-closed state to the ring-opened state) occurs, and the spectral characteristic changes from the broken line to the solid line in FIG. Is referred to as a decolored state, and the broken line state is referred to as a colored state).

Thus, the benzothiophene type material is A
r = 458 nm from a laser light source and 514.5 n
m can be recorded / erased using emitted light of m.
It can be performed by detecting a change in absorbance (reflectance) due to 5 nm radiation.

Next, 2- as the second material of the optical medium
(1,2-dimethyl-3-indolyl) -3- (2,
3,5-Trimethyl-3-thionyl) maleic anhydride (hereinafter abbreviated as asymmetric thiophene-indole type) structural formula of ring-opened state and absorbance characteristics of ring-opened state (solid line) and ring-closed state (dashed line) Is shown in FIG.

This material is, for example, λ = 4 from an Ar laser.
When light of 58 nm is irradiated, the state changes from the ring opening state to the ring closing state, and the absorption spectrum characteristic changes from the solid line to the broken line in FIG.

Λ = 633 nm from a He—Ne laser
When the light is irradiated, the reverse reaction (return from the ring-closed state to the ring-opened state) occurs, and its spectral characteristic changes from the broken line to the solid line in FIG.
The dashed state will be referred to as the colored state).

As described above, the thiophene-indole type material can be obtained by irradiating light of λ = 458 nm from an Ar laser with He-
Recording and reproduction can be performed using the radiation of λ = 633 nm from the Ne laser, and the operation can be performed by detecting a change in absorbance (reflectance) due to the radiation of λ = 633 nm.

When an optical recording medium as shown in FIG. 4 is formed by using the above two kinds of materials, first, about 15 wt% is mixed with polyvinyl butyral resin, and this mixture is dissolved with anone. Is formed on a glass substrate 1 by using a spin coating method to form a recording layer 2. In this case, the thickness of the recording layer 2 is about 1 μm.

Next, an Al reflective layer 3 was formed on the recording layer 2 by a vacuum evaporation method to form an optical recording medium. The entire surface of the optical recording medium thus formed was sufficiently irradiated with light of λ = 458 nm, and the colored state of the photochromic material of the recording layer 2 was set to an initial state (unrecorded state), and the absorbance was measured by reflected light. However, the absorption spectrum characteristic shown by the solid line A in FIG. 5 was obtained.

After irradiating the medium in the colored state again to the initial state with light of λ = 633 nm, the absorbance was measured in the same manner as described above. The absorption spectrum characteristic indicated by the dotted line B in FIG. was gotten.

Further, in the same manner, when only light of λ = 514.5 nm is irradiated (dotted line C in FIG. 5), λ = 51
The absorbance was measured when light of 4.5 nm and λ = 633 nm were simultaneously irradiated (broken line d in FIG. 5).

From these measurement results, λ = 514.5 nm
It is evident that the absorbance under each condition is different under irradiation of light of λ = 633 nm.

This means that the benzothiophene type material is irradiated with light of λ = 514.5 nm to cause a ring opening reaction to perform recording, and that the thiophene-indole type material is irradiated with light of λ = 633 nm. When the ring opening reactions occur and are recorded, these reactions show that, although wavelength selectivity exists, it is not complete.

Next, a crosstalk generated by performing recording / reproducing on the optical recording medium formed by the above-described method using an optical recording / reproducing apparatus schematically shown in FIG. 6 was measured.

Here, the optical system of FIG. 6 will be described.
4 is the optical recording medium described in FIG. 6, 5 is an objective lens system, 6 is a dichroic mirror, 7, 8 are λ / 4 plates,
9 and 10 are deflection beam splitters, 11 and 12 are beam expanders, 13 and 14 are AO modulators, 15 and 16 are ND filters, 17 is an Ar laser light source, and 18 is He-N.
e-laser light source, 19 and 20 are reproduction lens systems, 21 and
2 is a photodetector, 23 and 24 are IV converters, 2
5 to 28 are differential amplifiers, and 29 is an arithmetic circuit.

In the above arrangement, the Ar laser light source 1
7 is a wavelength λ = 514.5 nm, He-Ne laser light source 18
Have a wavelength λ = 633 nm, and radiate at a constant power of 30 mW in output.

The ND filters 15 and 16 adjust the laser intensity of the light emitted from each of the light sources 17 and 18, and the AO modulators 13 and 14 adjust these lasers during recording.
The light is intensity-modulated according to the recording signal.

The beam expanders 11 and 12 make the beam width of the intensity-modulated laser light appropriate.

The dichroic mirror 6 has λ = 51.
The light of 4.5 nm is totally transmitted and the light of λ = 633 nm is totally reflected, and these lights are combined with the same light source when irradiating the medium.

FIG. 6 omits a conventionally known optical system and circuit system for a focus servo and a tracking servo.

Thus, at the time of recording information, by using the ND filters 15 and 16 having a high transmittance, λ = 514.5 nm or λ = 63 with relatively strong power.
Optical recording medium 4 with 3 nm beam in initial state (colored state)
Irradiated to

On the other hand, at the time of reproduction, the ND filters -15 and 16
The laser beam having a low transmittance is used to irradiate the medium 4 with each laser beam of a constant level of weak power, and the reflected light is detected by the photodetectors 21 and 22.

At this time, since the reflectance of the reflected light changes according to each recording information, the output voltages of the photodetectors 21 and 22 and the IV converters 23 and 24 change accordingly. At this time, the outputs O1 and O2 of the IV converters 23 and 24 are output.
Is an output before the crosstalk reduction processing is performed.

Next, the crosstalk reduction operation circuit will be described. The differential amplifiers 25 and 26 subtract the equation (2) and the linear crosstalk component in the equation described in the section of the operation. However, at this time, it is necessary to adjust the input resistance of each of the amplifiers 25 and 26 in accordance with the values of the crosstalk parameters ξ and ξ ′.

Then, the product of the outputs of the amplifiers 25 and 26 is created by the arithmetic circuit 29 which performs the product, and the results are used by the other differential amplifiers 27 and 28 for the amplifiers 25 and 26.
Subtract from

Also at this time, it is necessary to adjust the input resistance of each of the amplifiers 27 and 28 in accordance with the values of the crosstalk parameters η and η '.

The differential amplifiers 27 and 28 have the above formula (3).
Outputs O3 and O4 with reduced linear / non-linear crosstalk components corresponding to the following equation are generated.

Using the apparatus shown in FIG. 6, crosstalk was measured under the following conditions. That is, the optical recording medium 4 shown in FIG. 4 is used. In the initial state, light of λ = 514.5 nm is irradiated at a power of 6 mW and a frequency of 300 kHz, and λ = 63.
Light of 3 nm was recorded at a power of 5 mW and a frequency of 200 kHz.

Reproduction was performed by irradiating light of λ = 514.5 nm at a constant level of power of −0.5 mW, and irradiating light of λ = 633 nm at a constant level of power of −0.5 mW.

Each of these lights is applied to the same spot on the medium 4 and the rotation speed of the medium 4 itself is 400 rpm.

The crosstalk was measured by measuring the output ratio of the crosstalk component to the main signal using a spectrum analyzer.

Table 1 shows O1, O2 before the crosstalk reduction operation.
Table 2 shows the outputs of O3 and O4 after the operation.

[0067]

[Table 1]

[0068]

[Table 2] As a result, both channels (O1 and O3 and O2 and O4 are the same channel) have a linear crosstalk component (3
00 or 200 kHz) is included at -7 dB, and the nonlinear crosstalk components (100 and 500 kHz) are -1 dB, respectively.
3 or -22 dB, but after the operation, it is linear (3
00 or 200 kHz), nonlinear (400 or 10 kHz)
0 kHz) component is reduced to -38 dB or less in both channels.

In this measurement, a crosstalk component of -44 dB or less was buried in noise and could not be measured.

As described above, in the apparatus shown in FIG. 6 using the present invention, the crosstalk in the wavelength multiplex recording on the optical recording medium could be greatly reduced. In the example of FIG. 6, an example relating to wavelength multiplex recording has been described. However, the present invention can be easily applied to multiplex recording of three or more wavelengths.

The present invention can be applied to the reduction of crosstalk in the case of polarization multiplexed recording.

FIG. 7 shows an optical reproducing apparatus in the case of polarization multiplexed recording. In the figure, components having the same functions as those in FIG.

In the figure, reference numeral 30 denotes a light source for reproduction, and 31 denotes a polarized light.
The optical beam splitter 32 has the same reflectivity of P wave and S wave.
It is a half mirror .

In the case of polarization multiplexed recording, a reproducing beam is irradiated on the medium 4 by the λ / 4 plate in the form of circularly polarized light (or random polarized light by the depolarizing plate), and is generated by the multiplexed recorded signal. The reflected light is intensity-modulated in two orthogonal directions due to optical anisotropy (particularly dichroism).

The orthogonal components thus obtained are separated by the polarization beam splitter 31 and detected independently by the photo detectors 21 and 22. In this case, the detected signal still contains a linear / non-linear crosstalk component caused by incomplete change selectivity during recording / reproduction.

Therefore, as in the case of FIG.
To 28 and the arithmetic circuit 29 to reduce such crosstalk components.

The present invention can be variously modified in addition to the above embodiments. As the photochromic material, various materials such as spiropyran-based fulgide-based materials as well as diarylethene-based materials can be used. Further, various calculations other than the above method can be used for the crosstalk reduction calculation.

[0078]

Since the present invention is as described above, it is possible to efficiently reduce both the linear and nonlinear components of the crosstalk generated at the time of multiplex recording.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of a method for reproducing an optical recording medium according to the present invention.

FIG. 2 is a diagram showing a structural formula and a light absorption characteristic of a first material of an optical recording medium.

FIG. 3 is a diagram showing a structural formula and light absorption characteristics of a second material of an optical recording medium.

FIG. 4 is a schematic sectional view of an optical recording medium.

5 is a diagram showing light absorption characteristics of the optical recording medium of FIG. 4 with respect to reproduction light having different wavelengths.

FIG. 6 is a diagram showing an optical system of a wavelength multiplex recording system.

FIG. 7 is a diagram illustrating an optical system of a polarization multiplex recording system.

FIG. 8 is a diagram illustrating the cause of the occurrence of crosstalk.

[Explanation of symbols]

 Reference Signs List 4 optical recording medium 9, 10 beam splitter 21, 22 photo detector 23, 24 IV converter 25-28 differential amplifier 29 arithmetic circuit

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-199676 (JP, A) Practical application Contents of the specification and drawings attached to the application form No. 62-180652 (Practical application No. 1-85913) Microfilms that photographed (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) G11B ^7/ 00-7/013 G11B 7/30

Claims (1)

(57) [Claims] 1. An optical recording medium on which information is multiplex-recorded is irradiated with reproduction light, and the reproduction light reflected by the medium is converted into light of a different wavelength or a different polarization component corresponding to the multiplex-recorded information.
Light, and detect the separated light independently.
Together to obtain a first recording signal and the second recording signal Te, the linear arithmetic process and the first recording signal between these signals
A method of reproducing an optical recording medium, comprising removing a linear crosstalk component and a non-linear crosstalk component from the first and second recording signals by performing a non-linear operation process for integrating the second recording signal.