JPH05250679A - Reproducing method and reproducing device for optical recording medium - Google Patents

Abstract

PURPOSE:To decrease medium noises and to obtain a reproduced signal of high C/N by subtracting the signal obtd. by irradiation with light of a wavelength at which information is not detected from the signal obtd. by irradiation of light of a wavelength at which the information signal is detected. CONSTITUTION:An optical recording medium 1 is irradiated with a slight laser beam outputted from a light source 2 and the signal A changing in accordance with a change in absorbance is detected by a detecting system 27. The signal B obtd. by irradiating the medium 1 with the laser beam outputted from the light source 3 is detected by a detecting system 37. The signal B is then subtracted from the signal A by a differential amplifier 6, by which the reproduced signal effectively decreased in noises is obtd. The disk noises by the surface roughness of the medium 1 included in the signals A, B is the same phases. Amplifiers 26, 36 or differential amplifier 6 of the detecting systems 27, 37 are previously adjusted and the signal B is subtracted from the signal A, by which the disk noises, laser noises, etc., are decreased and the reproduced output of the high C/N is taken out.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art In recent years, researches using photochromic materials for recording layers of optical recording media have been actively conducted. This photochromic material, when irradiated with light of a predetermined wavelength,
It has a characteristic that a molecular structure is changed by causing a photochemical reaction, and optical characteristics such as absorbance for light of a specific wavelength are changed.

The photochromic material also has a characteristic that when it is irradiated with light of a predetermined wavelength, the molecular structure changed as described above is restored. With such a property,
The reproduction of information recorded on an optical recording medium containing a photochromic material can be performed, for example, on pages 3 to 4 of "Optical Memory Symposium '88 Papers" published by Japan Optical Industry Technology Promotion Association (September 1988). Page, and Symposium “Current State and Future Prospects of Photochromism” sponsored by Kinki Chemical Society Functional Dye Subcommittee and Electronics Subcommittee (November 1990)
As disclosed on pages 39 to 40 of the collection of papers (March 20th), an optical recording medium is irradiated with a reproducing laser beam at a low power, and a change in absorbance due to a photochemical reaction is detected as a change in reflectance. This is generally done by doing.

However, if the information is reproduced as described above, the complete surface smoothness of the medium cannot be obtained in the manufacturing process of the optical recording medium (for example, at the time of forming a group or the like). There is a problem that a noise (hereinafter referred to as a disk noise) due to the surface roughness of the optical recording medium is present and a low noise (high C / N) reproduction signal cannot be obtained.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems of the prior art, and is a reproducing method for an optical recording medium in which medium noise is reduced and a high C / N reproduced signal is obtained. It is also an object of the present invention to provide a reproducing apparatus that realizes this.

[0006]

According to a first aspect of the present invention, which is a method for reproducing an information signal by irradiating an optical recording medium with light, a first wavelength of an information signal is detected. The signal obtained by irradiating the second reproducing light of the wavelength that does not detect or hardly detects the information signal is subtracted from the signal obtained by irradiating the reproducing light.

The second invention is a reproducing method of an optical recording medium for irradiating an optical recording medium with light to reproduce an information signal, which is obtained by irradiating a first reproducing light of a wavelength for detecting an information signal. The linear and non-linear operations are performed between the generated signal and the signal obtained by irradiating the second reproduction light of the wavelength that does not detect or hardly detects the information signal. A third invention is a reproducing method of an optical recording medium for irradiating an optical recording medium with light to reproduce an information signal, wherein a plurality of reproducing lights are irradiated on the same track of the medium, The transmitted or reflected light from the medium generated by the respective is independently detected, and each obtained detection signal together with the synchronization signal is temporarily stored in the storage unit, and each detection signal is simultaneously read by the synchronization signal, and these are simultaneously obtained. A read signal is obtained by performing a predetermined calculation on the read detection signal.

According to a fourth aspect of the present invention, in a reproducing method of an optical recording medium for irradiating an optical recording medium with light to reproduce an information signal, a plurality of reproducing lights are radiated on the same track of the medium, The transmitted or reflected light from the medium generated by the reproduction light is independently detected, and the distance between the spot positions on the medium of the reproduction light corresponding to the detected signal and the relative position of the spot are detected. After correction by giving a time delay amount corresponding to the speed, a predetermined operation is performed between these corrected detection signals to obtain a reproduction signal.

On the other hand, a fifth invention of the apparatus is a first reproducing light source for generating a light of a wavelength for detecting an information signal recorded on an optical recording medium, and a light of a wavelength for not detecting or hardly detecting an information signal. A second reproducing light source for generating a light, an optical system for irradiating the same spot on the optical recording medium with light of each wavelength, and a detection system for detecting light of each wavelength emitted from the optical recording medium. And a circuit for subtracting the signal obtained from each detection system.

A sixth aspect of the invention is to provide a first reproducing light source for generating reproducing light having a wavelength for detecting an information signal recorded on an optical recording medium, and reproducing light having a wavelength different from that of the first reproducing light source. A second reproduction light source for generating a light beam, a combining optical system for matching the optical axes of the respective reproduction lights, and a lens system for condensing the respective reproduction lights to the same spot on the medium.
A separation optical system that separates the reflected light or transmitted light from the medium by each reproduction light into two independent lights again, a detection system that independently detects the separated beams, and a detection system obtained from the detection system. And a circuit system for subtracting the output.

Further, a seventh invention is such that a first reproducing light source for generating a light having a wavelength for detecting an information signal recorded on an optical recording medium and a light for a wavelength not detecting or hardly detecting an information signal are generated. A second reproduction light source, a combining optical system for combining the light emitted from each of these light sources, a lens system for collecting the light emitted from the combining optical system on the medium, and the first and the second (2) Separation optical system that separates the reflected light and the transmitted light from the medium by the reproduction light source again, a detection system that independently detects each separated light, and a linear and nonlinear calculation between the signals obtained from these detection systems And a circuit system for performing.

A second reproducing light source having the same phase and a different size from the first reproducing light source is used, or a second reproducing light source having an opposite phase is used.
The reproducing light source is used, or the light separated from the first reproducing light source and not passing through the medium is used to perform linear or non-linear calculation between these lights.

[0013]

The signal obtained by irradiating the light of the wavelength for detecting the information signal includes the information signal and the disk noise, and the signal obtained by irradiating the light of the wavelength at which the information signal is not detected or hardly detected is the information Since it does not contain or hardly contains, and contains disk noise, it can be obtained by irradiating with light of a wavelength that does not detect or hardly detects an information signal from a signal obtained by irradiating light of a wavelength that detects an information signal. Since the medium noise can be reduced by subtracting the received signal, high C
A reproduction signal of / N is obtained.

The signal obtained by irradiating the light of the wavelength for detecting the information signal and the signal obtained by irradiating the light of the wavelength at which the information signal is not detected or hardly detected are linear (subtraction). And non-linear calculation (multiplication), noise can be effectively reduced at any information signal amplitude level, resulting in a higher C / N ratio.
Is obtained.

In particular, in the case of wavelength multiplexing recording, it is possible to reduce wavelength multiplexing crosstalk by a predetermined calculation with a reproduction signal obtained by irradiating light that can detect a plurality of multiplexed signals. ..

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A plurality of embodiments of the reproducing method and reproducing apparatus for an optical recording medium of the present invention will be described in detail below with reference to the drawings. [First Embodiment] FIGS. 1 to 7 show the drawings of the first embodiment.

First, the principle of the first embodiment will be described with reference to FIGS. The case of an optical recording medium using a photochromic material for the recording layer will be described with reference to the flowchart of FIG. The photochromic material has, for example, the absorbance characteristic as shown in FIG. 2, and when the state A having the absorbance characteristic shown by the solid line is irradiated with light of wavelength λa, the state changes to the state B having the absorbance characteristic shown by the broken line, and vice versa. When the light having the wavelength λb is applied to the state, the state changes to the state A.

First, the recording of information is performed in the conventional manner, for example, with or without the presence or absence of absorbance in the state B. For example, based on the recording information signal (recording information signal waveform) 100, ON / OFF of the light irradiation of the wavelength λa in the recording system 101.
Then, the information is recorded on the optical recording medium 102.

For reproduction of information, for example, a signal (for example, reflectance) A which changes based on a change in absorbance by irradiating light of wavelength λa with low power is detected by the detection system 103a, and the state A and the state B are changed. A signal (for example, reflectance) B obtained by illuminating light having a wavelength having the same absorbance, for example, light having a wavelength λc or wavelength λd, onto a spot irradiated with light having a wavelength λb is detected independently from the signal A by the detection system 103b. It is done by detecting.

Since the signal A is composed of an information signal and disk noise, and the signal B is a signal based on light of a wavelength having the same absorbance in the state A and the state B, it does not include the recorded information signal and the disc noise Is included. The disk noise included in the signal A and the signal B has the same phase because they are included in the signals A and B based on the light emitted to the same spot, and the signal A and the signal B are respectively amplified / attenuated by the amplification / attenuation systems 104a and 104a.
The disc noise has the same phase and the same amplitude because the disc noise is made to have the same magnitude by being appropriately amplified and attenuated by 4b.

Therefore, by subtracting in the subtraction circuit 105, the reproduced signal (reproduced signal waveform) 106 in which noise is effectively reduced can be obtained. Next, a specific configuration of the first embodiment will be described. The recording material used in this example includes spiropyran-based, azobenzene-based,
Although any photochromic material such as fulgide-based or diarylethene-based material can be used, for example, as shown in FIG. 3, a photochemical reaction occurs between state A and state B, and the absorbance characteristic changes as shown in FIG. A case where a spiropyran-based photochromic material is used for the recording layer will be described.

The optical recording medium used in this embodiment is 1.
A recording layer containing a spiropyran-based photochromic material having a thickness of 1 μm was formed on a glass substrate having a thickness of 2 mm by a vacuum vapor deposition method, and then a 1000 Å thickness of A was formed on the recording layer by a vacuum vapor deposition method.
1 A reflective layer is formed. Such an optical recording medium,
Information is obtained by irradiating the entire surface with ultraviolet light to bring it into a state B (erased / unrecorded state), and then irradiating it with a He-Ne laser beam having a wavelength of 633 nm at a high power (for example, power 10 mW) to bring it into a state A Can be recorded (formation of recording marks).

Therefore, it is possible to detect a change in reflectance (change in absorbance) according to a change in absorbance around 600 nm to obtain a signal containing disc noise and recorded information. Further, for example, a wavelength of 780 to 850 nm. Since there is no absorption in the region, the reflected light (or transmitted light) generated by irradiating the optical recording medium with light in this wavelength region does not contain recorded information, and a signal including disk noise can be detected.

FIG. 5 shows an example of a reproducing apparatus for reproducing the optical recording medium. In the figure, 1 is the optical recording medium. Reference numeral 2 denotes a light source such as a He-Ne laser device that outputs linearly polarized laser light having a wavelength of 633 nm, and 3 denotes a light source such as a semiconductor laser device that outputs linearly polarized laser light having a wavelength of 780 nm.

Reference numeral 4 denotes an objective lens system including an objective lens for two wavelengths designed to correspond to the light emitted from the light source 2 and the light source 3, and an actuator for driving the objective lens. Reference numeral 5 is a dichroic mirror that totally transmits the light output from the light source 2 and totally reflects the light output from the light source 3, so that the optical axes of the light emitted to the objective lens system 4 side coincide with each other. The optical system is set.

The weak laser light (for example, output of 1 mW) output from the light source 2 is expanded by the beam expander 21, and then transmitted through the polarization beam splitter 22,
It is converted from linearly polarized light into circularly polarized light by a quarter-wave plate (λ / 4 plate) 23, passes through the dichroic mirror 5, and is condensed as a spot on the recording layer of the optical recording medium 1 by the objective lens system 4.

The reflected light emitted from the optical recording medium 1 after being focused on the recording layer and then subjected to intensity modulation in accordance with the recorded information signal and the disk noise passes through the objective lens system 4 and the dichroic mirror 5. After being transmitted and converted into linearly polarized light orthogonal to the linearly polarized light by the quarter-wave plate 23, it is reflected by the polarization beam splitter 22 and is condensed by the objective lens 24 to generate an information signal and a signal including disk noise. It is detected by a detection system 27 including a photo detector 25 and an amplifier 26.

The laser light output from the light source 2 and the laser light output from the light source 3 at the same time (for example, output 1 mW)
Is shaped by the collimator lens 31, transmitted through the polarization beam splitter 32, converted from linearly polarized light to circularly polarized light by the quarter-wave plate 33, reflected by the dichroic mirror 5, and then by the objective lens system 4 as described above. Is focused on the same spot.

At the wavelength of the light output from the light source 3, the reflectance does not change according to the recorded information signal, and only the intensity modulation according to the disk noise is received. The reflected light that has undergone only the intensity modulation according to the disk noise is emitted from the optical recording medium 1. The reflected light passes through the objective lens system 4, is reflected by the dichroic mirror 5, is converted into linear polarized light orthogonal to the linear polarized light by the quarter wavelength plate 33, and is then reflected by the polarization beam splitter 32. A signal including the medium noise collected by the objective lens 34 is detected by a detection system 37 including a photodetector 35, an amplifier 36 and the like.

The disk noises included in the signals detected by the detection system 27 and the detection system 37 have the same phase, and the amplifiers 26 and 36 of the detection systems 27 and 37 or the differential amplifier 6 are adjusted in advance. Since the magnitudes of the disk noise are matched, the phases and amplitudes are the same. Therefore, the disk noise can be reduced by subtracting the output signal of the detection system 37 from the output signal of the detection system 27 by the differential amplifier 6. is there.

The adjustment of the amplifiers 26, 36 or the differential amplifier 6 is performed in the same state as that at the time of reproducing information. That is, at the same time, the light output from the light sources 2 and 3 is irradiated onto the optical recording medium at the same spot, and the resistance value of the amplifier 26 is observed while observing the noise of the signal output from the differential amplifier 6 with a measuring instrument such as a spectrum analyzer. , The resistance value of the amplifier 36 or the resistance value of the differential amplifier 6 is adjusted so that noise is reduced.

Before the signals detected by the detection systems 27 and 37 are input to the differential amplifier 6, circuits for subtracting the DC component from the signals are provided to remove the DC components of the signals. Is desirable. Also, in the above case, the light of wavelength 780 nm was used to detect the disk noise.
Since the difference in absorbance between the recorded state and the unrecorded state is small even with light near nm, and the information recording signal is hardly detected by the light, as shown in FIG.
A 458 nm Ar laser device is available. Incidentally, 31a is a beam expander.

Since the difference in absorbance between the recorded state and the unrecorded state is small even with light near the wavelength of 750 nm, it can be used for detecting disk noise. Furthermore, wavelengths 300-350
Since there is no difference in absorbance between the recorded state and the unrecorded state even with light of nm, it can be used for detecting disk noise.
As described above, while detecting the information signal including the disk noise as in the conventional case, the disk noise is detected by the light of the wavelength that does not or hardly detects the information signal, and the information signal including the disk noise is detected. Since disk noise is subtracted, noise can be reduced and high C /
N information reproduction signals were obtained.

In this embodiment, the amplifiers 26 and 36 are
Alternatively, the amplification characteristics of the differential amplifier 6 are changed so that the output levels of the disk noise are matched, but the amplification characteristics of the amplifiers 26 and 36 of the detection systems 27 and 37 and the differential amplifier 6 are made constant. By adjusting the power of the light output from the light source 2 or the light source 3 so that the magnitude of the disk noise is matched, there is no change in the phase characteristic or frequency characteristic due to the change in the amplification factor of the amplifier, so the noise is reduced. This is desirable because there is no risk of narrowing the frequency band that can be created.

When the output of the light source 3 is adjusted, even if the output is increased, light having a wavelength of 780 nm does not cause a photochemical reaction in the photochromic material, so that information is not destroyed, and therefore, the information is not destroyed. From this point, it is also suitable. Further, in the above embodiment, the light having the wavelength capable of detecting the information signal and the light having the wavelength not detecting the information signal or hardly detecting the information signal are simultaneously irradiated to the same spot. For example, the delay time of the signal based on each light may be adjusted to subtract the signals applied to the same spot.

However, the spot diameter d formed on the optical recording medium is approximately λ / NA when the wavelength λ of the light and the numerical aperture NA of the objective lens system 4 are set. Therefore, in the case of the above embodiment. However, there is a problem in that the spot diameters of the light having a wavelength of 780 nm and the light having a wavelength of 633 nm are different, and the removal of the disk noise is incomplete in the high frequency region. To solve this problem, for example, as shown in FIG.
From the light source 3 by the conventional super-resolution technology of inserting the disc-shaped shielding material 7 between the four-wave plate 33 and the dichroic mirror 5 to shield the central part of the light transmitted through the one-quarter wavelength plate 33. The size of the shielding material 7 may be selected so that the spot diameter due to the output light (wavelength 780 nm) is reduced and the spot diameter due to the light (wavelength 633 nm) output from the light source 2 is made the same.

When light having a wavelength longer than the wavelength of light for detecting information is used as light having a wavelength that does not detect or hardly detects an information signal, light and information that hardly detect or hardly detect information signal It is desirable that the wavelength of the light used for the detection is close because the frequency band in which the disk noise can be reduced becomes wider and the spot diameters can be easily matched by the above-mentioned super-resolution technique.

When light having a wavelength shorter than the wavelength of light for detecting information is used as light having a wavelength at which an information signal is not detected or hardly detected, some disc noise that cannot be removed remains due to different spot diameters. But
The light with a short wavelength can reduce the spot diameter, and thereby the disk noise can be detected over a wide frequency band, so that the disk noise reduction effect can be obtained over a wider frequency band.

Further, as the material of the recording layer of the optical recording medium, photochromic materials other than those in the above embodiments can be used.
In this case, it is necessary to appropriately change the wavelengths of the light from the light sources 2 and 3 and the optical elements so as to correspond to the wavelengths. In the above embodiments, the reflection type optical recording medium has been described, but the disk noise can be similarly reduced in the transmission type optical recording medium.

Further, even in the write-once type optical recording medium having the organic dye material in the recording layer, the disk noise can be detected by irradiating the material with light having a wavelength which is not absorbed, and thus the disk noise can be reduced by the same method. In addition, the same spot in this embodiment includes the case where the size and the position do not completely match.

[Second Embodiment] In the first embodiment, when the beam having the wavelength λa is irradiated in the state having the absorption spectrum shown in the state A as shown in FIG. At the same time, the change in the absorbance at that wavelength is detected with the beam of wavelength λb for reproduction, and at the same time, the second beam of wavelength λc, for which the absorbance does not change due to the photochromic reaction, is simultaneously irradiated to the same spot and the reflected light is detected. However, after appropriately amplifying or attenuating these two types of detection signals, the difference between them is used as the reproduction signal.

The mathematical description of this method is as follows. When the recording signal frequency is ω and the noise component is n (t), the output VB corresponding to the beam of wavelength λb is

[0043]

[Equation 1]

It becomes However, the component of the signal ω is standardized here. On the other hand, the output VC corresponding to the beam of wavelength λc
Does not include the signal, only the noise component is included in phase, so

[0045]

[Equation 2]

However, α is a constant depending on the detector efficiency and the beam power. Here, the above formula (1) -1 / α
× When formula (2) is created

[0047]

[Equation 3]

In principle, the disk noise is completely removed and the C / N becomes infinite, but in reality, the circuit system noise and the common mode rejection ratio of the differential amplifier are finite. C / N has a finite value due to the influence of laser noise, shot noise, and the like. Therefore, in the second embodiment, wavelengths having absorbance characteristics different from λb, such as λa and λc, are used.
However, it is a new finding that the improvement effect of C / N is produced.

For example, when the wavelength λd is used as the second beam, the output VC corresponding to the beam contains the signal component of the frequency ω and the noise component n (t) in the same phase as that of VB. The amplitude depends on the amount of change in reflectance, the sensor efficiency, and the difference in beam power. Therefore

[0050]

[Equation 4]

[Mathematical formula-see original document] Here, when formula (1) -1 / [gamma] * formula (4) is created,

[0052]

[Equation 5]

Therefore, the noise component can be removed, and C
/ N is improved. However, at this time, since the signal component is slightly lowered by the above subtraction, the C level is also lowered, and the value may be lower than the C / N given by the equation (3). On the other hand, consider the case where the wavelength λa is used as the second beam. In this case, the recorded absorbance at λa is
The change is opposite to the absorbance at λb. That is, the output VA corresponding to the beam of wavelength λa contains the signal component of the frequency ω in the opposite phase to that of VB, while the noise component is contained in the same phase.

Also, the amplitude depends on the reflectance change amount, the sensor efficiency, and the beam power as in the above equation (4).

[0055]

[Equation 6]

It becomes If formula (1) -1 / k × formula (6) is created here,

[0057]

[Equation 7]

This time, not only the noise component can be removed, but also the C component increases, so that a C / N larger than the C / N given by the equations (3) and (5) can be obtained. In this way, the first beam for reproduction and the second beam
The C / N can be improved if the wavelengths are different from each other, and especially if the changes in the absorbance due to the photochromic reaction are opposite in the first beam wavelength and the second beam wavelength, the C / N is improved. It can be said that the effect is great.

Now, as a reproducing optical system for carrying out the method of the present embodiment, basically the same device as shown in FIG. 6 may be used.

As the photochromic medium used in this embodiment, any of spiropyran-based, azobenzene-based, fulgide-based, diallylethene-based, etc. can be used, but especially as the wavelength of the second reproducing beam. If it is desired to enhance the C / N improvement effect by using the wavelength λa which causes the absorption characteristic change opposite to the wavelength of the first reproducing beam (λb in FIG. 2), 1 ', 3', 3'trimethyl- Instead of a material such as 6-hydroxyspiro [2H-1-1] benzopyrrillospirane, a material having a change in absorbance characteristics such as a diarylethene system may be used.

FIG. 8 shows the molecular structure of 2- (1,2-dimethyl-3-indolyl) -3- (2,3,5-trimethyl-3-thienyl) maleic anhydride, a typical diarylethene material. And shows the change in absorption spectrum.

As is clear from FIG. 8, the emitted light from the Ar laser having a wavelength of 458 nm is λa and the wavelength 6 is λb.
Emission light from a 33 nm He-Ne laser can be used.

Therefore, as the laser light source 2 of FIG.
A He—Ne laser may be used as the laser 3.

In this embodiment, information recording is first performed by the following procedure. First, the entire surface of the disk is sufficiently irradiated with Ar laser light of λ = 476.5 nm, and the absorption spectrum is made as shown by the broken line in FIG.

Then, recording is performed by irradiating a beam of λ = 633 nm with modulation at a linear velocity of 2 m / s and a power of 10 mW. Of course, without performing the above procedure, recording may be performed by modulating the beam of λ = 476.5 nm in a state having the initial full-line absorption spectrum.

In any of the above methods, the recording causes a change in the reflectance on the disc according to the information recording signal, which has opposite characteristics at the wavelength of 633 nm and the wavelength of 458 nm. Therefore, weak beam (~ 0.5mW) of λ = 633nm and λ
When a weak beam (= 0.5 mW) of = 458 nm is applied to the same spot and the reflected light is detected by independent detection systems 27 and 37, their output signal components have opposite characteristics.

On the other hand, since the noise components due to the disk surface property are in phase at the outputs of the detection systems 27 and 37, they are appropriately amplified or attenuated by the differential amplifier system 6 and then reproduced by subtraction processing. As an output, a signal level is high and an output with less noise can be obtained.

The above-mentioned 2- (1,2-dimethyl-3-indolyl)-
3- (2,3,5-Trimethyl-3-thienyl) maleic anhydride was mixed with polyvinyl butyral resin at 18 wt% and dissolved in anone solvent to give a film thickness by spin coating on a glass disk substrate with a diameter of 120 mmφ. A 1 μm recording layer was formed.

Next, Ag was vacuum-deposited from above to form a reflective film, and an optical recording medium was obtained. Recording and reproduction were performed using the optical system and pickup shown in FIG.

In this case, first, the disk sample 1 was irradiated with the λ = 476.5 nm radiated light from the Ar laser in an enlarged manner and irradiated over the entire surface to obtain the absorption spectrum characteristic shown by the broken line in FIG.

Then, the disk 1 was rotated at a linear velocity of 2 m / s, and He-Ne laser light having a power of 10 mW was modulated and irradiated at 100 kHz for recording. Note that this recording can be performed, for example, by installing an AO modulator between the He—Ne laser 3 and the beam expander 31a in the optical system of FIG.

For reproduction, a He-Ne laser (λ = 633 nm) was used.
CW (Continuou) with 5mW, Ar laser (λ = 458nm) at 0.5mW
s Wave) irradiation, and the output of the differential amplifier circuit 6 of FIG. 6 that performs the calculation between the detected signals obtained is analyzed by a spectrum analyzer, and the signal level (C level) is -5.
The dBm and noise level (N level) were -55 dBm, so that a good reproduction signal quality of 50 dB as the CN ratio was obtained.

On the other hand, as a first comparative example, the conventional method, that is, the Ar laser was not used, and the reproduction was performed only with the He—Ne laser. The disc conditions and other conditions for recording and reproducing at this time were the same as in the previous example. As a result, the C level is -11 dBm, which is 6 dB lower than the previous example.
Conversely, the N level increased by -51 dBm to 4 dB, and the CN ratio was 40 d.
It deteriorated to B.

As a second comparative example, instead of using the Ar laser (λ = 458 nm) as the second beam, λ = 780 nm light from a semiconductor laser having no absorption by the photochromic material was used. The power of this semiconductor laser is 1 mW, and other conditions are the same.

As a result, the C level was -11 dBm, which was the same as that of the first comparative example, but the N level was -55 dBm, which was about the level of this embodiment, and the CN ratio was 44 dB. .. Further, as another development example of the second embodiment, Y which emits a laser beam of λ = 532 nm as a light source of the second beam is used.
A light source capable of emitting SHG light of an AG laser was used. In this case, as explained in the equations (4) and (5), λ = 633 nm and λ = 5
At 32 nm, the signals due to reflectance changes are in phase, but λ = 633n
Since the amount of change in reflectance at m is larger, the signal level will drop slightly.

However, even in this case, if the N level can be reduced more than the reduction amount of the signal level, the obtained signal quality is improved.

In this example, λ = 53 as the second beam.
All were the same as in the previous example, except that irradiation with 2 nm light was performed at 0.5 mW. As a result, the C level was reduced to -13 dBm compared to the first comparative example, but the N level was -55 dBm and C
The N ratio was 42 dB, which was a reproduction output quality better than that of the first comparative example.

The above results are summarized as follows. That is, by performing an appropriate calculation (subtraction) between the detection signals obtained by irradiating the medium with the first beam for reproduction and the second beam having a different wavelength from the first beam, Conventional (1st
It is possible to improve the quality of the reproduced signal as compared with the comparative example.

Especially, the second beam is not limited to the wavelength range in which recorded information is not detected, and the effect of this embodiment can be expected as long as the wavelength is different from that of the first beam.

The most effective combination of the first and second beams is such that each beam has a wavelength at which information can be detected, and their detection signals have mutually opposite phases (directions of changes in reflectance are opposite to each other). ) Should be selected.

By the way, one of the problems of the photochromic medium is realization of nondestructive reading. This is because the photochromic reaction is a photon mode reaction, so there is no threshold in the reaction, so no matter how low the power is reproduced, by repeating this, the photochromic reaction progresses and finally the recorded information disappears. It's a problem.

That is, in the reproducing methods of the first and second embodiments, there is a problem that nondestructive reading cannot be realized because the change in absorbance is used. Therefore, in the present embodiment, non-destructive reading was realized by causing recording in the photochromic medium by causing changes in birefringence and optical rotatory power, and using a beam having a wavelength that the photochromic material does not absorb for reproduction.

The principle of such a method is described, for example, in Workshop on Organic Electronics Materials, workshop 91, "Characteristics and Problems of Organic Materials in Optical Recording Medium"('91 .7), Nos. 51-6.
It is disclosed in page 1 and JP-A-2-44536. First, a case where the third embodiment is applied to a nondestructive read method for detecting a change in birefringence will be described with reference to FIGS.

Since such a change in birefringence usually exists even in a region where there is no absorption, reflected light (transmitted light) can be obtained by simply irradiating a beam in such a wavelength range as the second beam.
Changes its polarization state under the influence of birefringence. Figure 9
(A) shows a state in which linearly polarized light whose polarization plane does not coincide with the neutral axis direction of birefringence is used as such a beam, and the reflected light becomes elliptically polarized light due to the influence of birefringence.

Since the ellipticity of the elliptically polarized light and the azimuths of the long axis and the short axis are affected by the magnitude of the birefringence of the recording layer, that is, the recording information signal, the first and second embodiments are left as they are. Can not use the principle of. This problem is the same when other polarization states (circularly polarized light or elliptically polarized light) are used. On the other hand, in FIG. 9B, a beam in a wavelength range in which the photochromic material does not have absorption is used as the second beam, and its polarization plane coincides with the neutral axis direction of the birefringence (slow axis in the figure). It shows a situation when linearly polarized light is used.

As described above, when the plane of polarization coincides with the birefringent neutral axis direction, the reflected light is not affected by the birefringence and has the same polarization state as the incident light. The principle of can be used. Similarly to this, a case where this embodiment is applied to a nondestructive read method for detecting a change in optical activity will be described with reference to FIGS.

FIG. 10 (a) shows a situation when linearly polarized light is used as such a beam. In the reflected light, the rotation of the polarization direction is caused by the optical rotatory power, that is, the recorded information is contained in the reflected light. There will be a signal effect, the first,
The principle of the second embodiment cannot be used. This tendency is the same when a polarization state other than circularly polarized light such as elliptically polarized light is used.

On the other hand, in FIG. 10 (b), a state in which a beam in the wavelength range in which the photochromic material does not have absorption is used as the second beam and a beam whose polarization state is circularly polarized is used. Shows. In this way, even if the circularly polarized light is isotropic in nature even if the polarization direction is rotated due to the effect of the optical rotatory power, the reflected light does not change as circularly polarized light. Therefore, the principles of the first and second embodiments are used. be able to.

Next, FIG. 11 shows the structure of the reproducing optical system / circuit system in the case of using the detection of birefringence change in the optical reproducing method of the third embodiment. The same components as those in are denoted by the same reference numerals. As the medium used at this time, a medium on which information is recorded can be used by irradiating linearly polarized light to induce birefringence, but the reproducing method of the present embodiment is not limited to this. Absent.

In the reproducing apparatus having such a structure, first, the first reproducing light (beam) for reproducing information will be described. Since many photochromic materials have absorption in the visible light region, the semiconductor laser 2 which emits laser light having a near infrared wavelength, for example, 780 nm can be used as the light source of the reproducing first beam. The emitted light from the semiconductor laser light source 2 is converted into parallel light by the collimator lens 28 and is transmitted as P waves to the polarization beam splitter 22 (the polarization azimuth at this time is 0 °).

Next, the azimuth is +45 with the λ / 2 wave plate 23.
The light is converted into linearly polarized light having a wavelength of λ = 780 nm while passing through a dichroic mirror 5 that reflects light having a wavelength of λ = 830 nm, and then is irradiated onto the photochromic medium 1 by the objective lens 2. Information is recorded on the photochromic medium 1 depending on the presence / absence of birefringence in which the neutral axis azimuth is 0 °. Therefore, the linearly polarized light irradiated to the medium at the azimuth angle of 45 ° has a portion in which birefringence exists. Then, under the influence, the reflected light is converted into elliptically polarized light, and the linearly polarized light is reflected as it is in the portion where birefringence does not exist.

The reflected light again passes through the objective lens 2 and the dichroic mirror 5, and the azimuth angle is converted again by the λ / 2 plate 23. At this time, the beam irradiated on the part where birefringence does not exist is converted again to linearly polarized light having the same azimuth angle of 0 ° as the original beam and passes through the polarization beam splitter 22, but the part where birefringence exists is irradiated. Since the converted beam becomes elliptically polarized light, a polarization component with an azimuth angle of 90 ° remains after the conversion of the polarization direction by the λ / 2 plate 23, and this component is detected by the photodetector 25.

Therefore, the output of the detector 25 changes depending on the presence or absence of birefringence, so that information is reproduced. On the other hand, due to the influence of the surface roughness of the medium 1, the reflected light intensity itself fluctuates, and this fluctuation also occurs.
It is detected at 25 and becomes noise. Next, the second noise reduction
The beam will be described. The wavelength of the second beam is also selected so that the photochromic material does not have absorption. For example, the semiconductor laser 3 having λ = 830 nm is used as a light source.

The emitted light from the semiconductor laser light source 3 is converted into parallel light by the collimator lens 31, then transmitted through the beam splitter 32, reflected by the dichroic mirror 5 and the same as the first beam by the objective lens system 2. It is focused on the spot. At this time, the azimuth of the light source 3 is adjusted in advance so that the second beam is irradiated with linearly polarized light whose polarization plane coincides with the neutral axis azimuth (0 ° in this case) of the birefringence in the medium.

Therefore, the second part is present in the part where the birefringence of the medium exists.
When the second beam is applied to a portion having no birefringence, the reflected light is reflected as linearly polarized light. After that, a part of the beam is reflected again by the beam splitter 9 through the objective lens 4 and the dichroic mirror 5, and is detected by the detector 35.

The amount of light reaching the detector 35 does not depend on the presence or absence of birefringence, but depends only on the roughness of the medium surface, and the output of the detector 35 is only a noise component. The output currents of the detectors 25 and 35 are the IV conversion units 26 and 36, respectively.
After being converted into a voltage by, the differential amplifier system 6 performs arithmetic processing (subtraction). At this time, in order to efficiently reduce the noise component, it is necessary to appropriately amplify or attenuate the output voltage of each of the detectors. This is realized by optimally selecting the resistance value used for the differential amplifier system 6. it can.

On the other hand, FIG. 12 shows the configuration of the reproduction optical system and the circuit system when the detection of the change in optical rotatory power is used for reproduction. As the medium used here, an optical recording medium in which a photochromic compound is dispersed in a twisted nematic liquid crystal as described in JP-A-2-44536 can be used.
The present embodiment is not limited to this.

First, the first beam for reproducing information will be described. The elements having the same functions as those in the first and second embodiments are designated by the same reference numerals. The difference from the reproducing apparatus of FIG. 11 is that the λ / 2 plate is not necessary.
As a result, the first beam irradiates the medium 1 with linearly polarized light, but the polarization plane of the reflected light is either rotated or reflected in the same direction depending on the presence or absence of the optical rotatory power generated in recording the information. To be done.

Therefore, the amount of light reflected by the polarization beam splitter 22 changes depending on the presence or absence of optical rotatory power,
This is detected by the detector 25 and becomes a reproduction signal output. The noise is the same as in the case of FIG. Next, the second beam for noise reduction will be described. This beam passes through the polarization beam splitter 32 as a P wave and then λ /
It is converted into circularly polarized light by the four plates 33.

Therefore, since the medium is irradiated with circularly polarized light, it is reflected without being affected by the presence or absence of optical rotatory power and is again converted into S polarized light by the λ / 4 plate 33, and the polarized beam splitter 32 Reflected by, Detector 35
Detected in. In this way, the detector 35 can obtain an output containing only the noise component. And the IV converter
The functions of 26 and 36 and the differential amplifier section 6 are the same as those in the case of FIG. 11, whereby a reproduction output with less noise can be obtained.

When the second beam has a polarization state that is considerably different from the circularly polarized light when the medium 1 is irradiated by the phase difference generated when the second beam is reflected by the dichroic mirror 5, these dichroic mirrors are used. It is necessary to provide a phase difference compensating plate between the 5 and the λ / 4 plate 33. As described above, in the third embodiment, the case where the recording is executed depending on the presence or absence of the birefringence and the presence or absence of the optical rotatory power is, of course, applicable to the case where the recording is executed depending on the magnitude of those.

Next, of the two development examples of the third embodiment, first, the details of the experiment and the result thereof regarding the method using the birefringence change will be described.

First, for the optical recording medium, a fulgide material (FF-10) was used as a photochromic material.
This material is disclosed in JP-A-2-260127 (G11B).
7/00), dichroism is generated by irradiating with linearly polarized light, and birefringence is caused by the anisotropy generated with it.

This FF-10 and polymethylmethacrylate were dissolved in methylene chloride, and spin-coated on a glass disk substrate having a diameter of 120 mm and a thickness of 1.2 mm to a thickness of 1
A μm recording layer was formed. Next, an Ag reflection film was formed by vacuum vapor deposition to form an optical recording medium structure.

Therefore, first, an Ar laser beam capable of emitting linearly polarized ultraviolet light having a wavelength of 360 nm is emitted to this medium with a spot diameter of 1.5.
Focus on μm, relative speed 2m / s, power 10mW, frequency 100kH
Recording was performed by irradiation with intensity modulation with z. The azimuth of the linearly polarized light at this time was matched with the relative velocity direction (this azimuth is 0 °).

Next, reproduction was performed using the reproducing apparatus described in FIG. The reproducing power of the first beam and the second beam was set to 1 mW respectively, and the relative speed was the same as during recording.
The other conditions of the optical system are the same as those described in FIG.

Then, the reproduction output was analyzed by a spectrum analyzer. Table 1 shows preamplifier 2 before noise reduction processing
6 shows the signal level (C level) and the noise level (N level) of the outputs of 6 and the differential amplifier 6 and the CN ratio. Looking at this Table 1, C
The N level is improved without changing the level, resulting in CN
It becomes clear that the ratio is improved and the reproduced signal quality is improved.

[0108]

[Table 1]

Although the fulgide-based photochromic material is used in this development example, the present invention is not limited to this, and can be similarly applied to a medium using various materials such as spiropyran-based, diarylethene-based, and azobenzene-based materials.

Next, the details and results of the experiment will be described below with respect to the second development example, that is, the method using the optical rotation change.

Various materials can be applied as the optical recording medium material used in the development example. For example USP 5,011,
The 756 specification discloses a large number of materials whose rotatory power is changed by irradiation with circularly polarized light, and JP-A-63-259850 discloses various spiropyran-based materials whose rotatory power is changed by light irradiation. Further, JP-A-2-44536 discloses an optical recording medium that causes a change in orientation of liquid crystal molecules having a helical structure due to a change in molecular structure due to photochromism, thereby causing a change in optical rotation in a recording layer. In the second development example, 1 ', 3', 3'-trimethyl-6-of spiropyran series
Nitrospiro [2H-1-benzopyran-2,2'-indoline]
A mixture of polystyrene (hereinafter referred to as NBPS) at 10 wt% and a solution of the mixture dissolved in methylene chloride solution was applied onto a glass disk substrate by spin coating to form a recording layer having a thickness of 1 μm.

NBPS has spiro carbon and has optical rotatory power, but it is converted to a merocyanine type by irradiation with ultraviolet rays and the optical rotatory power disappears. However, it has the property of becoming a spiropyran type again by irradiation with visible light or heating to generate optical activity. Then, Ag is formed on the recording layer having such a structure.
The reflective film was vacuum-deposited to obtain the same optical recording medium structure as in the first developed example.

This optical disk has a wavelength of 36 from a mercury lamp.
The entire surface was sufficiently irradiated with 5 nm ultraviolet light to make the photochromic molecule in the recording layer a merocyanine type.

Next, the emitted light of the He-Ne laser is emitted at a power of 10 m.
Recording was performed by irradiation with intensity modulation at W, spot diameter 1.5 μm, and frequency 100 kHz. The relative speed at this time is 2 m / s. Therefore, only the portion irradiated with the He-Ne laser has the photochromic molecule changed to the spiropyran type, and a recording mark having optical activity is formed.

Next, reproduction was performed using the optical system shown in FIG. Since both the spiropyran type and merocyanine type materials have no absorption in the near infrared wavelength region such as λ = 780 nm and λ = 830 nm, nondestructive readout is possible by using a semiconductor laser emitting a beam of these wavelengths as a light source. I can do it.

The powers of both lasers are both constant at 1 mW, the relative speed is the same as that at the time of recording, and other optical systems are the same as in the first developed example.

The reproduced output was analyzed by a spectrum analyzer. Table 2 shows the preamplifier 26 after noise reduction processing.
And the noise level (N level) and CN of the signal level (C level) and the noise output of the differential amplifier 6 after noise reduction processing.
It shows the ratio.

[0118]

[Table 2]

As is apparent from Table 2, the noise reduction process improves the N level without changing the C level,
As a result, it can be seen that the CN ratio is improved and the reproduction signal quality is improved. Needless to say, various photochromic materials can be used in the present development example as described above.

Summarizing the above results, in the reproducing method of the information recorded in the photochromic optical recording medium by the change of the birefringence or the optical rotatory power (or both),
A beam having a polarization state in which information can be reproduced is used as a beam for the above, and a beam having a polarization state in which the information is not reproduced is used as a second beam, and an appropriate calculation is performed between the detection signal outputs obtained thereby. As a result, the reproduction signal quality level can be improved.

[Fourth Embodiment] First, the principle of the fourth embodiment will be described with reference to (a), (b) and (c) of FIG. Figure 13
Discloses a method of storing detection signals by the first and second reproduction lights, which are independently detected, in a storage unit and reading the stored detection signals by a synchronization signal.

Here, in order to simplify the explanation, there are two detection systems, CH1 and CH2, respectively. These detection systems may be used for any method of reducing the noise of the reproduced signal, increasing the signal level, or wavelength multiplexing, but when using for reducing the noise or increasing the signal level, both detection systems may be used. The signal must include a sync signal.

When used for wavelength multiplexing recording,
It is necessary that the sync signal in each of the multiple-recorded signals be located at the same portion on the medium. Now, in FIG. 13 (a), each information signal (detection signal) is stored as a pair with the front and rear synchronization signals, and two beam spots for CH1 and CH2 are on the same track. , CH1 and CH2 depending on the distance between the spots and the relative speed between the optical head and the disk when they are not focused on the same spot.
A time difference T occurs in the detection signal of.

FIG. 13B shows a block diagram of the reproduction system of each of the detection signals. Each detection signal of CH1 and CH2 stores the information signal between the synchronization signals as one block. Are temporarily stored in the waveform memories 41 and 42, and are simultaneously read by using an external memory read sync signal (not shown) as a trigger. Therefore, FIG.
As shown in (c), it is possible to obtain the signals CH1 'and CH2' that have been read without a time difference T.

At this time, each of the memories 41 and 42 needs to have a capacity capable of storing detection signals for a plurality of blocks according to the time difference T. Then, the read signals CH1 ', C
For H2 ', it is sufficient to perform arithmetic processing corresponding to each method of reducing noise, increasing signal level, and reducing crosstalk by wavelength multiplexing.

On the other hand, FIG. 14 discloses a method of delaying one signal system corresponding to the time difference T shown in FIG. 13 and synchronizing and reading the detection signal. Figure 14 here
Since (a) is the same as FIG. 13 (a), the description is omitted.
FIG. 14B shows a block diagram of the reproduction method of each detection signal obtained in the above (A). It is assumed that the detection signals of CH1 and CH2 (the detection signal of CH2 is delayed from that of CH1). The timing of the sync signal is detected by each of the sync signal detectors 43 and 44, and the time difference determination unit 45 compares the timings with each other to obtain the time difference T, and the delay amount corresponding to this is calculated as the time difference T delay. Give to CH2 by part 46.

Here, various methods of delaying CH2 are conceivable. For example, a method of temporarily storing in the waveform memory and then delaying by the delay amount T and reading out may be used as in the example of FIG. The signal CH2 'and the detection signal CH1' thus delayed have no time difference as shown in FIG. 14C, and these signals CH1 ', CH
For 2 ′, it is sufficient to perform calculation corresponding to each method of reducing noise, increasing signal level, and reducing crosstalk by wavelength multiplexing.

FIG. 15 is a block diagram of an optical system / circuit system showing an example in which the detection signal reading method of this embodiment (example of FIG. 13) is adopted in the crosstalk reducing reproducing apparatus at the time of wavelength multiplexing recording. Components having the same functions as those in the first to third embodiments are designated by the same reference numerals. FIG. 16 shows how the objective lens system 4 irradiates the medium 1 with two beams at different spot positions, although they are on the same track. Here, 48 is a recording layer, 49 is a substrate, and 50 is a recording track.

In FIG. 15, in order to reproduce the multiplex-recorded signal, the beams of the corresponding wavelengths λa and λb are emitted from the light sources 2 and 3.
Are radiated with a constant power respectively, are combined by the dichroic mirror 5, and are focused on the same track 50 of the medium 1 by the objective lens system 10. The reflected light from the medium 1 passes through the objective lens system 10 again, is separated by the dichroic mirror 5, and reaches the photodetectors 25 and 35 by the actions of the λ / 4 wavelength plates 23 and 33 and the polarization beam splitters 22 and 32, Here, photoelectric conversion is performed and current-voltage conversion is performed by the IV conversion units 26 and 36, and a detection signal with large crosstalk is obtained.

Next, after converting these signals into digital signals by the A / D converters 38 and 39, the waveform memory parts 41 and 42 are converted.
Once stored in. Further, it is read out as a synchronized signal from the memory units 41 and 42 by using an external read synchronizing signal, converted into an analog signal by the D / A conversion units 46 and 47, and composed of two differential amplifiers 48 and 49. A predetermined crosstalk reduction calculation process is performed by the differential amplifier system 6.

Of course, the device having such a structure can be applied to a reproducing system such as a low noise and a high signal level other than the crosstalk reduction by changing the structure of the differential amplifier system 6, and in any case. Photo detector 25,35
After the detection signals are converted by the I / V conversion units 26 and 36 and before performing a predetermined calculation, the waveform memory units 41 and 42 are provided and the signals can be synchronized here.

Further, in the system in which one detection signal is delayed according to the time difference T, synchronization signal detection units 43 and 44 and a time difference T determination unit 45 are provided as shown in the block diagram of the optical system / circuit system of FIG. Needless to say, it is possible to delay one detection signal by the time difference T by the control circuit 40.

By using such a method, it is not necessary to exactly match the beam spots corresponding to the channels to be multiplex-recorded on the medium in order to reduce the crosstalk at the time of wavelength or polarization multiplex recording / reproduction. It is not necessary to exactly match the spots by arranging the spots on a straight line, which facilitates adjustment of the optical pickup.

As a concrete example of this embodiment, an experiment and a result of crosstalk reduction in dual wavelength multiplex recording will be shown below.
First, as photochromic materials, 2- (1,2-dimethyl-3-indolyl-3- (2,3,5-trimethyl-3-thienyl) maleic anhydride (hereinafter referred to as asymmetric indole / thiophene type) and 2,3- Bis (2-methylbenzo [b] thiophen-3-yl) maleic anhydride (hereinafter referred to as symmetrical benzothiophene type) was mixed with polyvinyl butyral resin at 9 wt% each and dissolved with anone to form a glass disk substrate with a diameter of 120 mmφ. Spin coating was performed to form a recording layer having a film thickness of 1 μm.

Changes in the molecular structure and absorption spectrum of these materials are shown in FIG. 8 and FIG. 18, respectively. Then, an Ag reflective layer was formed on this recording layer by a vacuum vapor deposition method to obtain a medium.

As shown in FIG. 8, when the asymmetric indolethiophene type is irradiated with light having a wavelength of 400 to 480 nm, the solid line changes from the solid line to the broken line.
Irradiation with light in the vicinity of 0 to 700 nm causes a reverse reaction, that is, a change from the broken line to the state shown by the solid line.

Therefore, for this type of material, for example, the entire surface is irradiated with blue light to change it to the state shown by the broken line in advance, and then the light of the He-Ne laser of λ = 633 nm is responsive to the recording signal with high power. Recording can be performed by irradiating with modulated light, and reproduction can be performed by irradiating the light of the He-Ne laser of λ = 633 nm at a low power of one low level and detecting the reflectance change.

Further, as shown in FIG. 18, the symmetric benzothiophene type compound changes from a state shown by a solid line to a state shown by a broken line when blue light having a wavelength of about 400 to 460 nm is irradiated, Irradiation with light of 500 to 600 nm causes a reverse reaction, that is, a change from the broken line to the state shown by the solid line.

Therefore, the medium containing this kind of material is irradiated with blue light over the entire surface to change it to the state shown by the broken line in advance, and then the light of the Ar laser of λ = 514.5 nm is recorded with high power. It is possible to perform recording by irradiating with a modulation depending on the condition, and similarly, by irradiating the light of the Ar laser of λ = 514.5 nm with a low power of a low level to detect the change in reflectance.

Therefore, 2 prepared by mixing these materials
The wavelength-multiplexed recording disk can be wavelength-multiplexed recorded / reproduced by irradiating the entire surface with blue light in advance and then using a He-Ne laser or an Ar laser as a recording / reproducing light source.

By the way, in a material system in which the wavelength selectivity is not perfect, crosstalk occurs between multiplexed channels. In the above-mentioned material system, the absorption spectrum change that causes such crosstalk is characterized as shown in FIG.

That is, in the figure, the solid line a is the initial state immediately after the blue light irradiation, the broken line b is the irradiation of the He-Ne laser light of λ = 633 nm, and the dashed line c is the irradiation of the Ar laser light of λ = 514.5 nm. The back and the broken line d represent the absorption spectra after the simultaneous irradiation with the He-Ne laser and the Ar laser, respectively. In the case of this system, the asymmetric indolethiophene type also absorbs at λ = 514.5 nm and causes a reaction, which causes such crosstalk.

Next, recording / reproducing was performed using the apparatus shown in FIG. As described above, the He-Ne laser is used as the light source 2, the Ar laser is used as the light source 3, and the AO modulators are installed between the light sources 2 and 3 and the polarization beam splitters 22 and 32 during recording. Power modulation is performed at a predetermined recording frequency.

In this development example, it is not necessary to make the respective laser beam spots perfectly coincide with each other on the medium 1, so that the adjustment can be easily performed.

At the time of recording, the λ = 514.5 nm light of the Ar laser was irradiated at a power of 6 mW and a frequency of 300 kHz, and the He-Ne laser light of λ = 633 nm was irradiated at a power of 5 mW and a frequency of 200 kHz.
During reproduction, λ = 514.5 nm light was radiated at 0.5 mW constant and λ = 633 nm light was radiated at 0.5 mW constant. The rotation speed of the disk 1 at this time was 400 rpm, and the crosstalk was performed by measuring the output ratio of the crosstalk component to the main signal with a spectrum analyzer.

As a result, as shown in Table 3, the outputs of the raw signals 26 and 36 before the crosstalk reduction calculation generated a large crosstalk of -7 dB in both channels.
As shown in Table 4, the output obtained by subtracting the signals read out simultaneously after storing them in the memory units 41 and 42 by the differential amplifiers 48 and 49 is -41 dB, and- We were able to reduce it significantly to 40 dB.

[0147]

[Table 3]

[0148]

[Table 4]

On the other hand, as a comparative example, the memory sections 41 and 42 are provided.
The optical system shown in FIG.
The description is omitted because it is the same as the above). In this case, it was difficult to adjust the laser optical axis in order to match the spots of the two lasers 2 and 3.

The results are shown in Tables 5 and 6. From this table, it can be seen that the −7 dB crosstalk included in the outputs of the preamplifiers 26 and 36 is improved to −38 dB in the differential amplifiers 48 and 49 for reducing the crosstalk component.

[0151]

[Table 5]

[0152]

[Table 6]

It should be noted from this result that not only the device of FIG. 15 is easier to adjust but the amount of crosstalk reduction is larger than that of the device shown in FIG. This is because it is difficult to make the laser spots perfectly match even if the laser spots are finely adjusted so as to match the laser spots as in the optical system of FIG. 20, and a mismatch amount of submicron order is inevitably generated, which reduces the crosstalk reducing effect. This is because it will be done.

On the other hand, in the optical system of FIG. 15, the read timing from the memory section of both laser optical paths can be electrically finely adjusted so that the crosstalk is minimized. Is possible.

An experiment similar to this was conducted for the optical system shown in FIG. 17, and the same results as those in Tables 3 and 4 were obtained.

As described above, in the fourth embodiment, two kinds of diarylethene materials are used and the light source is Ar.
Lasers and He-Ne lasers have been used, but the present invention is not limited to these, and various photochromic materials such as spiropyran-based, fulgide-based, azobenzene-based can be used, and the light source uses a semiconductor laser or an SHG element. Various other materials can be used.

Further, in the present embodiment, as the crosstalk reduction calculation, only the reduction of the linear crosstalk component by the subtraction is shown, but it can be easily applied to the reduction of the non-linear crosstalk component generally included in the wavelength multiplexing recording / reproducing. (To this end, the differential amplifiers 48 and 49 may be improved). [Fifth Embodiment] In the above-described first to fourth embodiments, optical recording is performed when performing calculation processing such as crosstalk reduction, noise reduction, and high signal level after synchronization of detection signals. The linear calculation is performed by subtracting the detection signal obtained by irradiating the second beam from the signal obtained by irradiating the medium with the first beam.

That is, the detection signal by the first beam contains both the recording information signal component and the disk substrate noise component, and the detection signal by the second beam contains only the disk substrate noise component. It is said that the noise component can be effectively reduced by taking the difference of. The state of noise reduction at this time is schematically shown in FIG.

In the figure, the simple subtraction of two detection signals has the following problems. That is, when the modulation degree of the medium (difference in reflectance between the unrecorded portion and the recorded portion) is small, as shown in (a) to (c), the detection signal from the first beam to the detection signal from the second beam is detected. By subtracting (b), a reproduced output signal (c) with noise efficiently reduced can be obtained.

On the other hand, as shown in (d) to (f), when the modulation degree of the medium is large, the absolute value of the photodetector output is large when the first reproduction light is reflected by the portion having a high reflectance, and therefore, The noise amplitude also becomes large as shown in (d) at the H level, while the absolute value of the photodetector output is small when the first reproduction light is reflected at the portion where the reflectance is small, so that the noise amplitude is also shown at the L level. Becomes smaller.

On the other hand, when the second reproduction light is reflected, its noise detection output always has a constant amplitude as shown in (e). Therefore, noise cannot be completely removed by simply subtracting the signal (e) from the signal (d). In this embodiment, attention is paid to this point, and a method is adopted in which noise can be efficiently reduced even when the modulation degree of the medium is high and the reproduction signal amplitude level continuously changes.

That is, as shown in FIG. 22, the first detection signal (a) contains noise whose amplitude varies depending on the reflection level. On the other hand, the second detection signal (B) contains only a noise component with a constant amplitude. Therefore, in order to correspond to the noise amplitude included in the first detection signal (a), the second
The noise can be effectively removed by changing the amplitude of the noise of the detection signal of (c) and subtracting it from the first detection signal (a), and as a result, the reproduced signal with small noise (d)
Is the principle that can be obtained.

A case where this principle is adopted as a disk noise reducing method will be described below. The first detection signal output V1 including the recording information signal (in this case, a single frequency of the angular frequency ω for simplification) and the disk noise is expressed by the following equation (8).

[0164]

[Equation 8]

Here, α is a proportional constant including preamplifier gain, pickup efficiency, detector efficiency, etc., P is the laser power on the medium surface, R is the average reflectance, M is the degree of modulation, and the fluctuation δR of R is Represents disk noise. Therefore, it becomes clear from this equation that the second term of the equation (8) becomes a disk noise component and the amplitude changes depending on the recording signal.

Normally, the low frequency component αPR used in various servo circuit systems is cut from the output V1, so that the output V1 further changes to the next V1 '.

[0167]

[Equation 9]

On the other hand, the disk noise output V2 is given by the following equation.

[0169]

[Equation 10]

Here, β is a proportional constant similar to α, and P ′ is laser power. And the low frequency components are cut like V2 to V1.

[0171]

[Equation 11]

Now, an offset voltage corresponding to the reflectance of the medium is added to the first detection signal containing the reproduction signal component,
Considering the operation of creating a product of the obtained signal and a second detection signal containing only a noise component and subtracting a signal obtained by appropriately amplifying or attenuating this signal from the first detection signal, An offset voltage (DC) corresponding to the average reflectance of the medium is added to the detection signal V1 ′ of 1 to restore the equation (8).

Then, V1 of the equation (8) restored in this way
Is multiplied by the second detection signal V2 'of the equation (11) to generate the signal V3. The signal V3 thus obtained is expressed as follows.

[0174]

[Equation 12]

Here, γ is a proportional constant. And using this signal V3

[0176]

[Equation 13]

When you create

[0178]

[Numerical equation 14]

Is obtained. The first term of the equation (14) represents the signal component and the second term of the equation (14) represents the disk noise component. In the equation (9), the second term is a linear equation of δR, whereas the equation (14) ) Is the quadratic expression of δR.
In addition, considering that normally δR is considerably smaller than the signal component, the square of (δR) is only a smaller contribution than that in the above equation (14), and therefore C / N Will be improved.

FIG. 23 is a block diagram showing an optical system / circuit system of a reproducing apparatus adopting the above-mentioned method.
Components having the same functions as those in the first to fourth embodiments are designated by the same reference numerals. In the figure, the detection system B27 outputs a first detection signal V1 containing noise and an information signal, and the detection system C37 outputs a second detection signal V2 containing only noise.

The low-frequency components for servo are separated by cutting the low-frequency components by the high-pass filters 51 and 52. That is, the first detection signal V1 'after passing through the filter 51 is added with an offset voltage by the differential amplifier section 48 and returned to V1, and then multiplied by the second detection signal V2' in the multiplication circuit section 53. As a result, an output corresponding to the signal V3 is obtained.

The signal V3 thus obtained is appropriately amplified or attenuated, and the first detection signal V is detected by the differential amplifier section 49.
By subtracting from 1 ', a reproduced output signal V4 with less noise is obtained. Various methods other than the above-mentioned calculation method can be considered. For example, from the first detection signal including the reproduction signal, the second detection signal including only the noise component is appropriately amplified or attenuated and subtracted, and then the obtained signal and the second signal
There is a method of calculating the product of the detection signal of 1), appropriately amplifying or attenuating the signal based on the calculation result, and subtracting again from the first detection signal.

In this case, depending on the noise recording signal, the second detection signal V2 'of the equation (11) is appropriately amplified or attenuated and subtracted from the first detection signal V1' of the equation (9). The components that do not do will be removed. That is,

[0184]

[Equation 15]

Next, when the product of this V3 'and the second signal V2' is created,

[0186]

[Equation 16]

Then, by appropriately amplifying or attenuating the obtained V4 ', and subtracting this from the above equation (15),

[0188]

[Numerical equation 17]

It becomes: Looking at this equation (17), the first term represents the signal component and the second term represents the disk noise component. However, like the above equation (14), the second term is a quadratic of (δR). It becomes clear that the C / N is improved. Figure 2
4 is a block diagram of an optical system / circuit system of a reproducing apparatus provided with a circuit system for executing this calculation, and the constituent elements having the same functions as those in FIG. 20 are designated by the same reference numerals.

23 and 24 show a differential amplifier 48,
The input and output of 49 and the input and output of the multiplication circuit section 53 are the same except for the above, and detailed description thereof will be omitted. Further, consider the case where the calculation method of the above equations (8) to (14) is applied to a circuit for reducing laser noise. Also in this case, the first detection signal output V11 containing laser noise is expressed as follows, as in the above example.

[0191]

[Equation 18]

Here, δP represents the fluctuation of the laser power, that is, the laser noise. On the other hand, the second detection signal output V12 containing only laser noise is given by the following equation.

[0193]

[Formula 19]

Comparing equations (8) and (18), and equations (10) and (19), the terms of the laser power fluctuation and the reflectance fluctuation are found apart from the proportional constant. You can see that they appear in the formula in exactly the same way. Therefore, it becomes clear that the laser noise is reduced by the same calculation as the calculation described in each of the above calculation methods.

Incidentally, FIGS. 25 and 26 show the above equations (18) and (19).
FIG. 3 is a block diagram showing two embodiments of a laser noise reduction type reproducing apparatus that employs the calculation according to the above. The components having the same functions as those of the reproducing apparatuses described above are denoted by the same reference numerals, and the description thereof will be omitted. .. In the figure, 54 is a half mirror, and 55 is a beam shaping optical element (collimating lens).
Is.

The above-described first embodiment provides a method / apparatus capable of effectively reducing noise even when the degree of modulation of the medium is high or the reproduced signal amplitude level continuously changes. The development example shown in (1) relates to reduction of laser noise among these noises. This example can be applied to any optical recording medium such as a write-once type optical recording medium using an organic dye, a photochromic type medium, and a read-only medium. Here, the molecular structure and absorption in FIG. The results of application to a medium containing a diarylethene-based photochromic material exhibiting a spectral change will be described.

Such materials are for example λ = from an Ar laser.
Irradiation with 458 nm radiation changes the state from the solid line in FIG. 18 to the state shown by the broken line, and, for example, irradiation with λ = 514.5 nm radiation of an Ar laser causes a reverse reaction, that is, the solid line from the dashed line. Change to a state.

Therefore, for example, recording is performed by setting the initial state of the medium to the state indicated by the broken line and irradiating the medium with high power and modulating the intensity of the Ar laser of λ = 514.5 nm. Reproduction can be performed by irradiating at a low level of 1 and detecting the reflectance change at λ = 514.5 nm. A photochromic material was mixed with polyvinyl butyral at 10 wt% and dissolved in an anon solvent and spin-coated on a glass disk substrate having a diameter of 120 mmφ and a thickness of 1.2 mm to form a recording layer having a thickness of 1 μm. Next, an Ag reflective film was formed by a vacuum evaporation method to prepare an optical recording medium.

[0199] First, Ar was added to the medium thus obtained.
Radiation light of λ = 458 nm from the laser was sufficiently irradiated on the entire surface to obtain the state shown by the broken line in FIG. Then, recording was performed by irradiating the emitted light of λ = 514.5 nm of the Ar laser with a power of 10 mW and a frequency of 100 kHz, and condensing and irradiating a spot of 1.5 μmφ on the medium. The relative speed at this time is 2m
/ s. Also, regarding this recorded medium, as shown in FIG.
Reproduction was carried out by the reproduction device indicated by.

At this time, the light source 2 is an Ar laser capable of emitting radiant light of λ = 514.5 nm, the power is 1.5 mW, and the relative speed is 2 m / s. The radiated light from the Ar laser usually causes large laser noise due to plasma instability, and therefore CN
The limiting factor for the ratio noise level is often laser noise rather than noise due to disk surface roughness.

When the reproduction output thus obtained was analyzed by a spectrum analyzer, the results shown in Table 7 were obtained.

[0202]

[Table 7]

Table 7 shows that when the laser noise reduction processing is not performed (output V1 'in FIG. 25), only subtraction (linear operation) is performed as the laser noise reduction processing (output V3 in FIG. 25), and laser noise reduction is performed. When a non-linear operation including subtraction and multiplication is performed as processing (output V5 in FIG. 25)
The signal level (C level) and the noise level (N level) of each are shown. From this table, the CN ratio is improved by the linear operation for laser noise reduction, but the nonlinear operation is further improved. I understand.

A similar experiment was conducted using the apparatus shown in FIG. 26, but the laser noise was reduced and the CN ratio was improved, as in the results shown in Table 7.

In this development example, the Ar laser was used as the light source, but the present invention is not limited to this and can be easily applied to other gas lasers, light sources using SHG elements, semiconductor lasers, and the like.

As described above, this embodiment separates the beam emitted from the light source into two, and irradiates the medium with the separated first beam; By performing appropriate arithmetic processing with the second detection signal obtained by directly detecting the other separated second beam, noise due to fluctuations in the emitted light intensity from the light source, that is, laser noise Is provided, and as a result, a reproducing method capable of obtaining good reproduced signal quality is provided.

[0207]

As described above, according to the reproducing method and reproducing apparatus of the present invention, it is possible to effectively reduce disk noise, laser noise and the like, and to take out a reproduction output having a high C / N. And the problem of crosstalk that occurs during wavelength multiplexing recording can be solved.

[Brief description of drawings]

FIG. 1 is a diagram showing a flowchart for explaining the principle of a first embodiment of the present invention.

FIG. 2 is a schematic diagram showing an absorbance characteristic of a general photochromic material.

FIG. 3 is a diagram showing a reaction formula of a photochromic material used in the first embodiment.

FIG. 4 is a diagram showing absorbance characteristics of the photochromic material of FIG.

FIG. 5 is a diagram showing an example of a reproducing apparatus according to the first example.

FIG. 6 is a diagram showing another embodiment of the reproducing apparatus according to the first embodiment.

FIG. 7 is a diagram showing still another embodiment of the reproducing apparatus according to the first embodiment.

FIG. 8 is a diagram showing the absorbance characteristics of the photochromic material used in the second embodiment.

9A and 9B are diagrams showing the principle of reproduction by birefringence used in the third embodiment.

10A and 10B are diagrams showing the principle of reproduction by optical rotation used in the third embodiment.

FIG. 11 is a diagram showing an example of a reproducing apparatus using birefringence according to a third example.

FIG. 12 is a diagram showing an example of a reproducing apparatus using optical rotation according to a third example.

13A to 13C are diagrams showing the principle of reproduction using an external read synchronization signal used in the fourth embodiment.

14A to 14C are diagrams showing the principle of reproduction using the detection signal delay method used in the fourth embodiment.

FIG. 15 is a diagram showing an embodiment of a reproducing apparatus using an external read synchronizing signal according to the fourth embodiment.

16 (a) and 16 (b) are diagrams showing the conditions of reproduction light and the positional relationship of spots on a track according to the detection signal delay system of the fourth embodiment.

FIG. 17 is a diagram showing an example of a reproducing apparatus using the detection signal delay system according to the fourth example.

FIG. 18 is a diagram showing an absorbance characteristic of a photochromic material different from that of FIG. 8.

FIG. 19 is a diagram illustrating a problem of crosstalk in wavelength multiplexing recording.

FIG. 20 is a diagram showing a reproducing device in the case of two-wavelength multiplex recording.

21A to 21F are waveform diagrams showing the noise reduction methods of the first to fourth examples.

22A to 22D are waveform diagrams showing a noise reduction method according to the fifth embodiment.

FIG. 23 is a diagram showing an example of a reproducing apparatus adopting the first operation method of reducing disk noise according to the fifth example.

FIG. 24 is a diagram showing an example of a reproducing apparatus adopting the second operation method of reducing disk noise according to the fifth example.

FIG. 25 is a diagram showing an example of a reproducing device adopting the first operation method of laser noise reduction according to the fifth example.

FIG. 26 is a diagram showing an example of a reproducing apparatus adopting the second operation method of laser noise reduction according to the fifth example.

[Explanation of symbols]

 1 Optical recording medium 2 Light source 3 Light source 4 Objective lens system 5 Dichroic mirror 6 Differential amplifier 22 Polarization beam splitter 32 Polarization beam splitter 23 λ / 4 plate 33 λ / 4 plate 25 Photodetector 35 Photodetector 26 Amplifier 36 Amplifier 27 Detection system 37 Detection system 41 Waveform memory 42 Waveform memory 43 Sync signal detection unit 44 Sync signal detection unit 45 Time difference T delay determination unit 51 Filter 52 Filter 46 Time difference T delay unit 53 Multiplying circuit unit

Claims (16)

[Claims] 1. A reproducing method of an optical recording medium for irradiating an optical recording medium with light to reproduce an information signal, wherein an information signal is converted from a signal obtained by irradiating a first reproducing light for detecting the information signal. A reproducing method for an optical recording medium, characterized in that a signal obtained by irradiating a second reproducing light which is not detected or hardly detected is subtracted. 2. The first and second reproduction lights are simultaneously irradiated, and the transmitted or reflected light from the medium due to the respective reproduction lights is independently detected, and the result of subtraction of these detection signals is calculated. The method of reproducing an optical recording medium according to claim 1, wherein the reproduction signal is used. 3. Information is recorded on the optical recording medium according to the magnitude of birefringence, and the second reproduction light is irradiated on the medium while being linearly polarized and aligned with the neutral axis direction of the birefringence. The method for reproducing an optical recording medium according to claim 1 or 2, characterized in that: 4. The light according to claim 1, wherein information is recorded on the optical recording medium according to the magnitude of optical rotatory power, and the second reproduction light is irradiated on the medium with circularly polarized light. Reproduction method of recording medium. 5. The first and second detected independently of each other
3. The method of reproducing an optical recording medium according to claim 2, wherein the detection signal by the reproduction light is temporarily stored in the storage unit together with the synchronization signal, and the detection signals are simultaneously read by the synchronization signal from the outside. 6. A method of reproducing an information signal by irradiating an optical recording medium with light to reproduce an information signal, wherein a signal obtained by irradiating a first reproducing light having a wavelength for detecting the information signal,
A reproducing method of an optical recording medium, characterized in that linear and nonlinear operations are performed with a signal obtained by irradiating a second reproducing light of a wavelength which does not detect an information signal or hardly detects an information signal. 7. The linear and non-linear operations multiply a signal by the first reproduction light by a signal by the second reproduction light,
7. The method for reproducing an optical recording medium according to claim 6, wherein the result is subtracted from the signal generated by the first reproduction light. 8. The linear and non-linear operations include a difference signal between a signal generated by the first reproduction light and a signal generated by the second reproduction light,
7. The method for reproducing an optical recording medium according to claim 6, wherein the signal by the second reproduction light is multiplied and the result is subtracted from the signal by the first reproduction light. 9. A reproducing method of an optical recording medium for irradiating an optical recording medium with light to reproduce an information signal, wherein a plurality of reproducing lights are radiated on the same track of the medium, and are generated by the plurality of reproducing lights. The transmitted or reflected light from the medium is independently detected, and each of the obtained detection signals is temporarily stored in the storage unit together with the synchronization signal, and the detection signals are simultaneously read by the synchronization signal from the outside. A reproducing method of an optical recording medium, characterized in that a read signal is obtained by performing a predetermined calculation on the read detection signal. 10. A reproducing method of an optical recording medium for irradiating an optical recording medium with light to reproduce an information signal, wherein a plurality of reproducing lights are radiated on the same track of the medium, and are generated by the plurality of reproducing lights. The transmitted or reflected light from the medium is independently detected, and for each of the obtained detection signals, the distance between the spot positions on the medium of the reproduction light corresponding to this and the time corresponding to the relative speed of the spot. A method of reproducing an optical recording medium, characterized in that after a specific delay amount is given and corrected, a predetermined operation is performed between these corrected detection signals to obtain a reproduced signal. 11. A first reproduction light source that emits light of a wavelength that detects an information signal recorded on an optical recording medium, and a second reproduction light source that emits light of a wavelength that does not or hardly detects an information signal. An optical system for irradiating the same spot of the optical recording medium with light of each wavelength, a detection system for detecting light of each wavelength emitted from the optical recording medium, and a detection system Apparatus for reproducing an optical recording medium having a circuit for subtracting the generated signal. 12. A first reproduction light source for generating reproduction light having a wavelength for detecting an information signal recorded on an optical recording medium, and a second reproduction light source for generating reproduction light having a wavelength different from that of the first reproduction light source. Of the reproducing light source, a combining optical system for matching the optical axes of the reproducing lights, a lens system for condensing the reproducing lights to the same spot on the medium, and the reproducing lights. A separation optical system that separates reflected light or transmitted light from the medium into two independent lights, a detection system that independently detects the separated beams, and a circuit that subtracts the output obtained from the detection system. And a reproducing apparatus for an optical recording medium having a system. 13. A first reproducing light source that emits light of a wavelength that detects an information signal recorded on an optical recording medium, and a second reproducing light source that emits light of a wavelength that does not or hardly detects an information signal. A combining optical system for combining the light emitted from each of these light sources, a lens system for condensing the light emitted from the combining optical system onto the medium, and the above-mentioned first and second reproduction light sources. A separation optical system that separates the reflected light and the transmitted light from the medium again, a detection system that independently detects each separated light, and a circuit system that performs linear and nonlinear operations between the signals obtained from these detection systems. A reproducing apparatus for an optical recording medium having. 14. A reproducing method of an optical recording medium for irradiating an optical recording medium with light to reproduce an information signal, wherein a first signal from the signal obtained by irradiating a first reproducing light for detecting an information signal A method for reproducing an optical recording medium, characterized in that a signal obtained by irradiating the second reproduction light having the same phase as that of the reproduction light of 1. 15. A reproducing method of an optical recording medium for irradiating an optical recording medium with light to reproduce an information signal, wherein the first signal is obtained by irradiating a first reproducing light for detecting an information signal. A method for reproducing an optical recording medium, characterized in that a signal obtained by irradiating a second reproducing light having a phase opposite to that of the reproducing light is subtracted. 16. A reproducing method of an optical recording medium for irradiating an optical recording medium with light to reproduce an information signal, wherein a signal obtained by irradiating a reproducing light for detecting an information signal and the reproduced light are separated. A method of reproducing an optical recording medium, characterized by performing a non-linear operation with a signal obtained from light which is not applied to the medium.