JPH0612671A - Recording method for optical recording medium - Google Patents

Abstract

PURPOSE:To provide the method for efficiently recording information by effectively using the energy of light for recording for a photon mode optical recording material. CONSTITUTION:This recording method for the optical recording medium can record information by photoirradiation. The irradiation power of the above- mentioned light for recording is so adjusted that the reproduced output signal attains <=-3dB with respect to the saturation level of the reproduced output signal based on the photochemical reaction of the material which generates the photochemical reaction by irradiation with the light for recording.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a recording method for a photon mode optical recording medium capable of high density recording.

[0002]

2. Description of the Related Art In recent years, research for applying a photochromic material as a rewritable optical recording medium has been actively pursued.

When a photochromic material is irradiated with light of a predetermined wavelength, the molecular structure is changed by a photochemical reaction, and accordingly, optical characteristics such as absorbance and refractive index with respect to light of a specific wavelength are changed, and at other wavelengths. It has a property that the changed molecular structure returns to its original state by applying light or heat.

Therefore, recording on the photochromic optical recording medium is carried out by a change in molecular structure caused by irradiation with light of a specific wavelength, and reproduction is carried out by detecting a change in optical characteristic associated with the change in molecular structure.

By the way, in order to perform recording on an optical recording medium, a heat mode type medium such as a conventionally known magneto-optical disc or a phase change type disc, or a photon mode type medium such as a photochromic type which is the object of the present invention. Regardless of the above, the recording laser beam is irradiated onto the medium with relatively high power.

However, it is not known that the higher the laser irradiation power, the better, and it is conventionally known that the optimum recording power exists especially for the heat mode medium. Regarding this, for example, “Measurement and Evaluation Technology for Optical Discs” published by Japan Industrial Technology Center (1988), P181 to 183, “The disc is rotated at a constant speed and the recording power is changed to record a constant frequency at a recording duty of 50%. The optimum recording power is the time when the second harmonic component during reproduction is minimized. "

Regarding the optimum recording power, according to the above-mentioned publicly known document, the second-order distortion, that is, the distortion of the reproduced output waveform, becomes larger even if the recording power is larger or smaller than the "optimal recording power". It is understood that it is desirable to set the recording power of the normal heat mode medium to this value because the error rate becomes large during recording.

FIG. 7 shows recording power and C / N ratio (CNR: C
Is a graph showing the relationship between the carrier signal level, N the noise signal level, R the ratio) and the second-order distortion. In the figure, when the optimum recording power is 4.9 mW, the C / N ratio reaches -1 dB, which is almost the saturation level with respect to the saturation level (here, the saturation level is 59 dB).

It has been known that the occurrence of the secondary distortion component in FIG. 7 is due to the influence of the temperature rise during recording. FIG. 8 is a diagram showing recording marks on the medium surface for explaining the influence of such temperature rise.
1) to (a3) are low power in heat mode recording,
The states of the recording marks at the time of recording with the optimum recording power and the high power are shown, and (b1) to (b3) show the states of the recording marks at the same time in the photon mode recording. As is clear from (a1) to (a3), in heat mode recording, the recording mark becomes relatively small at low power because the temperature rise of the recording layer is insufficient, and conversely at high power, thermal diffusion is strong. As a result, the recording mark becomes relatively large. As a result, the duty ratio of the reproduction signal is 50
The second-order distortion is generated with a deviation from%.

However, the recording power dependence of such secondary distortion is peculiar to the heat mode, and the definition of the optimum recording power does not apply to the photon mode recording. That is, as is clear from (b1) to (b3), when the power is increased, the color density is increased rather than the size of the recording mark, and when the power is decreased, the color density of the recording mark is decreased. / N ratio changes.

Therefore, from the viewpoint of the C / N ratio, it was considered desirable to irradiate as high a power as possible to saturate the C / N ratio, that is, to completely react the photon-mode material molecules for recording. .

[0012]

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method for efficiently recording information on a photon mode optical recording material by effectively using the energy of recording light.

[0013]

DISCLOSURE OF THE INVENTION The present invention is a recording method for an optical recording medium capable of recording information by irradiating light, wherein a material which causes a photochemical reaction by irradiation of recording light is The irradiation power of the recording light is adjusted so that the reproduction output signal becomes -3 dB or less with respect to the saturation level of the reproduction output signal.

[0014]

When information is recorded on the photon mode optical recording medium by changing the irradiation power of the recording light, the carrier signal level of the reproduction light (hereinafter simply referred to as signal level) is up to the saturation level -3 dB. Although the C / N ratio sharply rises with the increase in the irradiation power of, when the signal level is tried to approach the saturation level beyond (saturation level -3 dB), it is different from the time up to -3 dB. It is necessary to improve the irradiation power considerably.

In a conventional optical disk device, a semiconductor laser is generally used as a recording light source, but irradiating the semiconductor laser with a large irradiation power is a major factor of reducing the reliability of the output of the laser itself, which is not desirable. .

Therefore, when recording on the photon mode optical recording medium, it is most desirable to set the signal level to the saturation level of -3 dB or less from the viewpoint of improving the C / N ratio or the reliability of the apparatus.

[0017]

EXAMPLES First, a theoretical consideration on photon mode optical recording which led to the present invention will be described.

The following relationship is established between the reproduction signal level S (dB) and the irradiation power Prec (W) of the recording light (here, laser).

[0019]

[Equation 1]

FIG. 1 is a graph showing the relationship between S and Prec in the equation (1). The vertical axis represents the magnitude (dB) of the reproduction signal S when the saturation level is 0 dB, and the horizontal axis represents βP.
The magnitude (mW) of rec (β Prec is proportional to Prec because β is a constant coefficient and can be treated as a constant) is taken.

As described in the section of the prior art, in the case of the heat mode type, the signal level is -3d with respect to the saturation level.
To increase the power from B to -1 dB by 2 dB, it is necessary to increase the power of the irradiation light by about 17% from about 4.2 mW to about 4.9 mW, and from 4.9 mW to increase from -1 dB to the saturation level by 1 dB. While it is only necessary to increase the power by about 20% to 6 mW, in the case of the photon mode type, in order to increase 2 dB from -3 dB to -1 dB, it is necessary to increase the power twice as much as that of the heat mode type.
It is clear from FIG. 1 that a 1.5 times increase in power is required to raise the level from 1 dB to the saturation level by 1 dB compared with the heat mode type. From this, it is clear from the photon mode type medium that the signal level reaches the saturation level. The usefulness of setting below -3 dB can be proved.

Next, the results of a recording / reproducing experiment using an actual photochromic material will be described below. Accompanying the photochromic reaction, the molecular structure of 2- (1,2-dimethyl-3-indolyl) -3- (2,3,5-trimethyl-3-thienyl) maleic anhydride, which is the photochromic material used in this experiment. The absorption spectrum is as shown in FIG. 2, and when the light having the absorption spectrum shown by the broken line is irradiated with light in the vicinity of λ = 450 to 500 nm, a ring closure reaction occurs, and the absorption spectrum state as shown by the solid line in FIG. Changes to. Also, λ = 550 to 650n
A ring-opening reaction occurs by irradiation with light near m, and the absorption spectrum in the ring-closed state returns to the absorption spectrum in the ring-opened state.

Therefore, an optical recording medium containing this material can be reproduced, for example, by initializing it to a solid line state in advance and then irradiating a He-Ne laser of λ = 633 nm with low power to detect a change in reflectance. is there. Then, in the experiment, an optical recording medium using such a material was prepared as follows.

First, 10 wt% of the photochromic material is used.
% Of polyvinyl butyral resin, dissolved in a mixed solution of anone and benzene, and applied onto a glass disk substrate by spin coating. Thereafter, it is dried at 60 ° C. for 24 hours, an Ag reflective film is formed by vacuum vapor deposition, and an optical recording medium 8 having a structure composed of a glass substrate 80, a photochromic recording layer 81, and a reflective layer 82 is prepared as shown in the sectional view of FIG. did. The thickness of the recording layer 81 was about 1 μm.

FIG. 4 is a block diagram showing the configuration of the recording / reproducing apparatus used in this experiment. In the figure, 1 is a He-Ne laser light source having a wavelength λ = 633 nm, 2 is an ND filter, 3 is an AO modulator, 4 is a beam expander, 5 is a polarization beam splitter, 6 is a λ / 4 plate, 7 is a photodetector, Reference numeral 8 is an optical recording medium, 9 is an objective lens, 10 is a dichroic mirror, 11 is a half mirror, 12 is a semiconductor laser light source for focus tracking servo, and 13 is an optical system / circuit system for focus tracking servo.

In these configurations, the radiated light from the He-Ne laser light source 1 is power-adjusted by the ND filter 2, and the AO modulator 3 causes a predetermined frequency f during recording.
₁ (here, f ₁ = 100 kHz, duty 50%)
The intensity is modulated by and is set so that the power becomes constant during reproduction.

The intensity-modulated light has its beam width appropriately adjusted by the beam expander 4, passes through the polarization beam splitter 5 and the λ / 4 plate 6, is reflected by the dichroic mirror 10, and is reflected by the objective lens 9. It is focused on the medium 8.

The focus tracking servo light source 12 emits near infrared laser light, which is combined with the light from the light source 1 by the dichroic mirror 10 and condensed on the medium by the objective lens 9.

On the other hand, during reproduction, the reflected light from the medium 8 passes through the same optical path in the opposite direction, is reflected by the polarization beam splitter 5 and is detected by the photodetector 7.

As shown in FIG. 5, the relationship between the reproduction signal output level, the secondary distortion level (relative to the carrier level of the reproduction signal) and the recording laser power was examined by using the medium 8 and the optical system / circuit system as described above. The results were obtained. At this time, the relative speed of rotation of the optical recording medium 8 was constant at 1.4 m / s, and the reproducing power was set at 0.1 mW. The saturation level of the reproduction signal output, that is, 0 dB, is set by separately measuring and converting the macroscopic reflectance change of the medium 8 itself.

As can be seen from this figure, the recording power P
The reproduction signal output sharply increases until rec = 1 mW, and
It becomes (saturation level -3 dB) at mW, and from then on, the increase of the reproduction signal output becomes gradual,
It can be seen that the power reaches about 2.1 mW to reach B), which requires twice as much power as compared with the case of (saturation level-3 dB). This result is in good agreement with the result of FIG. 1 by the theoretical analysis described at the beginning of the section of this Example.

On the other hand, the secondary distortion is Prec = 0.6 to 2.1 m.
It maintained a substantially constant value at a low level of about -30 dB in the range of W, and the existence of the minimum value as shown in FIG. 2 for the heat mode medium was not seen.

From the above results, it can be seen that it is effective to set the irradiation power of the recording light so that the reproduction output signal level becomes (saturation level-3 dB) or less.

From the viewpoint that a higher C / N ratio is desirable, the reproduction output signal level is (saturation level-3 dB).
Although it is good to set it to a degree, in consideration of application to digital recording, for example, in order to improve the error rate, for example, if C / N ratio = 45 dB or less, it is generally C.
Although the error rate is improved with the improvement of the / N ratio,
It is well known that there is a threshold such that the error rate is hardly improved even if the C / N ratio is improved when it is larger than dB.

Therefore, if the C / N ratio is equal to or higher than the threshold value, the reproduction output signal level is set to (saturation level-3 dB).
It is not necessary to set to, and it is possible to set it to less than that.
Such a recording method is peculiar to the photon mode, and in the case of the heat mode type, as can be seen from FIG. 2, the secondary distortion increases below the optimum recording power, and therefore it cannot be adopted at all.

The present invention can be applied to any photon mode type material without being limited to the above-mentioned embodiments. Fig. 6 is 2,3-
Bis (2-methylbenzo [b] thiophen-3-yl)
The molecular structure and absorption spectrum of maleic anhydride are shown, and the same experiment as in the above-mentioned example was conducted using this material.

Such a material changes from a state shown by a broken line to a state shown by a solid line by irradiation with light having a wavelength of 400 to 460 nm, and from a state of a solid line to an original broken line by irradiation with light having a wavelength of 500 to 600 nm. It has the property of returning to. Therefore, as in the previous embodiment, recording / reproducing can be performed by irradiating the radiation of λ = 514.5 nm of Ar laser after setting the solid line state to the initial state.

Therefore, the light source 1 of the apparatus shown in FIG. 4 was an Ar laser, and the optical element was changed to correspond to λ = 514.5 nm, and recording / reproducing experiments were conducted. Here, the configuration of the medium and other conditions are the same as in the previous embodiment.

As a result, the graph as shown in FIG. 5 is obtained as in the previous embodiment, the recording power Prec = 1.2 mW at (saturation level-3 dB), and (saturation level-1 dB is slightly more than 2. It became 5 mW and the secondary distortion was about -32d.
It was constant as B. Therefore, also from the second embodiment, it is effective to set the reproduction signal output level to (saturation level-3 dB).

Furthermore, the present invention is not limited to the above two embodiments and can be applied to various materials. For example, a spiropyran-based material, which is also well known as a photochromic material, is known to react not only in the photon mode but also in the heat mode, but the present invention can be applied as long as the proportion of the photon mode reaction for recording is high. In addition, it can be applied to fulgide-based and azobenzene-based materials, and PHB (Photochemical Ho
abbreviation of le Burning) can also be applied to recording.

[0041]

As described above, according to the present invention, it is expected that the irradiation energy of the recording light can be effectively used for the photon mode optical recording medium to efficiently record information.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a recording power and a reproduction signal output according to a theoretical examination result.

FIG. 2 is a diagram showing a molecular structure and absorption spectrum characteristics of a photochromic material that can be used in the present invention.

FIG. 3 is a cross-sectional view showing the structure of an optical recording medium.

FIG. 4 is a block diagram showing a configuration of an optical recording / reproducing device.

FIG. 5 is a diagram showing a relationship between a recording power and a reproduction signal output, which is a result of a reproduction experiment by the optical recording method using the present invention.

FIG. 6 is a diagram showing a molecular structure and absorption spectrum characteristics of a photochromic type material which can be used differently from FIG.

FIG. 7: Recording power and C / N in heat mode recording
It is a figure S which shows the relationship of a ratio and secondary distortion.

8 (a1) to (a3) and (b1) to (b3) are diagrams showing a relationship between a recording power and a recording mark in heat mode recording or photon mode recording, respectively.

[Explanation of symbols]

 1 Light Source 2 ND Filter 3 AO Modulator 4 Beam Expander 5 Polarizing Beam Splitter 6 λ / 4 Plate 7 Photo Detector 8 Optical Recording Medium 9 Objective Lens 10 Dichroic Mirror 11 Beam Splitter 12 Semiconductor Laser

Claims (1)

[Claims] 1. A recording method for an optical recording medium capable of recording information by irradiation with light, wherein a material that causes a photochemical reaction by irradiation with recording light has a saturation level of a reproduction output signal based on the photochemical reaction. On the other hand, a recording method for an optical recording medium, characterized in that the irradiation power of the recording light is adjusted so that the reproduction output signal becomes −3 dB or less.