JPH0667364A - Recording method for optical recording medium - Google Patents

Abstract

PURPOSE:To enable information recording at a high density by specifying record ing conditions according to the SN ratio of the output of recording light. CONSTITUTION:A strong power laser is turned on/off according to recording signals and a medium 1 is irradiated with the beam at the time of recording with the optical recording and reproducing device, and therefore, the reflectivity changing parts according thereto remain as recording marks on the medium. The medium is irradiated with the DC light lowered in power by adjusting an ND filter 601 and the reflected light subjected to intensity modulation according to a reflectivity change is detected as photocurrent by a photodetector 7 at the time of reproducing. The photocurrent is subjected to current-voltage conversion by a preamplifier 8 and thereafter, the voltage is evaluated by a spectrum analyzer. The value of the recording light power Prec or beta satisfying the relation of equation when the SN ratio required at the time of reproducing is designated as SNR (reproducing) A and the SN ratio of the recording light as SNR (recording) R is set according to the SN ratio of the reproducing light at the time of recording and reproducing the information by using the photon mode optical recording medium.

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 an optical recording medium capable of recording information at high density.

[0002]

2. Description of the Related Art In recent years, researches for applying a photochromic material as a rewritable optical recording medium have been actively pursued. When this photochromic material is irradiated with light of a certain wavelength, the structure of the molecule changes in the photochemical reaction, and with this change, optical characteristics such as absorbance and refractive index for light of a specific wavelength change, and light of other wavelengths It has the property that the changed molecular structure returns to the base upon application of heat or heat. Therefore, recording on the photochromic optical recording medium is performed by a change in molecular structure caused by irradiation with light having a specific wavelength, and reproduction is performed by detecting a change in optical characteristics associated with this change in structure, particularly a difference in absorbance.

By the way, the SN ratio is a measure of the quality of a reproduction signal of a recording medium. This is defined by the ratio of the signal output power included in the reproduction output and the noise output power, and it is known that various noise levels must be sufficiently low in order to obtain a high SN ratio in the optical recording medium. . The causes of such noise are noise due to the disk surface property and the total recording surface property, laser noise due to fluctuations in the intensity of the laser light as recording light, photodetector and preamplifier system amplifier noise, and recording layer material specific There are media noises such as the polycrystal state and shot noise originally possessed by laser light in the quantum state represented by the coherent state.

Among the above-mentioned noises, the relationship between the reproduced signal quality and particularly the laser noise is related to, for example, the Institute of Electronics and Communication Engineers '85 / 3 Vol. J68-C No.3 P.216 and SPIE Vol.695 Opt.
icalMass Data StorageII (1986) P.72. However, in these documents, only the influence of laser noise at the time of reproduction is discussed, and the influence of laser noise at the time of recording is not mentioned at all. This is a problem that the medium disclosed in these documents is recorded by the heat mode, and that some fluctuation of the laser light intensity due to this recording is averaged when it is converted into heat on the medium. It will not happen.

On the other hand, the above-mentioned photochromic medium is characterized in that recording can be performed in the photon mode.
The inventor of the present application has found that the recording laser noise has an important relationship with the reproduction SN ratio during the photon mode recording,
The relationship between the required reproduction SN ratio and the recording laser noise (SN ratio of the recording laser) was clarified for the first time, and the present application was filed.

[0006]

SUMMARY OF THE INVENTION In view of the above problems of the prior art, it is an object of the present invention to provide a recording method for obtaining a high reproduction SN ratio in a photon mode optical recording medium.

[0007]

According to the present invention, the S / N ratio required for reproduction when recording and reproducing information using a photon mode optical recording medium is defined as SNR (reproduction), and the S / N ratio of recording light is set. Is the SNR (record),

[0008]

[Equation 2]

Recording optical power P satisfying the following relation
The value of _rec or β is set according to the SN ratio of the reproduction light.

SNR (playback) and SNR (recording)
Is displayed in dB,

[0011]

[Equation 3]

[0012]

[0013]

In photon mode optical recording, the amount of reactive molecules at the time of recording is accurately determined by the recording optical power P _rec . Therefore, the fluctuation of the recording power P _rec causes fluctuation of the amount of reactive molecules and fluctuation of the reflectance change amount. May cause noise during playback.

The inventor of the present application first _finds that the relational expression between P _rec and the reproduction signal amplitude when there is no noise in the recording light is expressed by the following mathematical formula 4.
It was derived by theoretical consideration that it is expressed as follows. Here, the recording light is turned on / off according to the recording signal.

[0015]

[Equation 4]

[0016] Now recording light power from the P _rec P _rec
When it fluctuates to (1 + δ), the reproduction signal output V _out becomes as follows.

[0017]

[Equation 5]

From this equation 5, the reproduction SN ratio is

[0019]

[Equation 6]

Is required.

On the other hand, when the average value P _rec of the recording light and the fluctuation magnitude P _rec · δ are used to define the SN ratio of the recording light as shown in Equation 7,

[0022]

[Equation 7]

After all, the following relational expression 8 is obtained.

[0024]

[Equation 8]

The above equation 8 is SNR (recording) and recording efficiency β
Is a formula that gives a certain fixed reproduction SN ratio, that is, the minimum required recording power for obtaining the SNR (reproduction), the reproduction SN ratio must be larger than the SNR (reproduction).

[0026]

[Equation 9]

[0027] FIG. 6 shows the SN ratio that requires the minimum SNR (recording) of laser light as recording light, that is, SN.
It is the figure which showed the product of recording power _Prec and (beta) when it is smaller than R (reproduction), and the obtained SNR (reproduction) / SNR (recording) by dB display. In the figure, if SNR (recording) ≧ SNR (reproduction), there is no problem, but for example, S
It can be seen that when NR (recording) is 10 dB smaller than SNR (reproduction), SNR (recording) and SNR (reproduction) can be made substantially equal by adjusting P _rec so that βP _rec becomes 2 or more.

Thus, even if the SN ratio of the recording light is poor,
The reproduction SN ratio can be improved by adjusting the recording conditions.

[0029]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A recording method for an optical recording medium according to the present invention will be described in detail with reference to the drawings with reference to the drawings. [First Example] 2- (1,2-dimethyl-3-indolyl) -3- (2,3,5-trimethyl-3-thienyl) whose molecular structure and absorption spectrum are shown in FIG. 1 as a photochromic material Maleic anhydride (hereinafter abbreviated as indolethiophene type material) was mixed with polyvinyl butyral resin (PVB) at 10 wt%, dissolved in anone solvent and spin-coated on a glass disk substrate to form a recording layer.

Next, Ag is used as a reflective film on the recording layer.
The layer was formed by vacuum evaporation. The photochromic material thus obtained changes from the characteristic state shown by the broken line in FIG. 1 to the characteristic state shown by the solid line by blue light irradiation, and from the characteristic state of the solid line by light irradiation at a wavelength of about 600 nm. The characteristic state changes to the broken line.

Therefore, for an optical recording medium containing such a photochromic material, a He--Ne laser beam of λ = 633 nm or λ = 630 has been prepared in advance as a solid line state.
Recording can be performed by irradiating a semiconductor laser beam on the order of nm with relatively high power, and reproduction can be performed by irradiating the same wavelength light with low power and detecting a change in reflectance.

FIG. 2 is a view showing the arrangement of an optical recording / reproducing apparatus used for measuring the SN ratio of the optical recording medium. In the figure, 6 is a light source unit, and as the light source 603, a He-Ne laser that emits light of λ = 633 nm with low noise is used. The AO modulator 602 has a function of turning on and off the laser according to a recording signal (in this case, a constant frequency f) at the time of recording, and transmitting at a DC level during reproduction.
Reference numeral 601 is an ND filter for adjusting the laser power of the light source 603. The laser beam emitted from the light source 603 is expanded to an appropriate width by the lens system 5, passes through the polarization beam splitter 4 and the λ / 4 plate 3, and is focused on the optical recording medium 1 by the objective lens 2.

At the time of recording, the high power laser is turned on and off according to the recording signal and is irradiated, so that the medium 1
The reflectance changing portion corresponding to this remains as a recording mark. During reproduction, the ND filter 601 is adjusted to emit low-power DC light, and the reflected light whose intensity is modulated according to the change in the reflectance is detected by the photodetector 7 as a photocurrent and converted into a current-voltage by the preamplifier 8. After that, it is evaluated by a spectrum analyzer (not shown).

When recording and reproducing a constant frequency f, the spectrum analyzer can evaluate the so-called CN.
It is called a ratio (CNR), and the SN ratio (SNR) is converted from the CN ratio as follows.

The bandwidth used in the system of FIG. 2 is B, and the RBW (Resolution Ba) of the spectrum analyzer is
nd Width) is Δf

[0036]

[Equation 10]

Therefore, in the obtained measurement result, the CN ratio can be used instead of the SN ratio.

The above description is based on the SN ratio at the time of reproduction, that is, C
Regarding the N ratio measurement method, the SN ratio of the recording laser light, that is, the CN ratio was measured as follows. That is, an optical recording medium (disk) 1 provided with only a reflection layer is set in a stationary state, and intensity modulation (on / off) is performed at a constant frequency.
Photodetector 7 and preamplifier 8 for reflected light when
The output of was evaluated simultaneously with the spectrum analyzer.
The comparison between the laser noise level at this time and the amplifier noise level is made such that the noise level when the laser is on is the laser noise level, and the noise level when the laser is off is the amplifier noise level. > I confirmed that it was the amplifier noise level.

As a result of measuring the CN ratio of the recording laser of the He-Ne laser by the above-mentioned method, this CN ratio does not depend on the power of the laser and is 80 dB (RBW: Δf = 30).
kHz, f = 1 MHz).

Next, recording was performed while changing the recording laser light intensity variously, and the reproducing CN ratio was measured. At this time, the relative rotation speed of the optical recording medium 1 is 5 m / s, and the laser spot system φ =
The recording sensitivity of medium 1 for 1.4 μm and λ = 633 nm, that is, ε · κ, is 1200 l / mol · cm, and as a result, β = 274 (W ⁻¹ ) is obtained.

For normal digital recording, the CN ratio is 50.
Although about dB is required, the result of the above measurement βP _rec =
CNR ≧ 50 dB in the range of 0.1 to 4.0
was gotten. This is the CN ratio of the recording laser
This is because R (record) = 80 dB ≧ 50 dB.

Next, a semiconductor laser emitting a laser beam of λ = 630 nm was used as a recording laser. In this case, the same semiconductor laser may be installed instead of the light source unit 6. When the CN ratio of this recording laser was measured in the same manner as the above case, it was found that there was much noise and
It was 4 dB. Regarding the reproduction, low noise He
-Ne laser.

According to FIG. 6, theoretically βP _rec ≧
It is 1.2, and it is expected that a reproduction CN ratio of 50 dB or more will be obtained. Here, the experimental results showing the relationship between P _rec and the reproduction CN ratio are shown in FIG. According to the figure, the CN ratio is 50 dB
The above is P _rec ≧ 4.2 (mW), and βP
The result was almost theoretically predicted as _rec ≧ 1.15. [Second Embodiment] Next, as a second embodiment, a case of using a photochromic material different from that of the first embodiment will be described. Such a photochromic material is shown in FIG.
A photochromic optical recording medium was prepared under the same conditions as the above medium using 2,3-bis (2-methylbenzo [b] thiophen-3-yl) maleic anhydride whose molecular structure and absorption spectrum were shown in Table 1. It was done. This optical recording medium changes from the characteristic state shown by the broken line in FIG. 4 to the characteristic state shown by the solid line by blue light irradiation. Therefore, after setting the characteristic state of the solid line in advance, Ar with λ = 514.5 nm
SHG of laser light or YAG laser of λ = 532 nm
Recording can be performed by irradiating light with relatively high power, and conversely, reproduction can be performed by irradiating the same laser with relatively low power and detecting a change in reflectance.

The recording / reproducing apparatus has the same structure as that shown in FIG. However, since the CN ratio of Ar laser light of λ = 514.5 nm is as low as 40 dB, and the CN ratio of the YAG-SHG laser (λ = 532 nm) is as high as 100 dB,
Recording was performed with an Ar laser, and reproduction was performed with a YAG-SHG laser. The relative speed of the optical recording medium 1 is 5 m /
s, laser spot diameter φ = 1.1 μm, spot area s = 9.5 × 10 ⁻⁹ cm ² , λ = 514.5, 532n
ε × κ = 1100 l / mol · cm for m,
β = 253 (W ⁻¹ ). Therefore, in this case, it is expected that a reproduction CN ratio of 50 dB or more is theoretically obtained with βP _rec ≧ 2.0.

FIG. 5 is a diagram showing the relationship between P _rec and the reproduction CN ratio under the above conditions. In the figure, P _rec > 8mW
Although the CN ratio becomes 50 dB, it is clear that βP _rec = 2.02, which is almost in agreement with the theoretical prediction.

As described above, the required SN is adjusted by adjusting the recording power of the recording light with which the optical recording medium is irradiated.
It can be seen that R, that is, SNR equal to or higher than SNR (reproduction) can be obtained. Moreover, it is possible to use other adjustment methods. That is, it is important to adjust βP _rec , so β = αλκ · εt / s instead of P _rec
May be adjusted. A typical parameter for this purpose is the relative speed of the optical recording medium, especially the disc.

In general, the higher the recording power, the lower the reliability of the light source and the greater the heat damage to the medium.

[0048]

[Equation 11]

It is desirable that

Of course, the present invention can be modified in various ways other than those described above. For example, the present invention can be easily applied to the case where the required CN ratio is 60 dB or more as required for analog moving image recording.

[0051]

According to the present invention, the recording condition can be optimally set according to the SN ratio of the output of the recording light, and thereby the SN ratio at the time of reproduction required for the optical recording medium is secured. The effect that can be easily achieved can be expected.

[Brief description of drawings]

FIG. 1 is a diagram showing a molecular structural formula and an absorption spectrum characteristic of a photochromic material used in a first embodiment of the present invention.

FIG. 2 is a diagram showing a structure of an apparatus used for recording and reproduction.

FIG. 3 is a diagram showing a relationship between a reproduction CN ratio and P _{rec in} the first embodiment.

FIG. 4 is a diagram showing a molecular structural formula and absorption spectrum characteristics of a photochromic material used in a second example of the present invention.

FIG. 5 is a diagram showing a relationship between a reproduction CN ratio and P _{rec in} the second embodiment.

FIG. 6 is a diagram showing the principle of the present invention.

[Explanation of symbols]

 1 Optical Recording Medium 2 Objective Lens 3 λ / 4 Plate 4 Polarization Beam Splitter 5 Lens System 6 Light Source Section

Claims (1)

[Claims] 1. When the SNR (reproduction) is the SN ratio required at the time of recording and reproducing information using a photon mode optical recording medium, and the SN ratio of the recording light is SNR (recording). , [Equation 1] A recording method for an optical recording medium, wherein the value of the recording optical power P _rec or β is set so as to satisfy the following relationship.