JPH06176363A - Optical recording method - Google Patents

Abstract

PURPOSE:To efficiently prevent the increase in an error rate by enlarging the length of the shortest recording mark formed on a medium than the diameter of a light spot for recording. CONSTITUTION:Although a reaction is caused large in the center of the spot without a precise recording threshold value as the time of heat mode recording at the time of photon mode recording, a photochemical reaction according to the intensity occurs even in a part of the periphery of the spot where optical power is weak, and the change of reflectance occurs. An intensity distribution in the recording light spot is shown as a figure (a), and a recording mark shape on a recording layer is shown as the figure (b), and a reflectance change distribution on the recording layer is shown as the figure (c). It is found that a boundary between a recording mark part and a nonmark part is not precise, and the reflectance is changed continuously between a high level and a low level, and the mark is spread from the figures. In such a manner, the increase in the error rate occurring in particular in the photon mode recording is prevented efficiently without remarkably lowering recording density.

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 information recording.

[0002]

2. Description of the Related Art In recent years, photochromic optical recording, which utilizes a photon mode reaction for recording, has been actively researched as a rewritable optical recording medium. When a photochromic material is irradiated with light of a predetermined wavelength, the structure of the molecule changes due to a photochemical reaction, which causes changes in optical characteristics such as absorbance and refractive index for light of a specific wavelength, and also light and heat of other wavelengths. Has the property that the changed molecular structure returns to its original state.

Therefore, recording on the photochromic 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 characteristics associated therewith, particularly a change in absorbance.

As described above, the characteristic feature of the photon mode recording is that the recording marks are formed by a photochemical reaction, so that the recording marks do not spread due to thermal diffusion unlike the conventional heat mode recording, and therefore, minute marks are recorded. Was considered suitable for. In other words, it has been considered that if recording is performed with a laser spot of the same size, smaller recording marks can be formed in photon mode recording than in heat mode recording, and higher density recording can be achieved.

[0005]

However, the inventor of the present invention is erroneous in the above-mentioned conventional idea, that is, the photon mode recording is inferior to the heat mode recording in the ability to record and reproduce a small recording mark. I found a new problem.

That is, in heat mode recording, a recording mark smaller than the size determined by the diameter of the recording laser spot can be clearly formed, but in photon mode recording, a recording mark smaller than the size determined by the diameter of the recording laser spot is formed. It was found that when formed, the contrast between the marked portion and the non-marked portion was lowered. This means that the output of the reproduction signal in the photon mode is more significantly reduced in the high-density recording area than in the heat mode.

Therefore, for example, when digital information recording is performed by a recording method similar to the heat mode recording, there is a disadvantage that an error rate increases due to deterioration of CN (Carrier vs Noise) ratio in a high frequency region.

Therefore, an object of the present invention is to provide a recording method which suppresses an increase in error rate peculiar to optical recording by a photon mode recording medium and prevents a significant decrease in recording density.

[0009]

According to the present invention, an optical recording medium recorded in a photon mode is irradiated with recording light in accordance with an information signal to be recorded, and the medium has various lengths corresponding to recorded information. The method for recording information by forming the recording mark, characterized in that the length dimension of the shortest recording mark formed on the medium is larger than the diameter dimension of the recording light spot.

[0010]

According to the recording method of the photon mode optical recording medium, the diameter of the recording light spot is φ (the spot diameter is usually defined where the light intensity of the element becomes e ⁻² times the spot center), and the medium is recorded by the recording. When the length of the shortest recording mark formed above is L _min ,

[0011]

[Equation 1]

It is desirable that RLL (Run Length) be used as a recording code in order to suppress a decrease in recording density.
(Limited) code, by setting d to be 1 or more and k to be 4 or more, it is possible to prevent the effect of a significant decrease in the CN ratio in the high frequency region and further reduce the bandwidth, so that the SN ratio of the entire system is also large. It is possible to improve and prevent an increase in error rate.

Further, since the SN (Signal vs Noise) ratio can be improved, R is increased without increasing the error rate.
It is possible to increase d in the LL code, which results in L
It is possible to suppress a decrease in recording density due to increasing _min .

[0014]

EXAMPLE First, the difference between the heat mode and the photon mode at the time of recording a minute mark will be described below. In heat mode recording, the laser light is focused on the medium with relatively strong power,
As a result, the temperature of the recording layer is raised, and only the portion that has reached a certain temperature (= threshold value) or higher is recorded as a recording mark on the medium. This temperature threshold is peculiar to the material, and is, for example, the melting point of the alloy in phase change recording, and the Curie point temperature or compensation temperature of the magnetic material in magneto-optical recording. Corresponding to the fact that the threshold value is clear, the boundary between the marked portion and the non-marked portion is also clear, and the physical characteristics such as reflectance and magnetization direction change rapidly.

FIG. 2 shows the intensity distribution (a) in the recording light spot, the temperature distribution (b) in the recording layer corresponding to this intensity distribution, the recording mark shape (c) on the recording layer, and the reflection on the recording layer. A rate change distribution (in the case of a phase change type) (d) is shown. From FIG. 2, even if the intensity distribution of the recording light spot (approximately a Gaussian distribution) continuously changes, the reflectance change of the formed recording mark / non-mark portion is discrete (high reflectance). And low reflectance). If the power of the recording light is adjusted appropriately, a recording mark smaller than the size of the light spot can be formed.

On the other hand, in the photon mode recording, it does not have a clear recording threshold as in the heat mode recording, and it reacts largely at the center of the spot, but a photochemical reaction corresponding to the intensity occurs even in a weak optical power area around the spot. A change in reflectance occurs.

FIG. 1 shows the intensity distribution (a) in the recording light spot, the recording mark shape (b) on the recording layer, and the reflectance change distribution (c) on the recording layer in comparison with FIG.
It can be seen from FIG. 1 that the boundary between the recorded mark portion and the non-mark portion is unclear, the reflectance continuously changes between the high level and the low level, and the mark becomes wider accordingly.

The size of the recording mark depends on the power level of the recording light as well as the spread of the recording mark, but the size of the recording mark is increased by adjusting the laser power as in the heat mode recording as shown in FIG. It is not possible to change it (especially to make it much smaller than the size of the light spot).

Now, the problem of the photon mode recording with respect to a small recording mark becomes clearer when considering recording a high frequency signal at a constant relative speed. FIGS. 3A to 3C show recording frequency dependence of recording marks and reflectance for heat mode recording.
As described with reference to FIG. 2, the boundary between the mark portion and the non-mark portion is clear, and even if the frequency becomes high (high density) from (a) to (c), the reflectivity becomes higher in the mark portion and the non-mark portion. Takes a constant value according to. In the figure, the spot diameter of the recording light is a size corresponding to the mark length of (a), but the reflectance change does not change even if the mark length becomes smaller than the spot diameter. Therefore, when it is considered that the recording marks in these high-density areas are irradiated with the spot of the reproducing light and the reflected light at that time is detected to reproduce, the output reduction of the reproducing signal is caused by the MTF (Modulation Transfer) of the reproducing light spot. Func
tion) will be governed only by the characteristics.

On the other hand, the recording frequency dependency of the recording mark and reflectance change in the photon mode recording is shown in FIG.
It becomes like (c). Also in this case, the frequency becomes high (high density) from (a) to (c) as in FIG.

The inventor of the present invention theoretically studied the recording frequency in the photon mode recording and the change in the molecular concentration due to the photochromic reaction in the recording layer of the medium, and as a result, the following formula was obtained.

[0022]

[Equation 2]

However, the left side C (X, Y) is the molecular concentration [mol / l] of the recording layer, X is the track direction, and Y is the coordinate system showing the track width direction. On the right side, C ₀ is the density in the initial (unrecorded) state, P _rec [W] is the power of the recording light, β≡αλεκ / π ^1/2 av, and α is a constant 1.9 × 1.
0 ⁴ [Jm / mol], λ [m] is the wavelength of the recording light, ε
[L / mol · cm] is the molecular extinction coefficient at the recording light wavelength, κ is the quantum yield of the reaction, v [m / s] is the relative velocity,
2 · 2 ^1/2 · a [m] represents the spot diameter of the recording light. Further, t [s] is time and ¼t ₀ [Hz] corresponds to the recording frequency.

The reflectance in FIG.
Was obtained by numerical calculation, and (a) to (c) in the figure are
These correspond to (a) to (c) of FIG. 3, respectively.
In FIG. 4A, the recording mark length is the optical spot diameter,
(B) is light spot diameter x 1/2 ^1/2 In addition, (c) is an optical spot
It corresponds to the diameter x 1/2. Because of these things
High density so that the recording mark length is smaller than the spot diameter.
The boundary between the marked part and the unmarked part becomes unclear in the degree area
Not only due to the spread of the recording light spot
Indicates that each mark is adjacent to a pulse for forming a recording mark.
And the amount of change in reflectance itself will also decrease.
I understand.

Therefore, in the photon mode recording, unlike the heat mode recording, the M of the reproducing light spot is
Not only the TF characteristics but also the decrease due to the spread of the recording light spot is added, and therefore the output is significantly decreased in the high frequency (high density) region.

FIG. 5 shows the experimental recording frequency (proportional to the reciprocal of the recording mark length) dependence of the reproduction signal level under the same conditions (the same recording / reproducing light wavelength, that is, the same spot diameter and the same relative speed). It is a thing. Specifically, a cyanine dye as a heat mode material is used as a recording layer, and a diarylethene photochromic material that is well known as a photon mode material that does not react with heat is used for the recording layer, and He-Ne of λ = 633 nm is used.
Recording / reproduction was performed by focusing on the disk medium with an objective lens having a relative velocity of 1.2 m / s and a numerical aperture of 0.5 by a laser. The spot diameter of the laser is 1.3 μm. It should be noted that, in FIG. 5, 0d with reference to a sufficiently low frequency (where the recording mark length equal to half the wavelength of the recording signal is sufficiently larger than the spot diameter).
B, the output reduction amount for this is shown. In the heat mode, the mark length when the output is reduced by 6 dB is 0.83 μm, that is, φ × 0.64 for the spot diameter φ = 1.3 μm, but in the photon mode, 1.2 μm, That is, φ × 0.92. This is in line with the theory that was previously derived.

In the normal heat mode recording, information recording is performed by using an area where the minimum mark length is considerably smaller than the recording laser spot (for example, in a recordable compact disc, the laser spot is 1.5 to 1.6 μm).
On the other hand, although the minimum mark length L _min is 0.87 μm), if the recording is performed by the same method in the photon mode recording, the error rate is significantly increased due to the decrease in the high frequency output.
From this, the minimum mark length L _min is the spot diameter φ of the recording light in order to prevent an increase in the error rate when the information signal is recorded in the photon mode recording.
It can be seen that it is desirable that the length is not less than the length given by.

Next, the experimental results of recording / reproducing a digital information signal on the photon mode recording medium will be described. The medium used contained the same diarylethene-based photochromic material as described in FIG. 5, and the recording / reproducing conditions were the same.
The code of the digital signal used for optical recording is, for example, the document ALAN.
B. MARCHANT, "OPTICAL RECORDING" (1990)
NRZ (Non- as described in P229-242.
Return-to-Zero), MFM (Modified Frequency Modula)
types), IBM (2, 7), EFM, etc., and these recording codes are unified by RLL (Run-Length-Limited) codes characterized by two parameters (d, k). It can be handled (see Table 1).

[0029]

[Table 1]

Normally, the RLL code is a clock from the reproduced signal.
The longest symbol length L that appears to ensure the stability of the pouring_max 
Is considered to be finite,
Is L _max Like NRZ, which has the potential to be infinite
The code is also an RLL code of (d, k) = (0, ∞).
I handle it, so I will follow it here as well.

It is the parameter d that determines L _min, which is relevant to this embodiment,

[0032]

[Equation 3]

Is given by In the above-mentioned document, the minimum time width is

[0034]

[Equation 4]

However, in the present embodiment, since it is defined as the minimum mark length, this is obtained by multiplying this equation 4 by the relative velocity v. Then, in order to perform high density recording, it can be seen from the following equation 5 and Table 2 that L _min should be decreased or parameters d and k should be increased.

[0036]

[Equation 5]

[0037]

[Table 2]

First, a signal given by (d, k) = (0,2) was recorded / reproduced on the photochromic medium. However, L _min was set to 0.9 μm, which corresponds to a spot diameter φ = 1.3 and about 0.7 times μm. From the above Table 2, the value of the numerator on the right side of the equation 5 is 0.879,

[0039]

[Equation 6]

Is obtained.

On the other hand, the eye pattern of the reproduced signal has a very poor eye aperture ratio as shown in FIG. 6 (a), and it can be said that the error rate is large and not practical.

Next, in the same manner, recording / reproducing was performed with L _{min set} to 1.3 μm, which is about the same as the spot diameter, and the result of observing the eye pattern of the reproduced signal is shown in FIG. 6 (b). As is clear from the figure, a good eye pattern with a high eye opening ratio is obtained, and it can be seen that the error rate is kept low.

The recording density at this time is shown in Table 2 and Equation 5 above.
From

[0044]

[Equation 7]

The recording density is lowered by about 31%.

Further, the recording code is (d, k) = (1,5)
Recording / reproduction was performed by changing to the RLL code. At this time, L _min is 1.3 μm. FIG. 6C shows the eye pattern at this time, but it can be seen that a good eye pattern is obtained and the error rate is suppressed low. The recording density at this time is from Table 2 and Equation 5 above.

[0047]

[Equation 8]

It is apparent that the decrease in recording density as shown in FIG. 6B can be prevented.

However, in order to adjust the recording code and increase the density of the recorded information as described above, it is necessary to have a margin in the SN ratio. That is, in order to obtain an error rate of 10 ⁻¹² or less, the relational expression between the SN ratio and the error rate is

[0050]

[Equation 9]

Therefore, it is necessary to improve the SN ratio in order to increase d.

Therefore, in this embodiment, by setting L _min to be equal to or larger than the recording light spot diameter φ, the SN ratio could be improved by the following two factors. 1. The area where the mark length is less than φ, which is a condition that the signal level is extremely lowered, is not used as a recording mark. 2. The bandwidth of the optical recording / reproducing system is narrowed by increasing the L _min level, and the total noise power is reduced.

Therefore, the minimum mark length L _{min is set} to the spot diameter φ.
By doing the above, the error rate of the reproduced signal can be improved,
At the same time, as a result of improving the SN ratio, it was possible to prevent the recording density from decreasing by adjusting the recording code (at least d ≧ 1).

[0054]

As described above, the present invention can be expected to effectively prevent an increase in error rate that is characteristic of photon mode recording, without significantly reducing the recording density.

[Brief description of drawings]

FIG. 1 is a diagram showing an intensity distribution (a) of a recording light spot, a recording mark shape (b), and a reflectance (c) on a recording layer in a photon mode recording medium.

FIG. 2 shows an intensity distribution (a) of a recording light spot on a heat mode recording medium, a temperature distribution (b) on a recording layer, a recording mark shape (c), and a reflectance (d) on the recording layer. It is a figure.

FIG. 3 is a diagram showing a relationship between respective recording mark shapes and reflectance levels when recording is performed while changing the frequency of recording light in the heat mode recording medium from (a) to (c).

FIG. 4 is a diagram showing a relationship between respective recording mark shapes and reflectance levels when recording is performed while changing the frequency of recording light in the photon mode recording medium from (a) to (c).

FIG. 5: Recording frequency of reproduction output (record mark length dependency)
FIG. 6 is a characteristic diagram comparing the cases of heat mode recording and photon mode recording.

FIG. 6 is a diagram showing an eye pattern when recording and reproducing a digital signal with an RLL code by changing recording conditions (a) to (c) in a photon mode recording medium.

Claims (1)

[Claims] 1. An optical recording medium for recording in a photon mode is irradiated with recording light according to an information signal to be recorded to form recording marks of various lengths on the medium according to recording information. The optical recording method is characterized in that the length dimension of the shortest recording mark formed on the medium is larger than the diameter dimension of the recording light spot.