JPH0877607A - Information recording medium - Google Patents

Abstract

PURPOSE: To control the reading of data from the adjacent tracks by providing the first group tracks and the second group tracks with alternate level differences in the radius direction. CONSTITUTION: A substrate 1 is formed of polycarbonate or acryl and recessed track 3 and projected track 4 are alternaly formed at the surf ace of the substrate 1. These tracks 3, 4 are respectively formed spirally or concentrically. At the surface 2 of the substrate 1, the recording layer consisting of a phase varying thin film which may change into the crystal condition and amorphous condition is formed. In both the projected tracks 4 and recessed tracks 3, amplitude modulation marks for changing the reflectivity of light are formed, but it may be obtained because the reflectivity of light differs depending on the crystal condition and amorphous condition of the recording layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an information recording medium such as an optical disc.

[0002]

2. Description of the Related Art A phase change type optical disk (hereinafter referred to as a PC disk) is an example of conventional technology. Figure 5
It is a figure which shows the structure of a PC disk. As shown in FIG. 5, the PC disk has a 1.2 mm-thick transparent substrate 51 on which an amplitude modulation mark array 52 for changing the light reflectance is spirally formed. This is the information track. Information is stored by associating the digital information with the position and length of the amplitude modulation mark.

When reproducing the stored information, first, the PC disk is rotated. Then, the light beam emitted from the reproducing head forms a light spot having a size about the width of the amplitude modulation mark on the information track. Then, the information is reproduced by detecting the diffraction state of the reflected light of the light spot irradiated on the information track.

When there is no amplitude modulation mark at the position of the light spot, most of the reflected light from the information track returns to the reproducing head. Therefore, the information detector incorporated in the reproducing head detects a large value. When there is an amplitude modulation mark at the position of the light spot, the reflected light from the information track is diffracted by the amplitude modulation mark. Since the reflectance of the amplitude modulation mark is lower than that of the surrounding portion, the amount of light returning to the reproducing head is reduced together with the diffraction effect. That is, when there is an amplitude modulation mark at the position of the light spot, the information detector incorporated in the reproducing head detects a small value.

In general, the radius of the light beam from the reproducing head can be narrowed down to the diffraction limit. Therefore, the radius of the light spot irradiated on the information track should be 1
It can be narrowed down to less than μm. Therefore, the pitch of the information tracks can be set to 1.6 μm.

[0006]

However, there is a potential demand for storing more information on the information recording medium. The inventors have made many studies in order to meet this demand. In general, it has been proposed that the reproduction beam diameter be made smaller and the track pitch narrower by shortening the wavelength of the reproduction beam.

However, there is a restriction on the light source used for the reproducing head, and it is impossible to shorten the wavelength infinitely. On the other hand, if the diameter of the reproduction beam is made constant and the track is narrowed, the light spot of the reproduction beam is also irradiated on the adjacent track. As a result, the light spot is also applied to the amplitude mark of the adjacent track, which causes a problem that the information of the adjacent track is read.

Therefore, it is an object of the present invention to provide an information recording medium capable of suppressing the reading of information from an adjacent track and reproducing only the information from a target track.

[0009]

[Means for Solving the Problems] In order to solve the above problems,
In the information recording medium of the present invention, the tracks of the first group and the tracks of the second group are concentrically or spirally provided, and the tracks of the first group and the tracks of the second group are alternately arranged in the radial direction. Amplitude modulation type marks having different light reflectances from the surroundings are formed on the tracks of the first group and the tracks of the second group, which are provided with steps, and the constituent elements of these marks are
Formula (34) described in the following action term is satisfied.

[0010]

Now, it is assumed that the tracks of the first group are convex tracks (convex tracks) and the tracks of the second group are concave tracks (concave tracks). Here, the concave track is a track that is recessed when viewed from the side where the beam is irradiated, and the convex track is a track that is protruded when viewed from the side where the beam is irradiated. To do.

Consider a case where a signal is not recorded on the convex track (there is no amplitude modulation mark) and a signal is recorded on the adjacent concave track (there is an amplitude modulation mark).
FIG. 2 is a diagram showing this state. In FIG. 2, the reproduction beam 21 is emitted from above to the center of the convex track. Now, assuming that the length of the amplitude modulation mark in the track direction in FIG. 2 is sufficiently longer than the reproduction beam size (spot diameter), the intensity modulation amount N of the return light (reflected light) by the mark of the adjacent track is shown in FIG. It is equal to the difference in the amount of returning light in each state of a) and (b).

On the other hand, the light reflected and diffracted on the disk surface returns to the objective lens again to form a diffraction pattern as shown in FIG. In FIG. 4, regions 41 and 42 are regions where the 0th-order diffracted light and the ± 1st-order diffracted lights interfere with each other. Area 43
Is a region where only the 0th order diffracted light exists. The amount of return light is obtained by adding the light intensities of the regions 41 and 42 and the region 43.

Now, the step of the uneven track is d, the light reflectance inside the amplitude modulation mark is r, and the light reflectance outside the mark is 1.
And Phase change δ when light is reflected by a track step δ
Where n is the refractive index of the medium and λ is the wavelength of the light beam,

[0014]

[Equation 12]

Is given by In addition, the track pitch
Let p be the mark width (width of the groove carved in the concave track in FIG. 3B) be g. At this time, the 0th-order diffracted light amplitude K ₀ and the 1st-order diffracted light amplitude K ₁ are

[0016]

[Equation 13]

[0017]

[Equation 14]

Is given by Where o (x) is the amplitude distribution function of the disk surface

[0019]

(Equation 15)

[0020] Therefore, Fig. 5 when there is a mark
Intensities I ₁ (ψ) of the regions 41 and 42 (the region where the 0th-order diffracted light and the ± 1st-order diffracted light interfere) are

[0021]

[Equation 16]

Thus, the intensity I ₀ (ψ) of the region 43 (region where only the 0th order diffracted light exists) in FIG.

[0023]

[Equation 17]

It becomes here

[0025]

(Equation 18)

[0026]

[Formula 19]

[0027]

[Equation 20]

[0028]

[Equation 21]

[0029]

[Equation 22]

Areas 41 and 42 when there is no mark
Intensity of (area where 0th order diffracted light and ± 1st order diffracted light interfere)
I ₁ (0), area 53 (area where only 0th order diffracted light exists)
The intensity I ₀ (0) of r in equations (16) and (17) is r
= 1, so

[0031]

[Equation 23]

[0032]

[Equation 24]

It becomes Now, here, in the region 41 of FIG.
Assuming that the area of 42 is α and the area of the region 43 is β, FIG.
The difference N of the return light in each state of (a) and (b) is

[0034]

(Equation 25)

[0035] In equation (25), N clearly represents the crosstalk level from the adjacent concave track.
Therefore, if N = 0 in the equation (25), a crosstalk-free conditional equation can be obtained. Up to this point, crosstalk mixing from the adjacent concave track to the convex track has been considered, but next consider the case where the amplitude modulation mark is present on the own track, that is, the convex track where the laser spot exists. In this case, the light intensity J in the regions 41 and 42 of FIG.
₁ (ψ) and the light intensity J ₀ (ψ) in the region 43 are

[0036]

(Equation 26)

[0037]

[Equation 27]

The signal level S given by

[0039]

[Equation 28]

Is given by However

[0041]

[Equation 29]

[0042]

[Equation 30]

[0043]

[Equation 31]

Using equations (25) and (28), the ratio N / S
This gives the relative crosstalk. Generally, it is desirable that this relative crosstalk amount be -20 dB or less.

[0045]

[Equation 32]

It is desirable to satisfy the following. With the above discussion,
The effect of Gaussian beam is not considered. In addition, the effect that the ± 1st order diffracted lights interfere with each other is not considered. But,
Without considering these, the equations (25) and (28)
The equation gives the signal and crosstalk level with sufficient accuracy.

If the reproduction beam does not cause the track offset, the areas 41 and 42 in FIG. 4 have the same intensity. Therefore, it is not necessary to separately consider the −1st order diffracted light. The above is the crosstalk when viewed from the convex track. Considering the crosstalk when viewed from the concave track, the sign of δ is reversed because the convex-concave relationship with the adjacent track is reversed. But (2
Since the variable δ appears only in the form of cos δ in the equations 5) and (28), the crosstalk free condition of the equation (32) does not change even if the sign of δ is reversed. Therefore, equation (3
The solution 2) can simultaneously minimize the crosstalk in the convex track and the crosstalk in the concave track.

Up to this point, the discussion has been made with the amplitude modulation type optical disk in mind, but the effect of the present invention is also effective for a magneto-optical disk using the polar Kerr effect. If the complex Kerr rotation angle is θ = θr + iχ, then φ = arctan (χ / θ
r) The reflected light becomes linearly polarized by inserting a phase plate with a phase difference of r) into the reproducing optical system.At this time, assuming the symmetric differential method, the magneto-optical mark has an amplitude given by the following equation after passing through the analyzer. It can be used as a modulation mark.

[0049]

[Expression 33]

[0050]

1 is a diagram showing the structure of an information recording medium according to a first embodiment of the present invention. In FIG. 1, the substrate 1
Is made of polycarbonate, acrylic, or the like. The concave tracks 3 and the convex tracks 4 are alternately formed on the surface of the substrate 1. The two types of tracks are each formed in a spiral shape. It may be formed in a concentric shape. On the surface 2 of the substrate 1, a recording layer made of a phase change thin film capable of changing into both a crystalline state and an amorphous state is formed.

Marks (amplitude modulation marks) for changing the reflectance of light are formed on the convex track and the concave track. This is because the light is different depending on whether the recording layer is in the crystalline state or in the amorphous state. It is obtained by having different reflectances. The head 8 for reproducing the information recorded on the medium has the following configuration. The light beam emitted from the semiconductor laser 9 passes through the beam splitter 10 and is focused on the track surface 2 by the objective lens 11. The reflected light (return light) reflected by the track surface 2 passes through the objective lens again, is reflected by the beam splitter 10, and enters the photoelectric converter (for example, photodiode) 12. Information can be reproduced by the photoelectric converter 12 according to the level of the electric signal photoelectrically converted. In the substrate 1 of FIG. 1, the reflectance of the mark, the step between adjacent tracks, the track pitch, the mark width, the wavelength λ of the semiconductor laser 9 and the numerical aperture NA of the objective lens 11 are determined so as to satisfy the expression (32). Therefore, good signal reproduction with less crosstalk becomes possible.

[0052]

As described above, according to the present invention, the reading of information from the adjacent track can be suppressed, and thus the track pitch can be substantially halved as compared with the conventional one. Therefore, twice as much information can be recorded as compared with the conventional recording medium. A breakthrough that doubles the recording density without changing the playback head.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an information recording medium according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a surface of an information recording medium.

FIG. 3 is a diagram showing a simplified model of the surface of the information recording medium.

FIG. 4 is a diagram showing a diffraction pattern of return light from the surface of a recording medium.

FIG. 5 is a diagram showing a configuration of a phase change optical disc according to a conventional technique.

[Explanation of symbols]

 1 ... substrate 2 ... substrate surface 3 ... concave track 4 ... convex track 5, 6 ... amplitude mark 7 ... protective layer 8 ... reproducing head 9 ... ... Semiconductor laser 10 ... Beam splitter 11 ... Objective lens 12 ... Photoelectric converter

Claims (1)

[Claims] 1. A track of the first group and a track of the second group are concentrically or spirally provided, and the track of the first group and the track of the second group are alternately stepped in a radial direction. The first group of tracks and the second group of tracks are formed with amplitude modulation marks having different light reflectances from the surroundings, and when the amplitude modulation pits are illuminated by a laser spot,
The amplitude reflection coefficient of the light reflected inside the mark is r, the amplitude reflection coefficient of the light reflected outside the mark is 1, and the light reflected at the unmarked portion of the track of the first group and the second When the phase difference of the light reflected at the unmarked part of the track of the group is δ, the mark width is g, the track pitch (track interval of the first group) is p, and the circular constant is π, the quantity N, S is defined by the following formula, and [Equation 2] Where α is the area of the region where the 0th-order diffracted light and the ± 1st-order diffracted light interfere with each other in the far-field diffraction pattern of the reflected light of the light beam applied to the first group or the second group β; the first group or the second group Area of the region where only the 0th order diffracted light exists in the far-field diffraction pattern of the reflected light of the light beam irradiated on [Equation 4] (Equation 5) (Equation 6) (Equation 7) [Equation 8] [Equation 9] [Equation 10] When An information recording medium characterized in that