JPH087887B2 - Flat record carrier - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a flat information recording medium (hereinafter referred to as a disk) capable of optically and magnetically and optically recording and reproducing information.

2. Description of the Related Art Optical discs capable of recording / reproducing information optically or magneto-optically are being developed as recording media capable of recording a large capacity. Among them, a magneto-optical disk capable of rewriting information is considered to be applied to a data file or an image file of a computer.

At present, plastics such as polycarbonate and acrylic are regarded as promising substrates for such magneto-optical disks. It is known that when these plastics are used to form a substrate by injection molding, a large birefringence is generated, and this is a serious problem particularly in a magneto-optical disk that detects a signal by changing the polarization state.

Therefore, in order to reduce the absolute value of the birefringence of the substrate, efforts have been made to strictly control the molding conditions.

However, in the conventional method of reducing the absolute value of the birefringence of the substrate in order to improve the characteristics of the magneto-optical disk, the material itself has a large intrinsic birefringence (for example, polycarbonate or the like). ) Is not applicable. In addition, when the diameter of the disk is large, the distortion due to molding remains large, and it is very difficult to improve the characteristics by reducing the birefringence.

In view of the conventional problems, it is an object of the present invention to provide a flat information recording medium capable of reproducing a good recording signal even when a substrate having a large intrinsic birefringence such as polycarbonate is used. Is.

Means for Solving the Problem In order to solve the above problems, the present invention provides, at each position of a disk, either a fast axis or a slow axis of birefringence with respect to a light beam of a specific wavelength that is transmitted in the thickness direction of a substrate. And the angle between this position and the direction connecting the center of the board
The substrate is set to 12 degrees or less and the phase difference due to this birefringence is set to 20 degrees or less.

With such a configuration, when the reproducing light beam passes through the substrate having the cover refraction, if the light beam is DC polarized light, the polarization direction is the fast axis direction or the slow axis direction of the birefringence of the substrate. It can be made to substantially coincide with the phase axis direction, and the polarization state of the light beam can be prevented from changing. From this, even when the signal is reproduced from the magneto-optical disk, by setting the optical system so that the polarization direction of the reproduction light beam and the fast axis direction (or the slow axis direction) of the birefringence of the substrate always match, Even if the absolute value of the birefringence of the substrate is large, it is possible to sufficiently suppress the disturbance of the polarization state received by the reproduction light beam, and it is possible to perform good signal reproduction.

Example A magneto-optical disk of an example of the present invention will be described below with reference to the drawings. The side structure of the disk of this embodiment is shown in FIG. An amorphous thin film of TbFeCo, which is an alloy of a rare earth metal and a transition metal, is used as the recording medium layer 1, and is formed on a polycarbonate substrate 2 with an inorganic thin film 3 interposed therebetween, and further covered with a protective layer 4. are doing.

The substrate 2 has a variation range of birefringence in the slow axis direction within ± 8 degrees with respect to the radial direction from the disk center, and the absolute value of the birefringence is set to a maximum of 76 nm by a double path (reciprocal path). Using. The wavelength of the reproducing beam is 780 nm. The absolute value of this birefringence corresponds to a phase difference of 18 degrees.

As a comparative example of this example, a magneto-optical disk using glass as a substrate was prepared. A photocurable resin is used to form the guide groove, and the absolute value of birefringence of the substrate is 5 nm or less.

Table 1 shows the measured values of the CN ratio when continuously recording bits of 1 μm in size for these two examples. As is clear from this, in the present embodiment, a sufficiently high signal reproduction characteristic was obtained while using the substrate 2 having a very large intrinsic birefringence of polycarbonate.

Although polycarbonate is used as the substrate 2 in this embodiment, the material of the substrate 2 may be another plastic or the like. Further, as the material of the recording medium layer 1, a magnetic material other than the rare earth-transition metal alloy as in this embodiment may be used.

Next, CN ratio (signal to noise ratio) due to substrate birefringence when polycarbonate is used for the substrate 2 of the magneto-optical disk.
Fig. 2 shows the result of the numerical calculation of the change in the.
The phase difference δ due to birefringence represents the phase shift between the polarization component in the fast axis direction of the substrate birefringence and the polarization component in the direction perpendicular to it, of the reproduction beam that passes therethrough. Means a value. From FIG. 2, it is found that the angle between the plane of polarization of the reproduced light and the fast axis direction (or slow axis direction) of the substrate birefringence is 12 degrees or less, and the phase difference due to the birefringence is 20 degrees.
If it is less than 100 degrees, the decrease in CN ratio is within 2 dB, and it is clear that sufficient characteristics can be maintained.

At the time of actual signal reproduction, the substrate is rotated while moving the reproduced beam in the radial direction of the substrate. Therefore, even if the reproduced beam and the substrate are in any positional relationship, the radial direction of the substrate (each substrate point and the disk center The direction formed by the direction (connecting) and the polarization direction of the reproduction light beam is not changed.

The birefringent fast axis direction (or slow axis direction) of plastic tends to be along the molecular orientation direction, so the birefringent fast axis direction (or slow axis direction) during injection molding of a substrate. The disk of the present invention can be manufactured by performing molding so as to be aligned radially with respect to the center of the substrate. That is, at least the signal should be recorded during the molding of the substrate for the means for setting the fast axis direction (or the slow axis direction) of the birefringence of the substrate to 12 degrees or less with respect to the radial direction from the center of the substrate. In the region, the plastic material may be radially flown from the center position of the substrate so that the orientation directions of the molecules are aligned radially. In the outer peripheral area of the substrate, since the material flow is limited by the outermost mold wall surface, the radial molecular orientation state cannot be obtained up to the end. Again, this area should not be used as a signal recording area. In addition, it can be realized by increasing the size of the substrate with respect to the signal recording area.

Regarding the means to reduce the phase difference of light rays due to birefringence to 20 degrees or less, molding of mold temperature, pressure holding time, etc. using a special molding machine with a material with good fluidity even for polycarbonate with large birefringence It can be realized by selecting the conditions.

As described above, according to the present invention, the direction of birefringence of the substrate with respect to the center of the disk is ± 12 degrees or less, and the phase difference due to birefringence is 20 degrees or less, which makes it difficult to use conventionally. Even if a substrate having a large birefringence such as polycarbonate is used, it is possible to obtain a high quality disc such as a magneto-optical disc having good reproduction characteristics.

[Brief description of drawings]

FIG. 1 is a side view of a flat information recording medium according to an embodiment of the present invention, and FIG. 2 is a diagram showing the fast axis (slow axis) direction of the birefringence of the substrate and the amount of change in the CN ratio of the reproduced signal. It is a characteristic view which shows a relationship. 1 ... Recording medium layer, 2 ... Substrate.

Claims (1)

[Claims] 1. A recording medium layer having a recording medium layer formed on a substrate, wherein information recorded on the recording medium layer is changed by changing a polarization state of a light beam passing through the substrate and irradiated on the recording medium layer. Which is a record carrier for reproducing the information, wherein either one of a fast axis direction and a slow axis direction of birefringence of the substrate with respect to a light beam passing in a thickness direction of the substrate, and a radial direction from a center of the substrate. The flat information recording carrier is characterized in that the angle formed by each of them is 12 degrees or less at each ray passing position, and the phase difference of the rays due to the birefringence is 20 degrees or less.