JPS61283041A - Method for reproducing optical disk - Google Patents

Abstract

PURPOSE:To improve the C/N value of a high birefringence part and to make an optical disk applicable in a wide area by setting reproduced luminous energy with respect to the high birefringent part on a transparent substrate larger than a low birefringent part. CONSTITUTION:A reproduced light beam 9 from a semiconductor laser 3 is condensed on the information medium layer of the substrate 1 by an objective lens 7. The reflected light beam becomes a circularly deflected light beam if there is no birefringence on the substrate 1 and reflected by a beam spliter 5, and all split light enter a detector 8 as an effective signal light beam 13. If the substrate has the birefringence, the beam 11 becomes an elliptical light beam. Its vertical component becomes the effective light beam 13, and the horizontal one becomes an invalid signal light beam 14 to make a detection signal smaller. Since the birefringence becomes larger at the outer diameter side of the disk, the reproduced quantity of light 9 is increased or decreased in the disk diameter direction in accordance with the magnitude of the birefringence value to obtain the effective signal light beam 13 with the fixed quantity of light. Accordingly the high C/N value can be obtained even in the part at the high birefringence on the substrate, and the disk can be used in the wide area.

Description

[Detailed description of the invention] Industrial applications The present invention relates to a method for playing back an optical disc.

BACKGROUND OF THE INVENTION Birefringence is one of the important optical properties required for substrates of optical disks. FIG. 1 is an explanatory diagram showing the reproduction state of a general optical disk. Reproduction light 9 which is linearly polarized light emitted from the semiconductor laser 3 and vibrates in a direction parallel to the plane of the paper
passes through a collimator lens 4 and a polarizing beam splitter 5, and reaches a quarter-wave plate 6. Here, reproduction light 9 is circularly polarized light 1
The light is converted to 0 and focused on the information medium layer 2 on the substrate 1 by the objective lens 7 . Information media! Reflected light from 2 1
1 becomes circularly polarized light if the substrate 1 has no birefringence, but becomes circularly polarized light if it has birefringence. This reflected light 11 is 1/4
When passing through the wavelength plate 6, it is converted into reflected light 12.

When the reflected light 11 is circularly polarized light, the reflected light 12 becomes linearly polarized light vibrating in a direction perpendicular to the plane of the drawing, is reflected by the polarizing beam splitter 5, and all enters the signal detector 8 as an effective signal light 13. However, if the substrate 1 has birefringence, the reflected light 11 becomes circularly polarized light as described above, so the reflected light 12 becomes circularly polarized light consisting of a horizontal component and a vertical component as shown in FIG. It becomes polarized light. Therefore, only the vertical component of the reflected light 12 is reflected by the beam splitter 5 and enters the signal detector 8 as an effective signal light 13, while the remaining horizontal component passes through the beam splitter 5 as an invalid signal light 14. In other words, the magnitude of the detected signal is disadvantageously smaller than that in the case where the substrate 1 has no birefringence.

Therefore, in conventional optical disks, a glass plate or cast acrylic plate with low birefringence is used as the substrate 1, or only a portion of a resin substrate made of injection molded polycarbonate or the like with low birefringence is used. Among these, injection molded resin substrates are suitable for mass production and are inexpensive, so molding conditions that reduce birefringence are being studied. However, birefringence has not been reduced to near the outer peripheral edge.

FIG. 3 shows changes in birefringence, reproduction light amount, effective signal light amount, and C/N value depending on the radial position of an optical disk using an injection-molded polycarbonate substrate. This is a result of reproduction with the reproduction light amount of the semiconductor laser 3 of FIG. 1 constant, and the effective signal light amount becomes small near the outer periphery where the birefringence is large, and the C/N value also becomes small.

Problems to be Solved by the Invention As described above, in the conventional reproducing method, the C/N value becomes small in the portion of the substrate where the birefringence is large, so there is a problem in that the entire area of the optical disc cannot be used. Ta.

The present invention has been made in view of this point, and provides a method for reproducing an optical disc, which allows a high C/N value to be obtained even in a portion of a substrate with relatively large birefringence, and allows the optical disc to be used over a wide area. The purpose is to

Means for Solving the Problems The method for reproducing an optical disc of the present invention includes irradiating reproduction light from the transparent substrate side of an optical disc having an information medium layer on a transparent substrate and reading information by the reflected light from the information medium layer. In this case, when reading information from a portion of the transparent substrate where birefringence is large, the amount of light of the reproduction light is increased compared to when reading information from a portion where birefringence is small.

Function: According to this reproducing method, since the amount of reproducing light is controlled according to the birefringence, a high C/N value can be obtained even in a portion of the optical disc substrate where the birefringence is relatively large, making it possible to use the optical disc over a wide area. Can be done.

EXAMPLE Hereinafter, a specific example of the optical disk reproducing method of the present invention will be described.

In other words, in the configuration shown in Fig. 1, the light intensity of the reproduction light 9 is reduced by increasing the value of birefringence, which varies depending on the radial position of the optical disk, depending on the value, so that a substantially constant light intensity is maintained at all times. Effective signal light 13 is obtained. Thereby, as shown in FIG. 4, a high C/N value can be obtained even near the outer periphery where the birefringence of the substrate is large. In this embodiment, an optical disk having a polycarbonate substrate made by injection molding is used. In an injection molded substrate, the change in the magnitude of birefringence depending on the radial position is determined by the molding conditions, and a substantially homogeneous substrate is molded. Therefore, the amount of reproduction light can be determined in advance according to the radial position of the optical disc to be reproduced.

If the birefringence of the substrate of the optical disk to be played cannot be predicted in advance, or if there is a large variation, the amount of effective signal light is detected and the resulting signal is transmitted to the semiconductor laser. By feeding back to a drive circuit (not shown), the light intensity of the reproduction light is changed as needed to keep the light intensity of the effective signal light 13 substantially constant.

As described in the detailed description of the invention, according to the optical disc reproducing method of the present invention, a high C/N value can be obtained in a very simple manner even in a portion of the optical disc substrate where the birefringence is relatively large, and the optical disc can be played over a wide area. It can be used over a period of time.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an optical disc reproducing device, Fig. 2 is an explanatory diagram of circularly polarized light, Fig. 3 is an explanatory diagram showing the characteristics of an optical disc according to a conventional reproducing method, and Fig. 4 is an explanatory diagram of the reproducing method of the present invention. FIG. DESCRIPTION OF SYMBOLS 1... Substrate, 2... Information medium layer, 3... Semiconductor laser, 5... Polarizing beam splitter, 6... 1/4
Wave plate, 8... Signal detector, 13... Effective signal light,
14... Invalid signal light agent Yoshihiro Morimoto Fig. 1 Fig. 2 Fig. 3 Inner blade 4 ---- A6 knee 1 - Tato circumference Fig. 4

Claims (1)

[Claims] 1. When reproducing light is irradiated from the transparent substrate side of an optical disk having an information medium layer on a transparent substrate and information is read by reflected light from the information medium layer, the birefringence of the transparent substrate is A method for reproducing an optical disc, in which when information from a large portion is read out, the light intensity of the reproducing light is made larger than when reading information from a portion where birefringence is small. 2. The light amount of the reproduction light is adjusted so that the amount of light incident on the signal detector out of the light reflected from the information medium layer is approximately the same in the portions of the transparent substrate where the birefringence is large and the portion where the birefringence is small. A method for reproducing an optical disc according to claim 1.