JPH04182935A - Optical disk reproducing method - Google Patents

Abstract

PURPOSE:To diminish aberration of a laser spot converged on a disk surface by making a laser beam incident approximately parallel to an optical axis of an objective lens and receiving all the light transmitted through the objective lens out of reflected light from V-groove slant surfaces. CONSTITUTION:A spot position is controlled by a reflected beam 21a. In this case, a focus control is performed by a signal obtained by subtracting a sum signal of light receiving parts 32 and 34 from a sum signal of light receiving parts 31 and 32. That is, under the focus control of an astigmatic system, three reflected beams are rotated with their images on the surface of a photodetector by 90 deg., so that the image of a signal pit in the track direction on the photodetector is parallel to a line dividing the light receiving parts 31 and 34 or 32 and 33, and hence a tracking control of the V-grooved disk is performed by a signal obtained by subtracting a sum signal of the light receiving parts 33 and 34 from a sum signal of the light receiving parts 31 and 32. Reflected beams 22a and 23a are the reflected light from the adjacent V-groove slant surfaces, and all of them are received by the light receiving parts 35 and 36 to reproduce a signal. By this method, the aberration of the spot on the disk can be diminished.

Description

[Detailed Description of the Invention] Industrial Field of Application The present invention relates to a method for reproducing an optical disc using a V-groove blade type. Conventional technology The V-groove blade type has been proposed for increasing the density of optical discs. (For example, JP-A-56-58144, JP-A-56-58144,
7-105828, JP-A-58-102339, etc.).

Figure 2 shows a perspective view of a cross section of a replica of a V-groove disk. 1 is a transparent substrate, 2 is a V-groove, and even though the signal bit 3 is formed on the slope of the V-groove, the laser for signal reproduction is on the transparent substrate 1 side. The reproducing optical system will be briefly explained with reference to FIG. 3. The light from the laser 11 passes through a diffraction grating 12 and a half mirror.
13, becomes parallel light by the collimating lens 14, and is focused onto the V-groove disc by the objective lens 15.The reflected light from the disc passes through the objective lens 15 and the collimating lens 14 again, and is then turned into a parallel light by the half mirror 13. 16, the laser beam is given astigmatism and irradiated onto a photodetector 17, and the signal from this photodetector is used to control the position of the laser spot on the disk and reproduce the recorded signal.

The beam of the semiconductor laser 11 is split into three by the diffraction grating 12, and three spots are formed on the disk as shown in Figure 4. Although the spot 21 in the middle of FIG. 4 is focused on the peak or valley position of the V-groove and is used for focus control and tracking control, it is also used as a spot 22 and 23 for other 1st-order diffracted light and 1st-order diffracted light.
is focused on the adjacent slopes of the V-groove, and by receiving these reflected lights, the signal on the slope is reproduced.Since the signals on the two slopes can be reproduced simultaneously and independently, the transfer rate can be doubled. The system not only plays V-groove discs, but also plays C-groove discs.
Since it is also used to play back conventional flat disks such as D and video disks, the playback method will be briefly explained. FIG. 6 shows the configuration of the photodetector section of the photodetector 17 for a conventional disk and three reflected beams from the disk.

The spot position is controlled by the reflected beam 21a of the beam 21. From the sum signal of the light receiving parts 31 and 33, the light receiving parts 32 and 34
Focus control is performed using the signal obtained by subtracting the sum signal of
Signal reproduction L with the sum of 1, 32, 33, 34 35 and 36
Tracking is accomplished by comparing the amount of light received by L, It is necessary to reduce leakage signals (crosstalk) from slopes adjacent to the V-groove slope being played, and although several methods have been proposed for playing the V-groove disc, three representative methods are described below. I'll keep it.

First, Japanese Patent Application Laid-Open No. 56-58144 proposes a method in which a laser beam is irradiated perpendicularly to the inclined surface of a V-groove and all of the reflected light is received. With a high NA objective lens, the incident angle of the laser beam must be within 1 or 2 degrees; if it exceeds this angle, aberrations will occur and the reproduction signal quality will deteriorate due to insufficient focusing. The angle of inclination must be at least 5 degrees. However, it is extremely difficult to implement this method.Secondly, although the method proposed in JP-A-57-105828 is explained, the laser beam is aligned with the optical axis of the objective lens. It is also possible to focus the laser spot on the V groove with small aberrations by making the laser incident approximately parallel to the V groove. Here, approximately parallel means within 1 degree to the optical axis of the objective lens. The direction of the reflected light is deviated from the optical axis of the objective lens.Since the center of the laser spot is tracked on the reproducing groove slope, most of the light is irradiated onto the reproducing slope, and most of the reflected light is also directed toward the objective lens. Also, the reflected light from the slope adjacent to the reproducing slope is biased toward the inner half of the objective lens.Only the periphery of the laser spot is illuminated on the adjacent slope. , the amount of reflected light is also small. FIG. 7 shows the distribution of the reflected light from the laser spot on the V-groove slope after passing through the objective lens 15.

This reflected light distribution is an example of when a laser spot is irradiated to a place where there is no signal pit on the slope, and it is divided into a large reflected light part and a small reflected light part, but this larger reflected light part is from the reproduction slope. Even if the signal pit spreads over more than half of the objective lens, if there is a signal pit on the slope, each peak in the distribution in Figure 7 will decrease, and the reflected light distribution will be scattered around by the amount of the decrease. Although not shown in detail, the reflected light distribution Dζ in Fig. 7 is the force that reduces the beam diameter in the lens system of the reflected optical path in Fig. 3 (irradiation onto the photodetector 17 with a similar distribution). However, in Japanese Patent Application Laid-Open No. 57-105828, it is analytically shown that the crosstalk of the reproduced signal becomes small slightly outside the center of the optical axis (range of arrow X in Fig. 7) due to the interference effect of reflected light. The third method proposed in JP-A-58-102339 is method L, in which a laser beam is made approximately parallel to the optical axis of the objective lens. Most of the reflected light is concentrated in the half of the lens, but the proposed method is to receive the reflected light that passes through the objective lens in the inner half. It is always considered to be completely coherent. ■ A signal bit is created on the slope of the groove. A case in which a thin film of phase change material is formed on the V-groove surface and only the reflectance of the recorded area is changed by signal recording. There are no irregularities on the slope, and the assumption of coherence holds true to some extent.

The same is true when forming a magneto-optical material on the groove surface and changing the magnetization direction by recording.In the case of a read-only disk in which signal bits are formed on the LV groove surface, the signal bits on the disk. It is difficult to form it into an ideal shape, and the bottom surface and periphery may be slightly disordered, so the reflected light cannot be said to be completely coherent. It is necessary to reproduce the light and consider the optimal reproduction method.There are many components in the reflected light that are disturbed by scattering, and the coherence is partially broken and the coherence is reduced.However, the scattered light does not have a specific direction. However, the crosstalk components due to these scatterings are scattered throughout the reflected light, and even though they are almost uniformly included, the tendency for the crosstalk to be the smallest in a portion of the reflected light decreases. If only a portion of the light is received, the reproduced signal will not be at its maximum, which is disadvantageous for obtaining high signal quality.a Means for solving the problem Inject the laser beam approximately parallel to the optical axis of the objective lens, The direction of incidence of the laser beam is approximately parallel to the optical axis of the objective lens, and the aberration of the laser spot focused on the disk surface can be small. is tilted, so the direction of the reflected light is also offset from the optical axis of the objective lens.The distribution of reflected light from the disk surface consists of a large reflected light portion from the playback slope and a small reflected light portion from the slope adjacent to the playback slope. Even if it is divided into two parts, the large reflected light part corresponding to the reflected light from the reproduction slope will spread over more than half of the objective lens, but if the entire part of the reflected light is received, the small reflection from the reproduction slope and the adjacent slope will occur. This will also reproduce the crosstalk part included in the light part.The amount of light in that part is small, so
The crosstalk is acceptable for reproducing digital signals.Example: A V-groove disc with signal bits formed on the slope is played back, and the signal quality (C/N and crosstalk) is measured to optimize the reflected light. We investigated the light-receiving area and explained the experimental results.However, when reproducing the signal from the reproduction slope, the signal from the adjacent slope is also mixed.The circle in Fig. 1(a) shows the reflected light from the V-groove slope.

When reproducing a signal from a slope that slopes upward to the right as shown in Figure 7, most of the reflected light is concentrated in the left half of the circle in Figure 7. Therefore, the reflection on the left side of the vertical line To reproduce the light (hatched area), the position of the vertical line is also plotted at the intersection with the ξ axis passing through the center 0 of the reflected light.The reproduction signal S of the reproduction slope and the reproduction signal C of the adjacent slope for the position of the vertical line The measured values are shown in FIG. 1(b). The difference between the reproduced signal of the reproduced slope and the adjacent slope is considered to be crosstalk. According to the analysis simulation, the optimal light receiving area is the part of the reflected light that is slightly smaller than a semicircle, as shown by the arrow X in Figure 7. On the other hand, in actual experiments, it has been found that the amplitude of the reproduced signal is larger and the crosstalk is smaller when the signal is regenerated by receiving a portion of the reflected light that is slightly larger than a semicircle. This corresponds to receiving most of the reflected light portion of the larger reflected light distribution.A The reason why the analysis and experimental results differ is briefly explained, but in conventional analysis, laser light is always completely coherent ( (coherence).In the case where no signal bit is created on the slope of the nine-groove groove, and only the reflectance of the recorded area is changed by recording a signal by depositing a phase change material on the surface of the groove, (
Since there are no irregularities on the V-groove slope that disturb the coherence, the assumption holds true to some extent.The analysis results can explain the experimental results, and may be true in the case of a read-only disc that forms signal bits on the LV groove surface. It is difficult to form a signal bit into an ideal shape, and the bottom and surrounding areas may be slightly disordered, so there are many components in the reflected light that are disturbed by scattering. Therefore, the reflected light is perfect. It cannot be said that there is coherence, but the coherence is partially broken and the drying property is reduced.The crosstalk components due to these scatterings do not have a specific direction of reflection, but rather are almost uniformly included in the overall reflected light. The tendency for crosstalk to be lowest in a portion of the reflected light is decreasing.If only a portion of the reflected light distribution is received, the reproduced signal is not at its maximum, which is disadvantageous for obtaining high signal quality. The distribution of reflected light from the groove slope has a large portion corresponding to the reflected light from the reproducing slope and a small portion corresponding to the reflected light from the slope adjacent to the reproducing slope (distribution D in Fig. 7).

By receiving most of the larger reflected light portion, the reproduced signal can be brought close to the maximum, but at that time, the crosstalk component tends to be uniformly included in the overall reflected light, so the crosstalk relative to the magnitude of the reproduced signal However, the optical system would be complicated to partially reproduce the reflected light. If the light is received, the crosstalk part included in the small reflected light part from the reproduction slope and the adjacent slope will also be reproduced.The amount of light in that part is small, so the crosstalk is acceptable for reproduction of digital signals, etc. The reproduction optical system of the present invention can also be used for reproduction of digital signals in which a reproduction signal of a level of quality is obtained even if the entire reflected light is received, and the requirements for crosstalk are not strict. To receive all of the reflected light from the V-groove slope that passes through the objective lens, the photodetector shown in Figure 6 can be used, and the reflected light above it can also be configured in the same way as shown in Figure 6. That's fine. The spot position is controlled by the reflected beam 21a in FIG. Focus control is performed using a signal obtained by subtracting the sum signal of light receiving sections 32 and 34 from the sum signal of light receiving sections 31 and 33.a Focus control using the astigmatism method causes the three reflected beams to have an image of 90° on the photodetector surface. Because of the rotation, the image of the signal bit on the photodetector in the track direction becomes parallel to the line dividing the light receiving sections 31 and 34 or 32 and 33.
Tracking control of the grooved disk is performed using a signal obtained by subtracting the sum signal of the light receiving sections 33 and 34 from the sum signal of the light receiving sections 31 and 32.

The reflected beams 22a and 23a are reflected lights from adjacent V-groove slopes, and the light receiving sections 35 and 36 receive all of them and reproduce signals.The same optical system as the conventional one is used to reproduce the Lx V-groove disc. Effects of the Invention: Since the laser beam is incident on the tapered objective lens almost parallel to the objective lens, the aberration of the spot on the disk can be reduced.
Moreover, it is possible to play V-groove discs using the same optical system as before.

[Brief explanation of the drawing]

FIG. 1(a) JA The light intensity distribution diagram in one embodiment of the optical disc reproducing method of the present invention is shown in FIG. 1(b).
Figure 2 shows a perspective view of a V-groove disc cross-section. Figure 3 shows the configuration of the V-groove disc playback optical system. Figure 4 shows the arrangement of a laser spot on a V-groove disc. Figure 5 shows a disc in a conventional playback method. Fig. 6 shows the configuration of a photodetector corresponding to a conventional disc and explains the reflected light on it. Fig. 7 shows a conventional reproducing method using the distribution of reflected light from the V-groove slope. In the explanatory diagram, there are... Transparent substrate 2... V groove 3... Signal pit 15... Objective lens 17... Photodetectors 31 to 36... Name of photodetector agent Patent attorney Small Akira Kaji and 2 others S-・San 11 Ai No. 1rlAQ・-Call signal/-jL Meiyocho Z-V wo Dew 3-°-Wound signal 0tsuto Figure 2 Evil Figure 4 Turtle 6 Figure 36 34 3 /
, 35117 Fig.

Claims (1)

[Claims] (1) A groove whose radial cross section is V-shaped is provided on the optical disk, a signal is recorded on the slope of the V-groove, and a signal is generated by irradiating a laser onto the disk with an objective lens and receiving the reflected light. An optical disc reproducing method characterized in that the laser is made incident substantially parallel to the optical axis of the objective lens, and the entire portion of the reflected light that passes through the objective lens is received.