JPH0492214A - Method for reproduction of optical disk - Google Patents

Abstract

PURPOSE:To decrease the aberration of the spot on a disk and to reproduce high-quality signals of small strokes by averting the irradiation of the photodetecting part of a photodetector with a part of reflected light. CONSTITUTION:The laser beam reflected from the disk is separated from an incident optical path by a half mirror 6. While the reflected beam shape is changed by the astigmatism of a cylindrical lens 9, the three beams are separated and the photodetector 10 is disposed in the position where the beam shapes thereof are circular. The three reflected beams are increased in the central spacing of the three reflected light beams on the photodetector to prevent the incidence of a part of the reflected light from the slopes of V-grooves on the detecting part. Not the whole but over a half of the reflected light transmitting an objective lens is received and the optimum region is eventually reproduced. The high-quality reproduced signals of the small strokes are thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for reproducing an optical disc using the groove method.

In response to the increasing density of conventional optical discs, for example,
Publication No. 58144 Japanese Patent Application Publication No. 57-105828
Although the V rowing force type has been proposed in Japanese Patent Application Laid-Open No. 58-102339, etc., Figure 3 shows a perspective view of a replica cross section of a conventional V-groove disk.
is a V-groove, and even if the signal pit 3 is formed on the slope thereof, the laser for signal reproduction is irradiated from the transparent substrate 1 side, and is reflected to the transparent substrate 1 side by a reflection film (not shown) formed on the surface of the substrate 1. reflected.

Next, conventional signal regeneration will be briefly explained using FIG. Even though the light is focused on the disk, the reflected light from the disk passes through the objective lens 8 and the collimating lens 7 again, is reflected by the half mirror 6, is given astigmatism by the cylindrical lens 9, and is irradiated to the photodetector 10. Based on the signal obtained from the photodetector 10 by the irradiation of light, the position of the laser spot on the disk is controlled and the recorded signal is reproduced.

In reality, the semiconductor laser beam is split into three by diffraction in the diffraction grating 5, and three spots are formed on the disk as shown in Figure 5. In order to do this, even if only one of these three beams is shown, it is narrowed down at the peak or valley of the IH1V groove, which is the middle spot among the three spots on the disk shown in FIG. 5, to perform focus control and tracking control.

Spots 12 and 13 of the other first-order diffracted light and the first-order diffracted light are focused on the adjacent slopes of the V-groove, and by receiving these reflected lights, the signal on the slope is reproduced.

In this way, the signals on two slopes of the seven sides can be reproduced simultaneously and independently, so the transfer rate can be doubled.

The optical system shown in Fig. 4 is not only used for reproducing V-groove discs, but also for playing C-groove discs.
It is also used to play back conventional flat disks such as D and video disks, and the playback method will be explained below. FIG. 7 shows the configuration of the photodetecting section of a photodetector in the case of a conventional flat disk and three reflected beams from the disk. Focus control of the spot position and signal reproduction are performed using the reflected beam lla. Focus control is performed using a signal obtained by subtracting the sum signal of light receiving parts 15 and 17 from the sum signal of light receiving parts 14 and 16.
The signal is reproduced by the sum of 5, 16, and 17. Also, the light receiving section 18
Tracking can be controlled by comparing the amount of light received by Narrow the gap between the peaks and reduce the track pitch (distance between the centers of adjacent slopes) l, % At that time, L It is necessary to reduce the leakage signal (crosstalk) from the V-groove slope being regenerated and the slope adjacent to it. Although several methods for playing V-groove discs have been proposed for this purpose, three representative methods are described below.

First, even though JP-A-56-58144 proposes a method of irradiating a laser beam perpendicularly to the inclined surface of the V-groove and receiving all of the reflected light, in this case 0
．． For an objective lens with a high NA of 5 or more, the incident angle of the laser beam must be within 1 or 2 degrees; if it is greater than that, the laser beam cannot be narrowed down sufficiently due to the effects of aberrations, and the quality of the reproduced signal will deteriorate. The angle of inclination of the slope of the groove must be at least 5 degrees, and the practical use of this method is extremely limited. The laser spot can be focused on the V-groove by making it approximately parallel to the optical axis of the objective lens to minimize aberrations. Here, approximately parallel means that it is within 1 degree to the optical axis of the objective lens, and the slope of the kV groove is Since it is tilted, 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 V-groove slope to be regenerated, most of the light is irradiated onto the regenerated slope.
Most of the reflected light also returns to the inner half of the objective lens. In addition, the reflected light from the slope adjacent to the reproduction slope is also biased toward the inner half on the opposite side of the objective lens (only the periphery of the laser spot is irradiated on the adjacent slope, so the amount of reflected light is small (°C). After the reflected light from the laser spot on the slope of the V-groove 2 in FIG. 8(B) passes through the objective lens 8, the distribution becomes like the curve shown in FIG. 8(A).This reflected light distribution is This is an example of a case where a laser spot is irradiated on a slope where there is no signal pit, and the reflected light is divided into a large intensity reflected light portion and a small intensity reflected light portion.This larger reflected light portion is reflected from the reproduction slope. corresponds to reflected light
It extends over half of the objective lens.

In addition, when there is a signal bit on a slope, each peak in the distribution in Figure 8 decreases, and the reflected light distribution scatters around the area by the amount of the decrease (not shown in Figure 8). Reflected light distribution D (Fig. 4) The beam diameter is reduced by the lens system of the reflected optical path in Figure 4. Even when the beam is irradiated onto the photodetector 10 while the shape of the distribution remains similar.
In Publication No. 7-105828, it was analytically shown that the crosstalk of the reproduced signal decreases due to the interference effect of reflected light slightly outside the center of the optical axis (range of arrow X in Figure 8). A method of regenerating the reflected light has been proposed.

The above is the outline shown in JP-A No. 57-105828, and now on to the third method: The method proposed in JP-A-58-102339: Injecting a laser beam approximately parallel to the optical axis of an objective lens; Much of the reflected light is also concentrated on half of the lens. In that proposal, the reflected light that passes through the objective lens is received in a semicircular area, which can reduce crosstalk to some extent, but it is not optimal. It is based on the assumption of complete coherence.

It is true that in recording by phase change, that is, a recording method in which signal bits are not formed on the slope of the V-groove (since there are no irregularities on the slope, the assumption of coherence holds true to some extent).

The same is true when forming a magneto-optical material on the surface of the L-V groove and changing the magnetization direction by recording.In the case of a read-only disk in which a signal focus is formed on the surface of the L-V groove (Yo). It is difficult to form the signal bit in an ideal shape, and the bottom surface and the surrounding area may be slightly disordered, so the reflected light (although not completely coherent (℃)) There are many components that are disturbed by scattering, and the coherence is partially broken, resulting in a decrease in coherence.Scattered light has a specific direction (although the crosstalk components due to these scattering are scattered throughout the reflected light, and are rather uniform). Contains close to 0.

Therefore, the tendency for cross-stiffness to be the smallest in a part of the reflected light distribution is reduced; on the contrary, when only a part of the reflected light distribution is received, the reproduced signal is no longer at its maximum, which is disadvantageous for obtaining high signal quality. .

Therefore, as a practical matter, it is necessary to provide an optimal reproduction method for a V-groove disk in which signal bits are formed on the slope.

Means for solving the problem Means for solving the above problem (Friend) Even if the laser beam is made to be incident approximately parallel to the optical axis of the objective lens, the reflected light from the V-groove slope will pass through the objective lens. According to the above method, the incident direction of the laser beam is approximately parallel to the optical axis of the objective lens, The aberration of the laser spot focused on the disk surface can be reduced.

■Since the groove slope is inclined, the direction of the reflected light is deviated from the optical axis of the objective lens. The distribution of reflected light from the disk surface can be divided into two parts: a large reflected light portion from the playback slope, and a small reflected light portion from the playback slope and the adjacent slope.A large reflected light portion corresponding to the reflected light from the playback slope spreads to more than half of the objective lens4.The optical system is configured so that a part of the reflected light from the slope does not enter the detection part of the photodetector, and the entire part of the reflected light that passes through the objective lens is However, by receiving light beyond the inner part of the objective lens, most of the part corresponding to the reflected light from the reproduction slope is received, and the reproduction signal can be maximized. At that time, the crosstalk component tends to be uniformly included in the overall reflected light, so the amount of crosstalk decreases relative to the size of the reproduced signal, and the quality of the reproduced signal can be improved. EXAMPLE Hereinafter, an example will be described with reference to the drawings.

We played back a V-groove disc with signal bits formed on the slope, measured the signal quality such as C/N and crosstalk, and examined the optimal receiving area for reflected light.The results are explained below. The circle shown in FIG. 1 (A) shows the cross section of the reflected light beam from the V-groove slope. ■ Reproduction of a groove When reproducing a signal from a slope, the signal from an adjacent slope is also mixed in. When reproducing a signal from a slope rising to the right as shown in Figure 8, within the left half of the circle in Figure 8. Most of the reflected light is concentrated in this area. Therefore, the reflected light () on the left side of the vertical line in Figure 1 (A)
The position of the vertical line is represented by the intersection with the ξ axis passing through the center O of the reflected light, and the reproduction signal S of the reproduction slope and the reproduction signal C of the adjacent slope are measured for the position of the vertical line. The values are shown in Figure 1 (B).The difference between the reproduced signals of the reproduced slope and the adjacent slope becomes crosstalk.

Based on analytical simulations, in the conventional example, the optimal light-receiving area was a portion of the reflected light that was slightly smaller than a semicircle, as shown by the arrow X in Figure 8.In other words, in Figure 1, the straight line was a point. If it is on the left side of O, the reproduction characteristics are good.4
This is the conventional thinking, but Figure 1 shows that if a portion of the reflected light that is slightly larger than a semicircle is received and the signal is reproduced, the amplitude of the reproduced signal will be thicker and the crosstalk will be smaller. . This area corresponds to receiving most of the reflected light portion of the two parts shown in FIG. 8, which have a larger reflected light distribution.

Based on the analysis, the above differences from the conventional example will be briefly explained.

Conventional analysis requires that laser light is always perfectly coherent (
This assumption is true to some extent if there are no irregularities on the V-groove slope.

However, in the case of playback-only discs that form signal pits on the LV groove surface, it is difficult to form the signal pits on the disc in an ideal shape, and the bottom and surrounding areas may be slightly disordered. , there are many components in the reflected light that are disturbed by scattering. Therefore, this reflected light cannot be said to be completely coherent, and the coherence is partially broken and the coherence is reduced. Even though the component does not have a specific direction of reflection, and rather is almost uniformly included in the overall reflected light, the tendency for crosstalk to be lowest in a part of the reflected light is reduced. In the case of receiving only There is a large part corresponding to the reflected light and a small part corresponding to the reflected light from the slope adjacent to the reproduction slope, and by receiving most of the larger reflected light part, the reproduction signal can be maximized. At that time, crosstalk components tend to be uniformly included in the overall reflected light, so the amount of crosstalk decreases relative to the size of the reproduced signal, and the quality of the reproduced signal can be improved. Not all of the power of the reflected light passing through the objective lens (
If more light is received than in the inner half of the objective lens, a high quality signal can be obtained.

The reproduction optical system in the above embodiment will be explained in detail. The configuration diagram of the optical system of the above embodiment is the same as that shown in FIG.

In Fig. 4, the wavelength of the semiconductor laser 4 is λ, the distance between its emission point and the diffraction grating 5 is d, the focal length of the collimating lens 7 is fl, the focal length of the objective lens 8 is f2, and the pitch of the diffraction grating is p. After passing through the I'L diffraction grating, the 0th -1st order laser beam is A = (λ/p) ・d...(1)
B = (f 2 / f 1) ・A ... (2)
It is squeezed into spots at intervals of .

The laser beam reflected from the disk is separated from the incident optical path by the half mirror 6, but the shape of the reflected beam changes due to astigmatism caused by the cylindrical lens 9.The three beams are separated and then moved to a position where their beam shapes become circular. Since the diameter of the reflected light beam on the photodetector is small, the distance between the centers of the 43 reflected beams is approximately the distance between the spots on the disk. fl/f2) times, which is approximately the value of A in equation (1).

The optical system is configured so that the distance between the centers of the reflected light on the photodetector is larger than that of the conventional disk as shown in Fig. 2, and the focus control ( Friend: As in the conventional case, the reflected beam 11a
From the sum signal of the light receiving parts 14 and 16, the light receiving parts 15 and 1 are
This is done using the signal obtained by subtracting the sum signal of 7.

In the astigmatic focus control, the images of the three reflected beams are rotated by 90 degrees on the photodetector surface, so the image of the signal pit in the track direction on the photodetector is the same as that of the light receiving sections 14 and 17 or 1.
It is parallel to the line dividing i15 and 16. Therefore, tracking control of the groove disk can be performed using a signal obtained by subtracting the sum signal of light receiving sections 16 and 17 from the sum signal of light receiving sections 14 and 15.

In an optical system configured in this way, as mentioned above, in astigmatic focus control, the images of the three reflected beams are rotated 90 degrees on the photodetector surface, so the center of the three reflected beams on the photodetector is By making the spacing larger than that of the conventional disk shown in Fig. 7, it is possible to prevent part of the reflected light (12a, 13a) from the groove slopes from entering the detection section, but not all of the reflected light that passes through the objective lens. By receiving more than half of the light and reproducing the optimal region shown in Figure 1, we can obtain a high quality reproduced signal with low crosstalk. The wavelength λ of the second semiconductor laser 4, the distance d between its light emitting point and the diffraction grating 5
, the pitch p of the diffraction grating is approximately (λ/p) ・
Therefore, the distance d between the light emitting point of the semiconductor laser 4 and the diffraction grating 5 can be increased simply by making the distance d larger than that of the conventional disk optical system. However, even with the configuration shown in FIG. 2, the amount of light received by the light receiving sections 18 and 19 is compared, and it is found that even though the conventional disk can be tracked, the effect is more than the effect of the invention. Since the beam is incident approximately parallel to the optical axis of the disk, it is possible to reduce the aberration of the spot on the disk and to reproduce a high quality signal with low crosstalk.

[Brief explanation of drawings]

FIG. 1 shows the reproduction area and signal level of the reflected light from the V-groove disk in an embodiment of the present invention. FIG. 2 shows the configuration of a photodetector for the V-groove disk in the embodiment and the light reflected thereon. Fig. 3 shows the perspective view of the V-groove disc of the embodiment. Fig. 4 shows the configuration of the reproduction optical system of the embodiment.
The figure shows the arrangement of the laser spot on the V-groove disk of the embodiment.1 Figure 6 shows the distribution of the laser spot on the conventional flat disk.Figure 7 shows the configuration of the conventional photodetector and the irradiation state of the reflected light on it. FIG. 8 is an explanatory diagram of a conventional reproduction method using the distribution of reflected light from the V-groove slope. 1... Transparent substrate 2... VIL 3... Pit 4... Semiconductor laser 5... Diffraction grating 8
...Objective lens
16.77.1B, /9...Year section diagram diagram diagram diagram diagram diagram ○ ○ ○ ○ ○ ○ ○ ○

Claims (2)

[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, a laser is irradiated onto the disk with an objective lens, and the reflected light is detected by a photodetector. 1. An optical disc reproducing method characterized in that a part of the reflected light is not irradiated onto a photodetecting section of the photodetector. (2) A diffraction grating is inserted between the laser light source and the objective lens to form multiple spots on the disk, and the photodetector consists of a photodetecting section that receives light from each of the multiple spots. 2. The optical disk reproducing method according to claim 1, further comprising setting a distance between the diffraction grating and the laser light source so that a part of the reflected light from the laser beam is not irradiated onto the photodetecting section.