JPH05325194A - Optical disk apparatus - Google Patents

Abstract

PURPOSE:To provide an optical disk apparatus wherein a high-density signal can be reproduced. CONSTITUTION:A laser beam which is radiated from a laser light source 1 is converted into parallel beams by means of a collimation lens 2; the beams are transmitted through a beam splitter 3. Light near the optical axis is shielded by means of a light-shielding body 4. The light is condensed on an optical-disk signal face 7 which is situated on the rear of an optical-disk substrate 6. Its reflected light is condensed by means of an object lens 5; after that, the light near the optical axis is shielded again by means of the light-shielding body 4; it is reflected by the beam splitter 3; it reaches a beam splitter 8. Light reflected by the beam splitter 8 is photodetected by means of a control signal detector 10 via a control-signal-detection optical system 9; a focusing signal and a tracking signal with reference to the optical-signal face are detected. The light which has been transmitted through the beam splitter 8 is condensed; its transmitted light is photodetected by means of a detector 13; a reproduction signal is obtained. The signal is amplified by means of an amplifier; it is converted into a detection signal on the basis of a first-order differentiation signal by means of a signal processing circuit 15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk device for reproducing a signal formed on an optical disk.

[0002]

2. Description of the Related Art In recent years, there has been a strong demand for higher density optical disc signals, and various approaches have been taken. The diameter of the light spot focused on the optical disk surface is generally expressed by the following equation.

D = 1.21λ / NA λ is the wavelength of the light source, and NA is the numerical aperture of the objective lens. Therefore, high density of the optical disc signal can be achieved by shortening the wavelength of the semiconductor laser light source and increasing the NA of the objective lens.

However, at present, the wavelength is shortened by GaAs system II.
It is possible for the IV group to reach 600 nm, but it is not noticeable thereafter. High NA also has problems such as difficulty in processing the objective lens and weakness in errors such as disc tilt and defocus.

On the other hand, there is a proposal to increase the density by adopting an annular opening as a method of increasing the NA and compensating for some of the drawbacks.

An example of a conventional optical disk device having the above-mentioned annular opening will be described below with reference to the drawings. For example, Figure 4 shows Nikkei Electronics (1991, No52
8 is a configuration diagram of the optical disc device described in p. In FIG. 4, 1 is a semiconductor laser light source, 2 is a collimator lens, 3 and 8 are beam splitters, 4 is a light shield, 5 is an objective lens, 6 is an optical disk substrate, 7 is an optical disk signal surface, and 9 is a control signal detection optical system. 10 is a control signal detector,
Reference numeral 11 is a converging lens, 12 is a pinhole, 13 is a reproduction signal detector, 14 is a signal amplifier, and 23 is a signal processing circuit. There is also a method in which the light shield 4 is placed between the collimator lens 2 and the beam splitter 3, but this is not taken up here.

In FIG. 4, the laser light emitted from the laser light source 1 is converted into a parallel beam by the collimator lens 2, transmitted through the beam splitter 3, the light in the vicinity of the optical axis is shielded by the light shield 4, and the optical disk is formed by the objective lens 5. The light is focused on the optical disk signal surface 7 on the back surface of the substrate 6. After the reflected light is condensed by the objective lens 5, the light in the vicinity of the optical axis is shielded again by the light shield 4, and reflected by the beam splitter 3 to reach the beam splitter 8. The light reflected by the beam splitter 8 passes through the control signal detection optical system 9 and is received by the control signal detector 10, and the focus signal and the tracking signal for the signal surface of the optical disc are detected. The light transmitted through the beam splitter 8 is condensed on the pinhole 12 by the converging lens 11, and the transmitted light is received by the detector 13 to obtain a reproduction signal. The reproduction signal is amplified by the amplifier 14 and converted into a detection signal by the signal processing circuit 23.

FIG. 5 shows the relationship between the signal mark on the signal surface of the optical disc, the reproduction signal and the detection signal.
Playback signal 1 due to the presence of 6a, 16b, 16c, 16d
7 changes. This reproduction signal 17 is detected at a certain detection level 17
Compared with R, 0 or 1 is determined depending on whether or not R is exceeded, and a detection signal 22 is obtained.

FIG. 6A is an explanatory view showing the effect of inserting the light shield 4. The light distribution immediately after exiting the aperture surface of the objective lens is on the annular zone by inserting the light shield 4, and the light intensity distribution on the focal plane is 24b (annular aperture NA = 0.45 to 0.70).
Calculated as). For reference, circular opening (N
The light intensity distribution on the focal plane (signal surface) according to A0.54) is 24
As shown in a, since the opening areas are unified, the Strehl intensities also match when the light amounts of the two are equal. The distribution 24b has a smaller main lobe diameter than the distribution 24a, but has a drawback that the side lobes rise. Therefore, the influence of the presence of the signal mark at the side lobe position is large, and crosstalk and intersymbol interference appear strongly in the reproduced signal.

FIG. 6B is an explanatory view showing the effect of inserting the pinhole 12. 24c is a light intensity distribution on the pinhole 12 and is similar to the light intensity distribution 24b on the signal surface. Therefore, if the pinhole 12 is made to transmit only the main lobe of the focused light 24c thereabove, even if the signal mark is at the side lobe position of the focused light 24b on the signal surface, the influence should not appear. Is. Therefore, it is considered that crosstalk and intersymbol interference are small and high-density signals can be reproduced.

In order to judge whether or not it is possible to reproduce a high density signal, let us look at how the signal mark pattern shown in FIG. 7 is reproduced. In FIG. 7, the laser spots 18 have a mark length of 0.4 with the intervals of 0.9 μm.
Scanning is performed on a pit row of 5 μm (track pitch 0.8 μm). FIG. 6A shows the case where the pit pattern of the adjacent track is synchronized with that of the scanning track,
(B) is the reverse pattern.

FIG. 8 is based on the principle of FIG.
FIG. 3 is a diagram in which the observed waveform (eye pattern) of the reproduced signal by the above-mentioned conventional optical disk device is drawn by theoretical calculation for the two types of patterns, the light source wavelength is 780 nm, and the annular aperture NA is 0.45 to 0. It was set to 70. The same figure (a)
Is a defocus of 1.14 μm (Strehl intensity of beam spot is 20% down)
The vertical axis is the signal amplitude, which is standardized by the amount of light detected when the signal surface is a mirror surface, and the horizontal axis is the time axis. X indicated by symbol a in the figure
The rhombus-shaped area surrounding the mark is the eye, and 1 or 0 is judged depending on whether the signal level is high or low for the x mark, so the eye is in the amplitude direction (vertical direction), the time axis direction (horizontal direction).
Both are preferably open.

[0013]

The conventional optical disk device as described above has the following problems.

First, even if the pinhole 12 is inserted,
That is, the effect of reducing crosstalk and intersymbol interference cannot be obtained. As a result of the calculation, it is confirmed that there is almost no change in FIG. 8 regardless of the presence or absence of the pinhole 12 (the size of the pinhole diameter is the presence or absence) (FIG. 8 shows the case without the pinhole). ..

Second, as can be seen from FIG. 8A, a large jitter d is caused by the difference in the pit pattern of the adjacent tracks.
Occurs. Although the signal pattern of a single period is shown in FIG. 8A, the jitter further increases when a random signal pattern is included. Further, as can be seen from FIG. 8B, the eye is largely shifted upward by adding defocus, and the jitter d is further increased. This tendency is common not only to defocus but also to other error factors. The generation of this large jitter makes it difficult to reproduce a high-density signal by a conventional optical disk device.

In view of the above problems, it is an object of the present invention to provide an optical disk device capable of reproducing high density signals.

[0017]

In order to solve the above problems, an optical disk device of the present invention is provided with a laser light source, a laser light source, and laser light from the laser light source on an optical disk signal surface on which signal marks are formed. The light collecting means includes a light collecting means for converging the light on the signal surface, a detecting means for reflecting the signal surface and detecting the return light passing through the light collecting means, and a differentiating circuit for differentiating the reproduction signal by the detecting means to obtain a first-order differential signal. The distribution of the converged light by the means is on the annular zone immediately after the exit from the aperture surface, and the center position of the signal mark having a uniform size is detected by the cross point between the first-order differential signal and the detection level.

[0018]

According to the present invention, since the main lobe diameter of the focused light on the signal surface can be reduced by the above-described structure, the extreme point of the reproduced signal can be made to correspond well to the center position of the signal mark, and the first-order differential signal. The center position of the signal mark can be accurately detected by detecting the cross point between the signal level and the detection level.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical disk device according to an embodiment of the present invention will be described below with reference to the drawings. The members having the same functions as those in the conventional example are designated by the same reference numerals, and detailed description thereof will be omitted.

FIG. 1 shows the configuration of an optical disk device according to an embodiment of the present invention, which is exactly the same as the conventional example except for a signal processing circuit 15. In FIG. 1, laser light emitted from a laser light source 1 is converted into a parallel beam by a collimator lens 2 and transmitted through a beam splitter 3,
The light in the vicinity of the optical axis is blocked by the light shield 4, and the objective lens 5
Thus, the light is focused on the optical disk signal surface 7 on the back surface of the optical disk substrate 6. After the reflected light is condensed by the objective lens 5, the light in the vicinity of the optical axis is shielded again by the light shield 4, and reflected by the beam splitter 3 to reach the beam splitter 8. The light reflected by the beam splitter 8 passes through the control signal detection optical system 9 and is received by the control signal detector 10, and the focus signal and the tracking signal for the signal surface of the optical disc are detected. Also, the beam splitter 8
The light passing through is condensed on the pinhole 12 by the converging lens 11, and the transmitted light is received by the detector 13 to obtain a reproduction signal. The reproduction signal is amplified by the amplifier 14 and converted into a detection signal based on the first-order differential signal by the signal processing circuit 15.

FIG. 2 shows the relationship between the signal mark on the signal surface of the optical disc, the reproduction signal and the detection signal. When the beam spot 18 scans the signal mark 16 having a single length, the reproduction signal 17 changes ( (The scale is on the left). The signal 19 is a first-order differential waveform of the reproduction signal 17 (the scale is on the right side). A gate signal 20 is obtained from the reproduction signal 17 and the detection level 17R, and the first-order differential signal 19 and the detection level 19 are obtained in the gate.
A cross point with R is detected, and a pulse signal 21 having the cross point as a starting point is obtained. The detection signal 22 is obtained as a signal which is inverted at the starting point of the pulse signal 21.

The detection method itself is PPM (Pit P
osition Modulation) signal detection method,
The signal mark 16 is a signal mark having a uniform length. PP
M is PWM (Pit Width Mod) such as EFM shown in the conventional example.
It is said that the influence of pit size error on jitter is less than that of the modulation method, and it is effective for an optical disc system for recording and reproduction.

FIG. 3 shows an observed waveform (eye pattern) of the first-order differential signal with respect to the two types of patterns shown in FIG. 7 in order to determine whether or not a high density signal can be reproduced by the optical disk device in this embodiment. ) Is drawn by theoretical calculation, and the wavelength of the light source is 780 nm and the NA of the annular aperture is 0.45 to 0.70. In the figure, (a) shows the case where there is no aberration in the optical system, and (b) shows the case where a defocus of 1.14 μm (a defocus at which the Strehl intensity of the beam spot is reduced by 20%) is added. The amplitude is standardized by the amount of light detected when the signal surface is a mirror surface, and the horizontal axis is the time axis. The diamond-shaped area surrounding the x mark indicated by the symbol a in the figure is the eye.

As in the case of FIG. 8, it is preferable that the eye is open in both the amplitude direction (vertical direction) and the time axis direction (horizontal direction), but FIG. 3 (a) is clearer than FIG. 8 (a). The jitter d is small, and this tendency is the same even for a random signal pattern. Further, as can be seen from FIG. 3B, the eye does not shift up and down even if defocus is applied, and the jitter d does not increase. This tendency is common not only to defocus but also to other error factors.

This can be explained as follows. Figure 6 (a)
As shown in, the annular opening has a large side lobe and a small main lobe diameter. The large side lobe appears as an effect that a DC component is added to the reproduced signal (that is, it fluctuates overall) due to crosstalk or intersymbol interference.
Error factors such as defocus also appear as an effect of adding a DC component to the reproduction signal. Therefore, the DC component can be removed by differentiating the reproduction signal, and the feature of having a small main lobe diameter can be utilized.

On the other hand, the fact that the main lobe diameter is small appears as an effect that the extreme point of the reproduced signal corresponds well to the center position of the signal mark. Since the extreme point of the reproduction signal appears as a cross point between the first-order differential signal and the detection level, this cross point corresponds well to the center position of the signal mark. Therefore, the eye pattern based on the first-order differential signal has a small jitter even if a signal mark exists at the side lobe position, and the jitter does not increase even if an error factor is added, so that a high density signal can be reproduced.

In the above embodiment, the presence or absence of the pinhole 12 (the size of the pinhole diameter in the case of existence) is shown in FIG.
It has been confirmed that there is almost no change (Fig. 3 shows the case without a pinhole), and the effect is the same even if the pinhole 12 is omitted (the effect of the pinhole is that the signal surface is defocused). However, there is little deterioration in signal detection characteristics, etc.).

Although the light shield 4 is placed between the beam splitter 3 and the objective lens 5 in the above embodiment, it may be placed between the collimator lens 2 and the beam splitter 3. In this case, the light is condensed on the signal surface of the optical disk by the annular aperture, but the reflected light is detected by the circular aperture, which shows the signal detection characteristics different from those of the above-mentioned embodiment, but the differential detection and the annular aperture are used. The principle that a high density signal can be reproduced by the combination of is not changed.

Further, in the above embodiment, the optical system using the bulk optical element such as the lens and the prism has been described, but the condensing element based on the other principle such as the grating lens and the focusing grating coupler may be used. If the distribution of the converged light by the condensing element is on the annular zone immediately after exiting the aperture, it is possible to reproduce a high density signal by combining this with differential detection.

[0030]

As described above, according to the optical disk device of the present invention, a stable eye pattern with small jitter can be obtained even when the signal density of the optical disk is high, and a large increase in signal density can be achieved.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical disk device according to an embodiment of the present invention.

FIG. 2A is an explanatory diagram of a signal mark on an optical disk signal surface in the same embodiment, and FIG. 2B is a relationship diagram between a reproduction signal and a detection signal in the same embodiment.

FIG. 3A is an eye pattern diagram of the first-order differential signal in the case where the optical system has no aberration in the configuration of the embodiment, and FIG. 3B is a diagram in which a defocus of 1.14 μm is added in the configuration of the embodiment. Eye pattern diagram

FIG. 4 is a block diagram of an optical disk device in a conventional example.

FIG. 5 is a relationship diagram of a signal mark on a signal surface of an optical disc, a reproduction signal, and a detection signal in a conventional example.

FIG. 6A is an explanatory view showing an effect of inserting a light shield, and FIG. 6B is an explanatory view showing an effect of inserting a pinhole.

FIG. 7A is an explanatory diagram showing the arrangement of signal marks, and FIG. 7B is an explanatory diagram showing the arrangement of signal marks.

FIG. 8 is an eye pattern diagram of a reproduction signal in a conventional optical disc device.

[Explanation of symbols]

 1 Semiconductor Laser Light Source 2 Collimator Lens 3, 8 Beam Splitter 4 Light Shield 5 Objective Lens 6 Optical Disc Substrate 7 Optical Disc Signal Surface 9 Control Signal Detection Optical System 10 Control Signal Detector 11 Converging Lens 12 Pinhole 13 Playback Signal Detector 14 Signal Amplifier 15 Signal processing circuit

Claims (2)

[Claims] 1. A laser light source, a condensing means for converging laser light from the laser light source onto an optical disc signal surface on which a signal mark is formed, and return light reflected by the signal surface and passing through the condensing means. And a differentiation circuit for differentiating the reproduction signal by the detection means to obtain a first-order differential signal, and the distribution of the converged light by the condensing means is on the annular zone immediately after the exit from the aperture surface, and An optical disk device, wherein a center position of a signal mark is detected by a cross point between the first-order differential signal and a detection level. 2. The optical disk device according to claim 1, wherein the signal marks have a uniform size.