JPH08221795A - Optical disk device - Google Patents

Abstract

PURPOSE: To substantially reduce the side robe of a light spot due to a super resolution effect by inserting a phase filter into an incident light path to control light to be circular and detecting reflected light flux from a recording medium by dividing the reflected light in a specified manner. CONSTITUTION: The optical disk 8 is irradiated with a laser beam from a semiconductor laser 1, a reflective beam is detected by light detectors 13a and 13b and differential-processed and a reproducing signal is formed. A phase filter 24 having a super resolution effect for controlling the transparency of the incident beam in a circular center part is inserted to the incident light path, the transparency of the center circular areas 131 of the detectors 13a and 13b are set lower than that of edge part areas 132 and side robes often produced in the area 13a reducing resolution due to crosstalk are substantially reduced. Thus, with resolution substantially increased, the recording information of a highly dense recording disk is reproduced keeping good S/N.

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 optically reproducing information from an optical recording medium, and more particularly to a technique for improving resolution characteristics of an optical system.

[0002]

2. Description of the Related Art The prior art is described, for example, in the Proceedings of the Symposium on Optics Union Hamamatsu '94 (1994), page 299. Here, it is described that a light blocking plate or a phase plate is inserted into an incident light flux of an objective lens used in an optical head to use a so-called super-resolution effect to improve the recording density of an optical disc.

[0003]

In the above-mentioned prior art, the size of the main maximum of the light spot can be reduced.
It cannot prevent the increase of the submaximum. The influence of the sub-maximum causes intersymbol interference in the operation direction of the optical disc and adjacent track crosstalk in the radial direction, and deteriorates the reproduction signal quality.

With respect to such a problem, an object of the present invention is to provide an optical disk device for suppressing a sub-maximum caused by a super-resolution effect of a light shielding plate or a phase plate and reproducing a high density optical disk. is there.

[0005]

In order to solve the above problems, a semiconductor laser, an optical system for condensing light from the semiconductor laser on a recording medium, a photodetector for detecting reflected light from the recording medium, and And a means for reproducing a signal recorded on an optical recording medium from an electric signal from the photodetector, and a light shielding plate or a phase filter is inserted into the light incident on the photodetector in a circular shape to reflect it from the recording medium. An optical head is used which is characterized in that the luminous flux is divided into a central portion and a peripheral portion to be detected.

Further, the effective detection sensitivity of the central portion is made lower than the effective detection sensitivity of the peripheral portion.

More specifically, the detection sensitivity of the central portion is set to 0.

A differential signal between the output of the central portion and the output of the peripheral portion is output.

Further, in order to change the detection sensitivity for the photodetector between the central portion and the peripheral portion, a filter having a different transmittance in the central portion is inserted into the reflected light beam from the optical recording medium.

Alternatively, the light-receiving portion of the photodetector is perforated.

Further, in the case of a magneto-optical disk, these means are applied to each of the two light beams split by the polarization prism for performing differential detection.

[0012]

[Function] A phase filter or a light shielding plate is inserted into the light flux from the semiconductor laser, the light is condensed on the recording medium, and the detection sensitivity to the reproduced signal is different between the central portion and the peripheral portion where the reflected light is divided into circles. In particular, detection is performed so that the detection sensitivity of the central portion is lower than the detection sensitivity of the peripheral portion. At this time, it is possible to reduce side lobes, which is a feature of the super-resolution effect and is a cause of crosstalk.
This principle will be briefly described below.

The side lobe of the light spot due to the super-resolution effect is wider than that of the main spot. This means that the side lobe mainly occupies a low resolution component in the spatial frequency component of the light spot. Shows. The low-resolution component mainly occupies a component near the center on the pupil plane. It is clear from the fact that the smaller the aperture, the wider the focused spot. Therefore, if the weight of the peripheral portion of the returned light of the super-resolution spot near the center is increased, the high-resolution component can be made relatively large, that is, the side lobe of the light spot can be effectively reduced. it can.

In order to increase the weight of the peripheral portion, it is more simple to set the detection sensitivity of the central portion to zero. Further, a filter having a changed central portion transmittance may be inserted into the reflected light beam from the optical recording medium. When the sensitivity is set to 0, it can be realized by forming the light receiving portion of the photodetector into a holed shape. In the case of a magneto-optical disk, these means are applied to each of the two light beams branched by the polarization prism for performing differential detection.

[0015]

FIG. 1 shows an embodiment in which the present invention is applied to a magneto-optical disk device. The light from the semiconductor laser 1 is converted into parallel light by the collimator lens 2, and the beam shaping prism 3,
At 4, the elliptical beam is converted into a circular beam. These prisms may be omitted if the emission intensity of the semiconductor laser can be made sufficiently high. The light then passes through a filter 24 to form an optical super-resolution spot. Examples of this filter include one that shields light from the central portion and one that adds a transparent film to the central portion to provide a phase difference between the central portion and the peripheral portion. Also, a transparent film may be added in a ring shape. The light emitted from the filter passes through the beam splitter 5 and the rising prism 6
And passes through the actuator-integrated objective lens 7,
It is focused on the optical disk 8. The reflected light passes through these elements again, is reflected by the beam splitter 5, and is separated by the beam splitter 9 into transmitted light and reflected light.

The polarization direction of the reflected light is rotated by 45 ° by the λ / 2 plate 10, passes through the condensing lens 11, and the polarization component which forms ± 45 ° with respect to the incident polarization direction is separated by the polarization beam splitter 12 and is transmitted respectively. Light and reflected light. The light is focused on the detectors 13a and 13b, respectively. The light is condensed by the lens in order to reduce the size of the detector to make the response faster, and for the purpose of the present invention, the detector position needs to be slightly deviated from the focus. In the central part of these detectors, there is a dead region, and only the peripheral part of the incident light is detected. The differential signal between the outputs of the detectors 13a and 13b is output by the differential amplifier 19a to obtain a magneto-optical signal. On the other hand, the light transmitted through the beam splitter 9 passes through a condenser lens 14 and is separated into transmitted light and reflected light by a beam splitter 15. Here, the reflected light is split and detected by the two-split photodetector, and the differential signal of its output is taken by the differential amplifier 19b to detect the tracking error signal. The transmitted light passes through the cylindrical lens and 4
The difference signal of the diagonal components detected by the split detector is output by the differential amplifier 19c, which is used as the defocus signal. By feeding back the tracking error signal and the defocus signal to the objective lens actuator 7, the light spot on the optical disc 8 is controlled. The above is an example, and these configurations are not necessarily fixed, and other configurations for obtaining basically the same signal may be used. At this time, it is necessary that the central portion of the light incident on the photodetector for detecting the magneto-optical signal is not received or the sensitivity is low.

FIG. 2 shows an enlarged view of the photodetector 13 in FIG. At the center, there is a dead zone 131 with no light detection sensitivity.
This dead zone may be made by attaching a mask after forming a uniform photodetector, or by making the light receiving part of the photodetector with a photomask in advance so that the central part does not detect light. You can also Further, the sensitivity is not always set to 0 and may be set lower than the peripheral portion. In such a case, a split photodetector is used, and the effective detection sensitivity can be changed by changing the gain of the signal amplifier. When it is better not to completely set the sensitivity of the central portion to 0, the intensity distribution of the parallel light flux incident on the optical disk is originally extremely low in the peripheral portion, and if only the peripheral portion is detected at this time, it is not diffracted light. This is because the next light component becomes extremely weak and the waveform distortion of the reproduction signal becomes large.

FIG. 3 shows an embodiment in which the present invention is applied to an apparatus for reproducing only an optical disk which is not a magneto-optical disk. The optical system is almost the same as in FIG. 1, but there is no polarization beam splitter for reproducing a magneto-optical signal, and only one signal photodetector 13a is required.

FIG. 4 is an enlarged view of only the part to which the present invention is applied to FIG. This is a diagram shown for comparison with another embodiment of the present invention shown in FIG. In FIG. 5, the density filter 23 is inserted in front of the condenser lens 11 to implement the present invention. The gray filter has a region 231 having a low transmittance in the central portion and a region 232 having a high transmittance in the peripheral portion. In this case, the photodetector 22 may have uniform sensitivity.
With such a configuration, the present invention can be applied to a conventional device at low cost because it can be realized even by simply inserting the grayscale filter into a conventional device.

6 and 7 show the MTF (Mo
(duration Transfer Function). The horizontal axis shows the spatial frequency of the structure on the optical disk, and the vertical axis shows the passing amount of the spatial frequency component normalized by the DC component. The spatial frequency is standardized by NA / λ, which is a measure of resolution determined by the optical system. Here, NA is the numerical aperture of the optical system, and λ is the wavelength of light. The resolution limit of the optical system is 2
NA / λ. First, the method of calculating the MTF and the calculation conditions will be briefly described below. Assuming a flat pit with a width of 0.41 μm and a length of 3 μm and a reflectance of 0, a light source with a full width at half maximum emission angle of 23 °, λ = 0.78 μm is collimated with NA 0.25 and a light spot focused with NA 0.55. After scanning on it, the response of the domain edge on one side is taken as the step response, and after this waveform is spline-interpolated,
The MTF is calculated by differentiating it into an impulse response and further performing a one-dimensional Fourier transform on this. FIG. 6 shows a case where a super-resolution effect is used in which a normal optical system receives light at the peripheral portion with a diameter ratio of 30% and shields the center of the incident light flux by 70% with a diameter ratio. According to this, although the peripheral light reception is inferior to the super-resolution effect in the high range, it shows a good response characteristic over the entire range. In this case, even if the peripheral light reception and the super-resolution are combined, there is no difference from the super-resolution alone. This is because the super-resolution effect of the light-shielding type has almost no return light to the center, so the peripheral light-receiving effect does not appear at 30% in the peripheral area against 70% light-shielding.

In FIG. 7, when the super-resolution effect obtained by shifting the phase of the central portion 40% by 180 ° is used together with the ordinary optical system, the peripheral light receiving of the peripheral portion 60% is also used. Indicate the case. From this, it can be seen that the low-frequency deterioration due to super-resolution is saved considerably by peripheral light reception. At this time, there is little change in the high range.

8, 9 and 10 are impulse responses obtained in the process of obtaining FIG. Here, the vertical axis shows the differential value of the original step response waveform as it is. FIG. 8 shows a normal case, FIG. 9 shows a case of super-resolution, and FIG. 10 shows a case of using peripheral light reception in combination. From this, it can be seen that the size of the sub-maximum is suppressed considerably without changing the main maximum of the center.

In the above description, all the MTFs are standardized and displayed with the DC component, but in reality, if only one part of the parallel light flux is detected, the detected signal amplitude naturally lowers. If the noise included in the reproduced signal when the present invention is not used is predominantly noise that is irrelevant to the light amount, such as amplifier noise, the S / N ratio may eventually deteriorate when the present invention is applied. There is. Therefore, the present invention needs to be applied within a range in which the light amount is not dropped too much. For example, in the calculation of FIG. 6, the total amount of light was 39% when the peripheral light was received by 30%. In an optical disk device, the S / N ratio deteriorated by 1.5 dB when the amount of light was reduced by this amount. Therefore, in this case, the improvement of the signal amplitude standardized by the total amount of detected light needs to be 1.5 dB or more, that is, about 1.2 times or more. FIG. 6 shows the display standardized by the DC component, but since the DC signal component with respect to the total light amount hardly changed in the normal case (normal) and the peripheral light reception, FIG. 6 was normalized as it is with the total light amount. It can be thought of as the signal amplitude. In a normal optical disk device, the spatial frequency is about 1.0 (NA / λ) as the closest recording frequency. Therefore, when compared in this vicinity, it can be seen that the amplitude is improved by about 1.5 times. Therefore, the present invention can be applied.

[0024]

According to the present invention, the resolution of the optical system can be improved, the cross-talk between adjacent tracks can be reduced, and the high-density recorded optical disc can be reproduced with high accuracy at low cost without complicating the circuit configuration.

[Brief description of drawings]

FIG. 1 shows an embodiment of a magneto-optical disk device.

FIG. 2 is an enlarged view of a photodetector according to the present invention.

FIG. 3 is an example of an optical disk device other than a magneto-optical disk.

FIG. 4 is a photodetector and collection optics according to the present invention.

FIG. 5 is an embodiment using a tone filter.

FIG. 6 shows MTF by peripheral light reception and amplitude super-resolution.

FIG. 7: MT when using both phase super-resolution and peripheral light reception
F.

FIG. 8 is a conventional impulse response.

FIG. 9: Impulse response by phase super-resolution.

FIG. 10 is an impulse response when peripheral light reception is used together with phase super-resolution.

[Explanation of symbols]

1 ... Semiconductor laser, 2 ... Collimating lens, 3, 4
Beam shaping prism, 5 Beam splitter, 6
・ ・ ・ Stand-up mirror, 7 ・ ・ ・ Objective lens integrated with actuator, 8 ・ ・ ・ Optical disk, 9 ・ ・ ・ Beam splitter, 1
0 ... λ / 2 plate, 11 ... Condensing lens, 12 ... Polarizing beam splitter, 13, 13a, 13b .. 14 ... Condensing lens, 15 ... Beam splitter, 16 ... 2-split photodetector, 17 ... Cylindrical lens, 18 ... 4-split photodetector, 19a, b, c ...
Differential amplifier, 20 ... Amplifier, 22 ... Photodetector, 23
・ ・ ・ Gray filter, 231, ・ ・ ・ Low transmittance part, 232
High transmittance part, 24 ... Super resolution filter.

Claims (7)

[Claims] 1. A semiconductor laser, an optical system for condensing light from the semiconductor laser on a recording medium, a photodetector for detecting reflected light from the recording medium, and an optical signal from the photodetector to an optical recording medium. And means for reproducing the recorded signal,
An optical disk device characterized in that a light shielding plate or a phase filter is inserted into a circular shape for light incident on the photodetector, and a reflected light beam from a recording medium is divided into a central portion and a peripheral portion to be detected. 2. The optical disk device according to claim 1, wherein the effective detection sensitivity of the central portion is lower than the effective detection sensitivity of the peripheral portion. 3. The optical disk device according to claim 1, wherein the detection sensitivity of the central portion is zero. 4. The optical disk device according to claim 1, wherein a differential signal between the output of the central portion and the output of the peripheral portion is output. 5. The transmittance of the central portion of the reflected light beam from the optical recording medium is changed in order to change the detection sensitivity of the photodetector between the central portion and the peripheral portion according to any one of claims 1 to 4. An optical disk device having a filter inserted therein. 6. The optical disk device according to claim 3, wherein the light receiving portion of the photodetector is perforated so as to change the detection sensitivity to the photodetector between the central portion and the peripheral portion. 7. An optical recording medium is a magneto-optical disk, and any one of claims 1 to 6 is applied to each of two light beams branched by a polarizing prism for performing differential detection. Characteristic optical disk device.