JPH0520710A - Optical pickup device - Google Patents

Abstract

PURPOSE:To reduce the influence of the double refraction of a disk board on the focusing and tracking error signals. CONSTITUTION:The focusing means 42 and 43 are provided to focus the reflected light sent from an optical recording medium together with a reflecting (transmitting) means 43 which reflects (transmits) partly the reflected light to supply it to a focusing/tracking error detector means 45 and leads the remaining reflected light to a polarizing means 44. Then the means 44 polarizes and separates the remaining reflected light led by the means 43 and supplies the separated light to a reproduced signal detector means 46. The outside part of the reflected light is not made incident on a photodetector and therefore an even distribution is secured for the received light quantity. Then no error is caused to the best focusing position and the track crossing noises are prevented regardless of the degree of the double refraction. Furthermore no offset is caused even though a beam moves on the photodetecting surface of the photodetector. At the same time, the mixture of the tracking errors can be reduced into the focusing error signal.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an optical pickup device, and more particularly to an optical system of an optical pickup device for reading information from an optical recording medium such as a magneto-optical disk using a laser beam.

[0002]

BACKGROUND ART Conventionally, an optical pickup device as shown in FIG. 6 has been known. Such an apparatus includes an optical system 2 for extracting reflected light by irradiating a magneto-optical disk 1 which is an optical recording medium with light from a light source provided therein, and a focus error signal and a tracking error based on a change in the reflected light. And a signal generating means 3 for emitting a signal and a reproduction signal.

The optical system 2 includes a semiconductor laser 11, which is a laser beam source, a collimator lens 12, a P-polarized reflectance Rp.
Is 20% and the S-polarized light reflectance Rs is 100%, and a beam splitter 13 and an objective lens 14. The signal generating means 3 is a polarization beam splitter (hereinafter, referred to as a PBS) that splits the half-wave plate 20, P and S into both polarized lights.
21), a detection lens 22a, a cylindrical lens 23 as an astigmatism generation element, a four-division photodetector 24 having four light-receiving surfaces divided into four by two orthogonal line segments, a detection lens 22b, and one line. It is composed of a two-division photodetector 25 having two light-receiving surfaces which are divided into two according to the distance. Here, in the signal generating means 3, the half-wave plate 20 is arranged with its optical axis tilted at an angle of 22.5 ° with respect to the incident surface of the beam splitter 13, and the P21 by the PBS 21 is used.
The separation of the S and S polarizations is made equal for the corresponding photodetectors 24 and 25. Even if the half-wave plate 20 is not provided in this way, the same effect as that of the half-wave plate 20 can be obtained even if the optical system 2 is rotated at an angle of 45 ° with respect to the incident linearly polarized light.

The operation of such a device is as follows. First, the semiconductor laser 1
The laser beam from the laser beam No. 1 is converted into a parallel laser beam by the collimator lens 12, passes through the beam splitter 13, and is focused by the objective lens 14 toward the magneto-optical disk 1.
The focused laser beam focused on the magneto-optical disk 1 is reflected by the perpendicular magnetization film 1b via the substrate 1a, and the reflected laser beam is converted into a parallel laser beam by the objective lens 14 and the beam splitter 13 of the signal generating means 3 is used. 1/2
It is directed to the PBS 21 via the wave plate 20.

The PBS 21 splits the light from the beam splitter 13 into P and S polarized lights. P transmitted through PBS21
The polarized light is condensed by the detection lens 22a and becomes a focused laser beam, which is passed through the astigmatism generation element 23 and is divided into four photodetectors 2
A spot is formed on the light receiving surface of No. 4. The S-polarized light reflected by the PBS 21 forms a spot on the light receiving surface of the two-divided photodetector 25 via the detection lens 22b.

The shape of the spot in the four-division photodetector 24 becomes a circle when the laser beam is focused on the magneto-optical disk 1, and when the laser beam is out of focus, it is in the direction of four-diagonal diagonal positions due to astigmatism. It becomes an elliptical shape that extends to. Further, the dividing line of the two-division photodetector 25 is optically aligned with the track direction of the disk, and when the laser beam is on the track, the light amounts of the two light receiving surfaces are the same, and the light beams deviate from the track. When there is a difference between the two light receiving surfaces.

The laser beam applied to the magneto-optical disk 1 is linearly polarized due to the characteristics of the semiconductor laser 11, and this linearly polarized light is either left or right depending on the magnetization direction (recording information) of the perpendicular magnetization film 1b. Is rotated in the direction of. Then, this rotation direction appears as a difference in the amount of received light in the four-division photodetector 24 and the two-division photodetector 25, and a magneto-optical reproduction signal is obtained from the difference in the amount of received light.

FIG. 7 is a circuit diagram of the signal generating means 3 and shows four light receiving surfaces 24a to 24a of the four-division photodetector 24.
The respective light receiving outputs by 4d are added for each of the two light receiving surfaces (24a and 24c, 24b and 24d) at two diagonal positions, and the difference between the added outputs is taken out as a focus error signal. Further, the difference between the light receiving outputs of the two light receiving surfaces 25a and 25b of the two-divided photodetector 25 is taken out as a tracking error signal. Further, the difference between the total received light output of the four-divided photodetector 24 and the total received light output of the two-divided photodetector 25 is taken out as a magneto-optical reproduction signal, whereby in-phase noise components are removed and linear polarization The information recorded on the magneto-optical disk 1 in the direction of rotation is differentially detected.

By the way, the transparent substrate 1 for the magneto-optical disk 1
As a, a glass substrate having excellent optical uniformity is suitable, but a plastic substrate is desired from the viewpoint of mass productivity and cost, and a polycarbonate substrate used for a compact disc or the like is influential. However,
Since this polycarbonate has a large birefringence, the following phenomenon occurs.

Since the focused laser beam focused on the disk is narrowed down into a conical shape, it is perpendicular to the substrate 1a at the center of the wavefront of the focused laser beam, but becomes closer to the substrate 1a at the periphery of the wavefront. Incident angle becomes large. Therefore, the light around the wavefront of the reflected laser beam is retarded by the birefringence of the substrate 1a and becomes elliptically polarized light.

Therefore, the photodetector 2 is caused by the elliptically polarized light component.
As shown in FIG. 8, the intensity distribution on the light receiving surfaces of Nos. 4 and 25 becomes non-uniform, and as shown in FIG. 24b, 24d) is the amount of light received by the other diagonal pair (24
A portion Sp that is larger than the received light amount of a, 24c) occurs.
Due to this phenomenon, an offset occurs in the focus error signal from the photodetector, or the track crossing component on the photodetector becomes unbalanced.

Therefore, when recording / reproducing a magneto-optical disk using a substrate having a large birefringence as a substrate, it is best to use a magneto-optical disk having a substrate having almost no birefringence such as glass or PMMA. There is a problem that the focus position shifts and track crossing noise to the focus error signal increases. Also, the light receiving surface 2
In the so-called push-pull method in which the difference between the light receiving outputs of 5a and 25b is used as a tracking error signal, if the orthogonal relationship between the disk recording surface and the laser beam is deviated, the projected position of the reflected light projected on each light receiving surface becomes one. , The received light outputs become unbalanced, and an offset occurs in the tracking error signal. Further, when the objective lens is deviated from the neutral position in the tracking direction, the reflected light is deviated by the amount deviated, so that the respective light receiving outputs are also unbalanced and an offset is generated in the tracking error signal.

Further, the push-pull method is based on the principle of detecting the change in the distribution of the diffracted light component due to the track in the reflected light, but this change in the distribution of the diffracted light component also affects the focus error signal, The tracking error signal sometimes leaked into the focus error signal.

[0014]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and an object of the present invention is to provide an optical pickup device in which the influence of birefringence of a disk substrate on a focus error signal and a tracking error signal is reduced. Is to provide.

[0015]

An optical pickup device according to the present invention comprises an optical system for irradiating an optical recording medium with light from a light source to extract reflected light, and a focus and tracking error signal and a reproduction signal based on a change in the reflected light. An optical pickup device including a focus and tracking error detecting means for emitting a light and a reproduction signal detecting means, the focusing means for focusing the reflected light, and the focus and focusing means for reflecting (or transmitting) a part of the reflected light. A reflection (or transmission) means that supplies the residual of the reflected light to a polarization means and supplies the residual of the reflected light to the tracking error detection means, and the polarization means includes a residual of the reflected light guided by the reflection (or transmission) means. It is characterized in that the polarized light is separated and supplied to the reproduction signal detecting means.

[0016]

In the optical pickup device having such a structure, the focusing light is focused by the focusing means, and a part of the reflected light is reflected (or transmitted) by the reflecting (or transmitting) means. The residual of the reflected light supplied to the error detecting means is guided to the polarizing means, and the residual of the reflected light guided by the reflecting (or transmitting) means is polarized and separated by the polarizing means and supplied to the reproduction signal detecting means. .

[0017]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a block diagram of an optical pickup device configured according to the first embodiment of the present invention. In addition,
The same parts as those in FIG. 6 are designated by the same reference numerals. In the figure, the optical pickup device includes an optical system 32 for irradiating the magneto-optical disk 1 which is an optical recording medium with light from a light source included therein to extract reflected light, and a change in reflected light from the magneto-optical disk 1. Signal generating means 3 for issuing a focus error signal, a tracking error signal and a reproduction signal based on
3 and 3.

The optical system 32 includes a semiconductor laser 35 as a light source for emitting a laser beam and a collimator lens 36.
A prism 37, a beam splitter 38 having a P-polarized reflectance Rp of 20% and an S-polarized reflectance Rs of 100%, and an objective lens 39. On the other hand, the signal generation means 33 includes a half-wave plate 41, a detection lens 42, a transparent parallel plate 43 as an astigmatism imparting means, and a polarization beam splitter (hereinafter, referred to as a polarization beam splitter for separating P and S polarizations). P
44) and so-called 6-division photodetector 45
And a pair of photodetectors 46 and 47. The parallel plate 43 has a non-polarized reflection film on its beam incident surface, and the reflection film has a reflectance (transmittance) of, for example, about 50% as shown in FIG. Two transmission surfaces 43a, 43b, 43c are formed, and the other portions except these are total reflection surfaces. As shown in the drawing, the transmission surfaces 43a to 43c are each formed in a circular shape and are arranged in the central portion within the beam spot (inside the dotted line) on the parallel plate 43. The parallel plate 43 imparts astigmatism to the laser beam by inclining it at a predetermined angle with respect to the optical axis of the laser beam incident on the parallel plate 43. A focusing means that imparts a focusing property to light is configured.

The 6-division photodetector 45 uses the tracking error signal for automatically controlling the displacement of the objective lens 39 in the tracking direction by the tracking servo mechanism, and the magneto-optical disk 1 and the objective lens 39 by the focus servo mechanism. The focus error signal for automatically controlling the in-focus state is obtained, and the other photodetectors 46 and 47 obtain the magneto-optical reproduction signal.

Next, the operation of this optical pickup device will be described. First, the laser beam from the semiconductor laser 35 is made into a parallel laser beam by the collimator lens 36, the optical path is bent by the prism 37, and then transmitted through the beam splitter 38, and the objective lens 39 is used for the magneto-optical disk 1.
Is focused toward. The focused laser beam focused on the magneto-optical disk 1 is reflected by the perpendicular magnetization film 1b through the substrate 1a, and the reflected laser beam is converted into a parallel laser beam by the objective lens 39 and the signal generation means 33 by the beam splitter 38. Of the half-wave plate 41. 1 /
The parallel laser beam that has passed through the two-wave plate 41 is detected by the detection lens 42.
Then, the parallel flat plate 43 is reached. The center portion of the optical axis of the focused laser beam reaching the parallel plate 43 is transmitted in the optical path direction by the transmission surfaces 43a, 43b, 43c of the parallel plate, and is given astigmatism. Reach

On the other hand, the focused laser beam reflected by the parallel plate 43, that is, the outer portion of the beam and the portion reflected by the transmitting surface pass through the PBS 44 without being transmitted, and thus a pair of photodetectors are detected. 46 and 4
It is separated into P and S polarized lights toward 7. Then, the magneto-optical reproduction signal is obtained according to the difference in the amount of light received by the photodetectors 46 and 47.

FIG. 3 shows a tracking and focus error signal generating circuit in the optical pickup device shown in FIG. As shown in the figure, the 6-division photodetector 45 has a 4-division photodetector 48 having four light-receiving surfaces divided into 4 by two orthogonal line segments, and a pair of light beams sandwiching the 4-division photodetector 48 and arranged in the track direction. It consists of detectors 49a and 49b. In the four-division photodetector 48, the addition output of each light receiving output of the light receiving surfaces 48a and 48c at one diagonal position is subtracted from the addition output of each light receiving output of the light receiving surfaces 48b and 48d at the other diagonal position. Then, a focus error signal is generated. Also, a pair of photodetectors 49a and 49b
, The photodetector 49a is based on the push-pull method.
The light receiving output of the photodetector 49b is subtracted from the light receiving output of 1 to generate a tracking error signal.

As described above, this 6-division photodetector 45
With respect to, due to the transparent surface applied to the parallel plate 43,
Of the reflected light from the magneto-optical disk 1, only the light at the center of its optical axis is guided. The light in this portion is reflected almost perpendicularly to the substrate 1a of the magneto-optical disk 1, and is hardly affected by the birefringence of the substrate 1a. Therefore, the intensity distribution of the light incident on the light receiving surface of the four-division photodetector 48 becomes uniform over the entire light receiving surface without including the elliptically polarized component due to birefringence.

Reflected light transmitted through the transmissive surfaces 43a and 43c is projected on the photodetectors 49a and 49b, respectively. However, movement of the beam due to disc skew or lens shift occurs in the photodetectors 49a and 49b. Even if there is, it does not affect the differential output, so that the tracking error signal is less likely to be offset. Also, the 4-split photodetector 4
Since the reflected light of the central portion having a small diffracted light component of the track is projected on 8, the phenomenon in which the tracking error signal leaks into the focus error signal is reduced.

In the first embodiment described above, the transmitting surface for transmitting a part of the reflected light from the magneto-optical disk 1 in the optical path direction is provided on the main surface of the parallel flat plate 43 for generating astigmatism. However, even with the configuration according to the second embodiment as shown in FIG. 4, the same effect as that of the first embodiment can be obtained. The optical pickup device shown in FIG. 4 is the same as the optical pickup device shown in FIGS.
The optical pickup device according to the embodiment has the same structure as that of the embodiment, and the operation thereof is also the same, so that the detailed description thereof will be omitted.

That is, in FIG. 4, the parallel flat plate 43 '.
Has three reflecting surfaces formed of, for example, a reflecting film having a reflectance of 50% on the surface opposite to the incident surface. This reflecting surface is also arranged in the vicinity of the central portion within the incident beam spot as in the case of FIG. 2, and the central portion of the optical axis of the focused laser beam incident from the detection lens 42 is caused by the parallel plate 43 '. Astigmatism is given and the reflecting surface 43a
6'because it is reflected to the side of the optical path by ', 43b', 43c '
It reaches the split photodetector 45.

On the other hand, the laser beam that has passed through the parallel plate 43 ', that is, the outer portion of the beam and the portion that has passed through the reflecting surface, is not reflected and passes through the PBS 44, so that a pair of photodetectors 46 and 47 can be detected. Polarized light is separated toward. The generation of the error signal and the reproduction signal performed by each photodetector is as described in the first embodiment.

In the present embodiment, contrary to the optical system (including the signal generating means 33) in the first embodiment, the transmission component of the parallel plate is guided to the photodetector for detecting the reproduction signal. The central portion of the optical path of the reflected light is guided to the photodetector for detecting the error signal, and the same effect as that of the first embodiment can be obtained. Further, FIG. 5 is a block diagram of an optical pickup device according to a third embodiment of the present invention.

In this embodiment, unlike the structure shown in FIG. 1, the half-wave plate 41 is provided in the reflection optical path which reflects the parallel flat plate 43. With such a configuration, the influence of birefringence can be reduced without using a non-polarization reflection film as the transmission surface formed on the parallel plate 43. For example, the reflectance of the P polarization component is 50% and the reflection of the S polarization component is 50%. Rate 100
% As the polarization reflection film. This reflectance is due to the Kerr effect in the magneto-optical reproducing method rotating the P-polarized component into the S-polarized component. This way,
It is possible to guide the magneto-optical signal component to the photo detectors 46 and 47 for detecting the read signal without using the non-polarized reflection film.

[0030]

As described in detail above, in the optical pickup device according to the present invention, the reflected light is focused by the focusing means, and a part of the reflected light is reflected (or transmitted) by the reflecting (or transmitting) means. A structure in which the residual of the reflected light supplied to the focus and tracking error detection means is guided to the polarization means, and the residual of the reflected light guided by the reflection (or transmission) means by the polarization means is polarized and supplied to the reproduction signal detection means. Therefore, the light in the outer portion of the reflected light which is affected by the birefringence of the disk substrate due to the converging property does not enter the photodetector. Therefore,
The intensity distribution of the light incident on the light receiving surface of the photodetector becomes uniform, and the best focus position is eliminated and the track is eliminated regardless of the size of the birefringence of the substrate material such as a glass substrate disc or a polycarbonate substrate disc. It is possible to prevent transverse noise.

Further, since only the central portion of the reflected light is used as the focused beam for tracking error detection, it is less susceptible to the influence of birefringence, and the offset beam due to the movement of the focused beam on the light receiving surface of the photodetector. It will prevent the occurrence. At the same time, the tracking error is less mixed into the focus error signal, which is favorable.

[Brief description of drawings]

FIG. 1 is a block diagram of an optical pickup device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an incident surface of a parallel plate.

FIG. 3 is a circuit diagram of signal detection means in the optical pickup device of FIG.

FIG. 4 is a block diagram of an optical pickup device according to a second embodiment of the present invention.

FIG. 5 is a block diagram of an optical pickup device according to a third embodiment of the present invention.

FIG. 6 is a block diagram showing an example of a conventional optical pickup device.

FIG. 7 is a circuit diagram of signal detection means in the optical pickup device of FIG.

FIG. 8 is a front view of a light receiving surface of a photodetector showing a state of a spot of a laser beam due to birefringence, which is a view for explaining nonuniformity of spot intensity distribution.

[Explanation of symbols for main parts]

32 ... Optical system 33 ... Signal generating means 35 ... Semiconductor laser 36 ... Collimator lens 37 ... Prism 38 ... Beam splitter 39 ... Objective lens 41 ... 1/2 wavelength plate 42 ... Detection lens 43, 43 '... Transparent parallel plate 44 ... Polarizing beam splitter 45 ... 6-division photodetectors 46, 47 for focus and tracking error detection ... Photodetectors 43a, 43b, 43c for reproduction signal detection ... Semi-transmissive film 48 ... Focus error detection photo detectors 49a, 49b ... Tracking error detection photo detectors

Claims (5)

[Claims] 1. An optical system for irradiating light from a light source onto an optical recording medium to extract reflected light, and focus and tracking error detection means for issuing a focus and tracking error signal and a reproduction signal based on a change in the reflected light. And an optical pickup device including a reproduction signal detecting means, wherein the focusing means focuses the reflected light, and a part of the reflected light is reflected (or transmitted) to be supplied to the focus and tracking error detecting means. A reflection (or transmission) means for guiding the residual of the reflected light to a polarization means, and the polarization means polarizes and separates the residual of the reflected light guided by the reflection (or transmission) means to detect the reproduction signal. An optical pickup device, characterized in that the optical pickup device is supplied to the means. 2. The focus (error) transmitting means includes astigmatism applying means for applying astigmatism to a part of the reflected light supplied to the focus and tracking error detecting means, and the focus error detecting means. Are orthogonal 2
The optical pickup device according to claim 1, comprising a photodetector having a light-receiving surface divided into four by line segments. 3. The astigmatism imparting means comprises a transparent parallel flat plate arranged in the optical path of the reflected light with an inclination with respect to the optical axis of the reflected light, and the reflecting (or transmitting) means. The optical pickup device according to claim 2, wherein is formed of a reflective (or transmissive) film formed on the main surface of the parallel plate. 4. The reflective (or transmissive) means has three reflective (or transmissive) surfaces, and the focus and tracking error detection means are formed by two orthogonal line segments.
4. The optical pickup device according to claim 1, comprising a photodetector having a divided light receiving surface and a photodetector having a pair of light receiving surfaces. 5. The optical pickup device according to claim 3, wherein the reflection (or transmission) film is arranged in the vicinity of the optical axis of the reflected light.