JPH05151634A - Optical head for magneto-optical recording medium - Google Patents

Abstract

PURPOSE:To simplify its structure, to reduce the number of parts and to increase an extinction ratio by utilizing effectively the double refraction plate of a parallel plane shape. CONSTITUTION:This optical head is an optical head for a magneto-optical recording medium having an optical system converging a light beam outgoing from a semiconductor laser 10 on a magneto-optical disk 15 and simultaneously converging the light beam reflected by relevant magneto-optical disk 15 on photodetectors 19, 20. The double refraction plate 18 being the parallel plane shape and whose crystal axis is cut out perpendicularly to the plane is arranged. The double refraction plate 18 is arranged so that the plane is inclined by about 45 degree to the optical axis of above-mentioned reflected light beam and above-mentioned reflected light beam is separated to two astigmatic luminous flux whose polarizing conditions are intersected orthogonally by relevant double refraction plate 18 and are converged on respectively different photodetectors 19, 20.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head for a magneto-optical recording medium which irradiates a magneto-optical recording medium with light to detect a magneto-optical signal recorded on the magneto-optical recording medium.

[0002]

2. Description of the Related Art Conventionally, as a readable / writable recording medium, there is a magneto-optical recording medium such as a magneto-optical disk (hereinafter referred to as "recording medium"). An optical head is used to read information from this recording medium and write information to the recording medium. FIG. 5 is a schematic configuration diagram showing a conventional optical head of this type.

In FIG. 1, the linearly polarized light emitted from the semiconductor laser 100 is collimated by the collimator lens 101, converted into a circular spot by the shaping prism 102, passes through the polarization beam splitter 103, and is further objective lens 104. Is collected by the magneto-optical disk 10
5 is focused on the recording track.

Next, the reflected light containing the recorded information, which is reflected by the magneto-optical disk 105, passes through the objective lens 104 again and is reflected by the polarization beam splitter 103. ° After being rotated,
The condensing lens 107 and the cylindrical lens 108 form focused light having astigmatism. Then, it is separated into P-polarized light and S-polarized light by the polarization beam splitter 109 and focused on the photodetectors 110 and 111, respectively. Photodetector 110,1
The light condensed on 11 is converted into an electric signal, and the recording information signal is detected from the differential signal between the two, and the photodetector 1
The focus error signal is detected from the state of the spot condensed on 10, 111. In order to obtain astigmatic light, a transparent parallel plate may be arranged obliquely with respect to the optical axis instead of the cylindrical lens 108.

However, in this optical head, since the reflected light is separated by the polarization beam splitter 109, its extinction ratio is small and there is a problem in signal quality. Further, the number of parts is large, and it is difficult to reduce the size and weight of the optical head.

On the other hand, in order to obtain a large extinction ratio, a Wollaston prism is used instead of the polarization beam splitter 109 to separate the reflected light into ordinary light and extraordinary light (for example, Japanese Patent Publication No. 61-10888). Publication).
However, since this Wollaston prism must be constructed by bonding two wedge-shaped prisms having crystal axis directions different from each other, two steps of cutting out and bonding the crystals are required, and the fabrication is not easy. There was a problem.

Further, instead of the polarization beam splitter 109 and the Wollaston prism, a parallel plane plate-shaped birefringent plate can be used to separate two orthogonal polarized waves. FIG. 6 shows this birefringent plate 1 instead of the polarization beam splitter 109.
3 is a schematic configuration diagram showing a main part of an optical head using 20. FIG.
In the figure, the optical system from the semiconductor laser 100 to the objective lens 104 of the optical head shown in FIG.

The birefringent plate 120 has a crystal axis (indicated by an arrow in the figure) of a single crystal plate (for example, a rutile plate) exhibiting birefringence, which is oblique to the optical axis and parallel to the paper surface. It is cut so as to have a parallel plane plate shape.

Then, as shown in FIG.
If the light is vertically incident on the birefringence plate 20, the birefringent plate 1
Since the crystal axis of 20 is tilted by about 45 ° with respect to the optical axis direction, the light is split into polarized waves in two directions orthogonal to each other and received by the photodetectors 110 and 111, respectively, and the differential signal thereof is obtained. Be done.

The half-wave plate 106 shown in the figure rotates the plane of polarization of the reflected light by 45 °, thereby tilting this light about 45 ° in the rotation direction with respect to the crystal axis of the birefringent plate 120. , Is used to detect the differential signals efficiently by making the light amounts of the respective separated lights substantially equal.

However, the birefringent plate 1 is formed by such a method.
Even when 20 is used, the half-wave plate 106, the cylindrical lens 108, etc. are required, and the number of parts is large,
There is a problem that the optical head cannot be reduced in size and weight, and the cost cannot be reduced.

The present invention has been made in view of the above points, and an object thereof is to effectively use a birefringent plate in the form of a plane-parallel plate to simplify the structure and reduce the number of parts.
Moreover, it is an object of the present invention to provide an optical head for a magneto-optical recording medium which can have a high extinction ratio.

[0013]

In order to solve the above problems, the present invention focuses light emitted from a light source on a magneto-optical recording medium (magneto-optical disk 15) and reflects it from the magneto-optical recording medium. Alternatively, the transmitted light is detected by the photodetectors 19, 2
In an optical head for a magneto-optical recording medium having an optical system for converging to 0, the optical system was configured as follows.

The optical system includes a birefringent plate 18 having a plane-parallel plate shape and a crystal axis of which is cut out perpendicularly to the plane, and the birefringent plate 18 is used to transmit the reflected light or the transmitted light. The birefringent plate 18 separates the reflected light or the transmitted light into two light beams having astigmatism orthogonal to each other in polarization state by arranging the plane so as to incline at a predetermined angle with respect to the axis, and respectively detects different lights. It is configured so that the light can be focused on the containers 19 and 20.

The optical system is a plane-parallel plate-shaped birefringent plate 2.
5 or 30, and the crystal axis of the birefringent plate 25 or 30 is about 45 with respect to the optical axis direction of the reflected light or the transmitted light.
Inclination of about 4 with respect to the plane of polarization of the reflected or transmitted light
The birefringent plate 25 or 30 separates the reflected light or the transmitted light into two luminous fluxes having different polarization states, which are arranged in the optical system so as to be rotated by 5 ° and have different photodetectors. It was constructed so that the light could be focused on 19, 20.

The optical system has a plane-parallel plate shape, a crystal axis of which is cut out perpendicularly to the plane, and a birefringent plate 32 having a half mirror film 33 formed on one surface thereof. A finite objective lens 34 arranged on the optical axis at a position facing the recording medium, and two photodetectors 3
6, 37, the birefringent plate 32 is oblique to the optical axis so that a part of the light emitted from the light source is reflected by the half mirror film 33 and directed toward the objective lens 34. The light arranged, reflected by the half mirror film 33, condensed on the surface of the magneto-optical recording medium through the objective lens 34, and reflected by the surface of the magneto-optical recording medium is transmitted through the objective lens 34 and the birefringent plate 32. 2 with different polarization states
It is configured such that the light is separated into two astigmatic focused lights and focused on the two photodetectors 36 and 37, respectively.

[0017]

When the birefringent plate 18 in the form of a plane parallel plate is used to split the light into two as described above, the extinction ratio can be made higher than in the case where a polarization beam splitter is used. Further, when the light is separated by using the polarization beam splitter, the separated light goes in the orthogonal direction, but by using the birefringent plate, it goes in the same direction, so that the device can be downsized and the structure can be simplified.

Particularly, when the birefringent plate 18 is arranged obliquely with respect to the optical axis, not only the light transmitted through the birefringent plate 18 can be separated but also astigmatic light can be obtained.

If the crystal axes of the birefringent plates 25 and 30 are tilted by 45 ° in the rotation direction with respect to the plane of polarization of the reflected or transmitted light, the birefringent plates 25 and 30 themselves will be half-wave plates. To act.

A finite objective lens 3 is used as an objective lens.
If 4 is used, only one lens is required for this optical system.

[0021]

Embodiments of the present invention will now be described in detail with reference to the drawings. 1 is a schematic configuration diagram showing an optical head according to a first embodiment of the present invention. As shown in the figure, the optical system of this optical head includes a semiconductor laser 10, a collimator lens 11, a shaping prism 12, a polarizing beam splitter 13, an objective lens 14, a half-wave plate 16, a condenser lens 17, and a birefringent plate. 18 and photodetectors 19 and 20.

The difference between this optical head and the conventional optical head shown in FIG. 5 is that instead of the polarization beam splitter 109 and the cylindrical lens 108 used for separating the light or converting it into astigmatic light. That is, the refraction plate 18 is used.

Here, the birefringent plate 18 is formed by cutting a single crystal material such as a crystal, a rutile plate, or a calcite into a plane parallel plate and cutting its crystal axis perpendicular to the plane. ing. The plane of the birefringent plate 18 is oblique to the optical axis of the reflected light (45 ° in this embodiment).
It is arranged so that

Next, the operation of the entire optical head will be described.
First, the light emitted from the semiconductor laser 10 is collimated by the collimator lens 11, and then the shaping prism 1
A circular spot is formed by 2 and is condensed and reflected on the recording track of the magneto-optical disk 15 via the polarization beam splitter 13 and the objective lens 14. Then, the reflected light is halved through the objective lens 14 and the polarization beam splitter 13.
It is incident on the wave plate 16. Then, after the plane of polarization is rotated by 45 ° by the half-wave plate 16, it is incident on the birefringent plate 18 through a condenser lens 17.

Here, the birefringent plate 18 is 45 ° with respect to the optical axis.
Since they are arranged obliquely, the crystal axis thereof is inclined by 45 ° with respect to the transmitted light in the optical axis direction, whereby the ordinary ray and the extraordinary ray are separated into two. These are S-polarized light and P-polarized light, which are detected by the photodetectors 19 and 20, respectively, and the difference is obtained by the differential amplifier 21 to obtain a magneto-optical signal.

On the other hand, since the birefringent plate 18 is installed obliquely with respect to the optical axis, the reflected light transmitted therethrough is not reflected as in the case where a parallel plate of glass is installed obliquely with respect to the optical axis. It becomes point aberration light. Therefore, the photodetectors 19, 2
It is possible to detect the focus error signal on either side of 0 and the tracking error signal on the other side.

That is, according to this embodiment, the birefringent plate 18 can not only split the light into two, but also can produce astigmatic light. Therefore, the cylindrical lens used in the optical head shown in FIGS. 5 and 6 is used. 108 is unnecessary. Further, when the light is split by using the polarization beam splitter, the split light goes in the orthogonal direction, but by using the birefringent plate 18, the split light goes in the same direction, so that the device can be downsized and the structure can be simplified.

FIG. 2 is a schematic diagram showing an optical head according to the second embodiment of the present invention (only the birefringent plate 25 is shown in a perspective view in the figure for convenience of explanation). As shown in the figure, the optical system of this optical head includes a semiconductor laser 10, a collimating lens 11, a shaping prism 12, a polarization beam splitter 13, an objective lens 14, a condensing lens 23, a cylindrical lens 24, a birefringent plate 25, and an optical system. Detector 1
9 and 20.

The difference between this optical head and the conventional optical head shown in FIG. 5 is that the polarization beam splitter 109 used for separating the light and the half-wave plate that rotates the polarization plane of the light by 45 °. The point is that a birefringent plate 25 is used instead of 106.

The birefringent plate 25 is in the form of a plane parallel plate and is cut out so that its crystal axis is inclined at 45 ° with respect to the plane. That is, it is the same as the birefringent plate 120 shown in FIG. However, the installation position of the birefringent plate 25 used in this embodiment is different from that of the conventional example shown in FIG. That is, the birefringent plate 25 is installed by rotating it so that its crystal axis is rotated by 45 ° about the optical axis with respect to the polarization plane of the light incident on the birefringent plate 25.

Next, the operation of the entire optical head will be described.
Light emitted from the semiconductor laser 10 in the same manner as in the embodiment shown in FIG. 1 is reflected by the magneto-optical disk 15 and enters the condenser lens 23 and the cylindrical lens 24 through the objective lens 14 and the polarization beam splitter 13. The light is condensed, converted into astigmatic light, and then incident on the birefringent plate 25.

Since the crystal axis of the birefringent plate 25 is tilted at an angle of 45 ° with respect to the optical axis, the light incident on the birefringent plate 25 is separated into orthogonal ordinary light and extraordinary light.

On the other hand, since the crystal axis of the birefringent plate 25 is rotated by 45 ° about the optical axis with respect to the plane of polarization of the incident light,
Even if the half-wave plates 16 and 106 used in FIGS. 1 and 5 are not used, the light amounts of the separated lights can be made substantially equal, and the differential signal can be efficiently detected.

That is, according to this embodiment, not only can the light be separated into two by one birefringent plate 25, but also the 1/2 wavelength plate can be eliminated.

In the first embodiment shown in FIG. 1, if the birefringent plate 18 is rotated by 45 ° about the optical axis, the half-wave plate 16 of the optical system becomes unnecessary for the same reason. ..

By the way, in this embodiment, the birefringent plate 2 is used.
Since 5 is arranged so as to rotate by 45 ° about the optical axis, its assembling accuracy becomes poor. For this reason, the cutting angle with respect to the crystal axis of the birefringent plate may be as shown in FIG. Here, FIG. 3A is a front view of the birefringent plate 30 viewed from the optical axis direction, FIG. 3B is a plan view, and FIG. 3C is a right side view.

That is, the birefringent plate 30 is cut out so that its crystal axis is inclined by 45 ° with respect to the optical axis and further rotated by 45 ° with respect to the optical axis. According to this structure, even if this birefringent plate 30 is installed in a normal state (a state where it does not rotate), the same effect as that of the second embodiment can be obtained.

Next, FIG. 4 is a schematic configuration diagram showing an optical head according to a third embodiment of the present invention. This optical head comprises a high output (50 mW or more) semiconductor laser 31, a birefringent plate 32, an objective lens 34, and two photodetectors 36 and 37. Here, the birefringent plate 32 is in the form of a plane parallel plate, and its crystal axis is cut out perpendicularly to the plane (that is, the same as the birefringent plate 18 shown in FIG. 1). The half mirror film 3 is formed on one surface of the birefringent plate 32.
3 is provided. And the plane is 4 with respect to the optical axis
It is arranged to incline at an angle of 5 °. On the other hand, the objective lens 34 is composed of a finite system objective lens.

A part of the light emitted from the laser light source 31 is reflected by the half mirror film 33 of the birefringent plate 32, is focused by the objective lens 34 and is condensed and reflected on the magneto-optical disk 35. It The reflected light is
After being focused by the objective lens 34, it is transmitted through the half mirror film 33 and the birefringent plate 32.
Since the crystal axis of is inclined by 45 ° with respect to the reflected light in the optical axis direction, the reflected light is separated into two orthogonal components of ordinary light and extraordinary light,
At the same time, it becomes astigmatic light. Therefore, the respective lights are focused on the photodetectors 36 and 37, and the difference between them is obtained by the differential amplifier 38 to obtain a magneto-optical signal, and a focus error signal and a tracking error signal.

In the figure, a part of the light emitted from the laser light source 31 passes straight through the half mirror 33 and the birefringent plate 32, and this light is used for APC (auto laser power control) monitor, for example. The photodetector can be used by receiving light.

In this embodiment, a half-wave plate is arranged between the objective lens 34 and the birefringent plate 32, or instead, the crystal axis of the birefringent plate 32 is changed as in the embodiment shown in FIG. The differential signal can be efficiently detected by rotating and installing the device at a position rotated by 45 ° with respect to the polarization plane of the incident light.

That is, according to this embodiment, not only can the light be separated into two by the single birefringent plate 32, but also the light with astigmatism can be obtained, and the objective lens 34 can be one lens. The number of parts can be greatly reduced, the device can be made smaller and lighter, and the structure can be further simplified.

[0043]

As described in detail above, the optical head for a magneto-optical recording medium according to the present invention has the following excellent effects. Since the light is split into two by using the birefringent plate in the shape of a plane parallel plate, the extinction ratio can be made higher than that in the case of using the polarization beam splitter. Further, when the light is separated by using the polarization beam splitter, the separated light goes in the orthogonal direction, but by using this birefringent plate, it goes in the same direction, so that the device can be downsized and the structure can be simplified.

When the birefringent plate according to the present invention is arranged obliquely with respect to the optical axis, the light transmitted through this birefringent plate can be astigmatic light. Therefore, it is not necessary to separately prepare an optical component (for example, a cylindrical lens) for converting light into astigmatism light, which is required in the related art, and the structure is simple and the number of components can be reduced.

Further, if the direction of the crystal axis of the birefringent plate is rotated by 45 ° in the optical axis direction with respect to the plane of polarization of the incident light, the 1/2 wavelength plate becomes unnecessary.

If a finite objective lens is used as the objective lens, only one objective lens is required for this optical system, the number of parts can be greatly reduced, and the size and weight of the device can be reduced and the structure can be simplified. It can be achieved further.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an optical head according to a first embodiment of the present invention.

FIG. 2 is a schematic configuration diagram showing an optical head according to a second embodiment of the present invention.

3 (a) is a front view of the birefringent plate 30 seen from the optical axis direction, FIG. 3 (b) is a plan view, and FIG. 3 (c) is a right side view.

FIG. 4 is a schematic configuration diagram showing an optical head according to a third embodiment of the invention.

FIG. 5 is a schematic configuration diagram showing a conventional optical head.

FIG. 6 is a diagram showing an optical head configured using a birefringent plate 120.

[Explanation of symbols]

 10, 31 Semiconductor laser 15 Magneto-optical disk (magneto-optical recording medium) 18, 25, 30, 32 Birefringent plate 19, 20, 36, 37 Photodetector 34 Objective lens 33 Half mirror film

Claims (3)

[Claims] 1. A magneto-optical recording medium having an optical system for condensing light emitted from a light source onto a magneto-optical recording medium and condensing light reflected or transmitted from the magneto-optical recording medium onto a photodetector. In the optical head, the optical system includes a birefringent plate having a plane-parallel plate shape and a crystal axis of which is cut out perpendicularly to the plane, and the birefringent plate is an optical axis of the reflected light or the transmitted light. With respect to the plane, the plane is inclined by a predetermined angle, and the birefringent plate separates the reflected light or the transmitted light into two light beams with astigmatism whose polarization states are orthogonal to each other and separates them into different photodetectors. An optical head for a magneto-optical recording medium characterized by being condensed. 2. A magneto-optical recording medium having an optical system for condensing light emitted from a light source onto a magneto-optical recording medium and condensing light reflected or transmitted from the magneto-optical recording medium onto a photodetector. In the optical head, the optical system includes a plane-parallel plate-shaped birefringent plate, and the crystal axis of the birefringent plate is inclined about 45 ° with respect to the optical axis direction of the reflected light or the transmitted light. Alternatively, the reflected light or the transmitted light is arranged in the optical system at a position rotated by about 45 ° with respect to the polarization plane of the transmitted light, and the reflected light or the transmitted light is separated into two light beams having different polarization states by the birefringent plate. An optical head for a magneto-optical recording medium, characterized in that each of them is focused on a different photo detector. 3. A magneto-optical recording medium having an optical system for condensing light emitted from a light source on a magneto-optical recording medium and condensing light reflected or transmitted from the magneto-optical recording medium on a photodetector. In the optical head, the optical system has a plane-parallel plate shape, a crystal axis of which is cut out perpendicularly to the plane, and a birefringent plate having a half mirror film formed on one surface thereof, and a magneto-optical recording. It comprises a finite objective lens arranged on the optical axis facing the medium and two photodetectors, and the birefringent plate reflects a part of the light emitted from the light source by its half mirror film. Is arranged diagonally with respect to the optical axis so as to face the direction of the objective lens, is reflected by the half mirror film, and is condensed on the surface of the magneto-optical recording medium via the objective lens. The reflected light is Magneto-optical recording medium an optical head for, characterized in that it has been caused to the condenser each is separated into converging light of two astigmatism different polarization states the two photodetectors by passing through the birefringent plate and.