JPH0650580B2 - Erasable optical head - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to an erasable optical head, and more particularly to an erasable optical head for optically recording / reproducing information on / from an information recording medium by utilizing a magneto-optical effect. Optical system.

[Technical Background of the Invention and Problems Thereof] Optical information is already recorded / reproduced from an information recording medium (hereinafter simply referred to as an optical disk) by utilizing a magneto-optical effect such as the Kerr effect or the Faraday effect. The device is known. In such an apparatus, for example, an optical head having an optical system as shown in FIG. 2 or 3 is adopted.

In the optical system shown in FIG. 2, the laser beam from the laser semiconductor 2 is converted into a parallel light flux and introduced into the objective lens 7 via the first beam splitter 4 and the mirror 6, and by this objective lens 7. It is converged on the optical disk 8. At the time of reproduction, a laser beam having a constant light intensity is generated from the semiconductor laser, and at the time of recording, a light intensity-modulated laser beam is generated from the semiconductor laser.
At the time of erasing, a laser beam having a constant light intensity higher than that at the time of reproducing is generated from the semiconductor laser. Digital information is recorded on the tracking guide of the optical disk 8 made of an amorphous magnetic alloy as the magnetization direction of the magnetic domain. That is, at the time of recording, the static magnetic field generated from the magnet device 10 is applied to the optical disk 8, and when the tracking guide of the optical disk 8 is traced by the recording laser beam, the magnetization direction of a specific area is changed and the information is magnetized. Is recorded in the magnetic domain area. Further, at the time of reproduction, when the tracking guide is traced by a reproduction laser beam and the converged laser beam is irradiated to the magnetic domain region magnetized in a specific magnetization direction, the polarization plane of the laser beam is slightly rotated. At the time of erasing, the static magnetic field generated from the magnet device 10 is applied to the optical disk 8, and when the erasing laser beam is applied to a specific region whose magnetization direction has been changed, the magnetization direction changes so as to be the same as other regions. To be done.

The laser beam reflected from the optical disk is reflected by the mirror 6 and the first beam splitter 4, and the reflected laser beam is split into two systems by the second beam splitter 12, and one laser beam is 1 / By passing through the two-wave plate 14, the plane of polarization is rotated by about 45 degrees and is converged by the first convex lens 16. The laser beam converged by the convex lens 16 is polarized beam splitter 18
Are separated into the P component and the S component, and the P component and S component laser beams are respectively separated by the first and second photodetectors 20, 22.
Is incident on. The other laser beam separated by the second beam splitter 12 is incident on the third photodetector 30 via an astigmatism lens system 28 including a second convex lens 24 and a cylindrical lens 26.

The information recorded on the optical disc 8 is read from the difference in the light intensity of the P component and S component laser beams, that is, the difference in the output signals of the first and second photodetectors 20 and 22, and the third photodetector is read. A focus signal is generated from 30, and a tracking signal is generated from one of the first and second photodetectors 20 and 22.

The optical system shown in FIG. 2 has a problem that the number of optical components is large and the arrangement thereof is complicated as is apparent from the figure. Therefore, the optical system becomes large and the time required for assembly and adjustment becomes long, There is a problem that the optical head becomes heavy.

For such a problem, an optical system as shown in FIG. 3 has been proposed. In the optical system shown in FIG. 3, the second beam splitter 12 is eliminated and the second beam splitter 1
The focus signal is generated by the first photodetector 20 which detects the P component laser beam separated by the polarization beam splitter 18 without separating the laser beam into two systems by 2 and detects the S component laser beam. A tracking signal is generated from the second photodetector 22. The information recorded on the optical disk 8 is read from the difference in the light intensity of the P component and S component laser beams, that is, the difference between the output signals of the first and second photodetectors 20 and 22. The optical system shown in FIG. 3 is certainly smaller than the optical system shown in FIG. 2 because the optical system is simplified and the second beam splitter 12 is removed. Although the optical head becomes lighter, there is a problem that the arrangement of the optical system is still limited and the optical system cannot be sufficiently simplified, and the weight of the optical head cannot be reduced. Further, in the optical system shown in FIGS. 2 and 3, since the laser beam incident on the polarization beam splitter 18 is converging, the light beam incident on the polarization plane is directed to this polarization plane. Will be incident at different incident angles. As a result, there is a possibility that the polarized light is not reliably polarized, the separation ratio of the P component and the S component is not constant, and the information recorded on the optical disc 8 cannot be read accurately.

[Object of the Invention] An object of the present invention is to provide an erasable optical head having a downsized and simplified optical system.

According to the present invention, according to the present invention, an objective lens for condensing a light beam on an erasable information storage medium and a light beam returned from the information storage medium via the objective lens are separated into two polarization components. In the erasable optical head including a polarization beam splitter for polarization, the polarization beam splitter includes a polarization plane that reflects the first polarization component and a reflection surface that reflects the second polarization component that has passed through the polarization plane. The surface and the light-reflecting surface are non-parallel and reflect the first and second polarization components in slightly different directions, respectively, and form a lens system in which the first and second polarization components are single. An erasable optical head is provided which is characterized by being introduced via a single photodetector.

Embodiment of the Invention FIG. 1 shows an optical system of an erasable optical head adopting the astigmatism method according to an embodiment of the present invention. In this optical system, the laser beam from the laser semiconductor 42 is converted into a parallel light flux by the collimator lens 40, introduced into the objective lens 46 via the beam splitter 44, and converged on the optical disc 48 by the objective lens 46. . The optical disc 48 has a recording layer made of, for example, an amorphous magnetic alloy, and a tracking guide is formed in a concentric or spiral shape on its surface in a concave or convex shape. Further, in this tracking guide, preliminary information such as a track address and a sector address, that is, a preformat signal is previously formed as concave or convex prepits. In the recording layer of the optical disc 48, all the magnetic domains are aligned in a fixed direction in a state where no information is recorded, and during recording, when a magnetic field is applied to rapidly heat, the magnetization directions of the magnetic domains in the region are reversed. It The semiconductor laser 42 is driven by a drive circuit (not shown), a laser beam having a constant light intensity is generated at the time of reproduction, and a light intensity is modulated according to information to be recorded at the time of recording. A laser beam is generated, and at the time of erasing, a laser beam having a constant light intensity larger than that at the time of reproduction is generated.

During recording, a static magnetic field generated from the magnet device 50 is applied to the optical disc 48, and when a tracking laser beam for tracing is used to trace the tracking guide of the optical disc 48, a specific area is rapidly heated, the magnetization direction is reversed, and information is recorded. Will be recorded. Further, at the time of reproduction, when the tracking laser beam for reproduction is traced on the tracking guide to irradiate the converged laser beam on the domain region which is inversely magnetized, the polarization plane of the laser beam is slightly rotated. When erasing, magnet device 5
The static magnetic field generated from 0 is given to the optical disc 48,
When the erasing laser beam is applied to a specific region in which the magnetization direction is reversed, this region is gently heated and is reversed again so that the magnetization direction is the same as the other regions.

The laser beam reflected from the optical disc 42 passes through the objective lens 46 and is reflected by the beam splitter 44. The reflected laser beam is passed through the half-wave plate 54 to increase the S component ratio and has a polarization plane of approximately 4
It is rotated 5 degrees. The laser beam whose plane of polarization is rotated is
The parallel light flux is introduced into the polarization beam splitter 56 as it is.

As shown in FIG. 1, the polarization beam splitter 56 is formed by joining a wedge prism to a right-angle prism, and the joint surface is formed on a polarizing surface 56A and the surface of the wedge prism facing the joint surface. Is reflective surface 56B
Is formed in. Therefore, when the parallel laser beam including the P component and the S component is incident on the polarization beam splitter 56, the S component is reflected by the polarization plane 56A and P
The component passes through the polarizing surface 56A, reaches the reflecting surface 56B, and is reflected by the reflecting surface 56B. Reflection surface 56B and polarization surface 56
The laser beams of the P component and the S component are directed in different directions because they are non-parallel to A and form a slight angle.
The P-component and S-component laser beams emitted from the polarization beam splitter form slightly different angles, and the convex lens 5
8 and the astigmatism lens system 62 including the cylindrical lens 60, and the light is converged and enters the photodetector 64. Therefore, when the objective lens is in focus, the P component and S
Converging points of the component laser beams are formed in different regions on the light receiving surface of the photodetector 64.

As shown in FIG. 4, the photodetector 64 is divided into four detection regions 66-1 and 6-6 in which the convergence point of the S component laser beam is formed at the center when the objective lens is in focus.
First detection unit 66 including 6-2, 66-3, 66-4
And a detection area 68-1, 68-2, 68-3, 68-4 which is divided into four and is formed at the center of the convergence point of the P component laser beam when the objective lens is in focus. It has two detectors 68. The signals from the detection areas 66-1, 66-2, 66-3, 66-4 of the first detection section 66 are added by the first adder 70, and similarly the detection areas of the second detection section 68 are detected. 68-1, 68-2, 68-3, 6
The signals from 8-4 are added by the second adder 72.
The addition output of the first and second adders 70 and 72 is the subtractor 7
4 and the information reproducing signal is generated as an output from the subtractor 74. That is, the difference between the S and P components that changes depending on whether the plane of polarization of the laser beam is rotated is generated from the subtractor 74 as an information reproduction signal. First
The addition outputs of the first and second adders 70 and 72 are the adder 76.
And the reproduction signal of the preformatted signal is generated as an output from the adder 76. That is, the light intensity of the laser beam whose light intensity is modulated in the pre-pit is generated from the adder 76 as a reproduction signal of the pre-format signal. Further, the signals from the first and second detection areas 66-1 and 66-2 of the first detection unit 66 and the second detection unit 68.
The signals from the first and second detection areas 68-1 and 68-2 are added by the third adder 78, and the first detector 66
Signals from the third and fourth detection regions 66-3 and 66-4 and the third and fourth detection regions 68- of the second detection unit 68.
The signals from 3, 68-4 are added by the fourth adder 80. The addition signals from the third and fourth adders 78 and 80 are supplied to the subtraction circuit 82, and the subtraction circuit 82 generates a track error signal. Signals from the first and third detection areas 66-1 and 66-3 of the first detection section 66 and the first and third detection areas 68- of the second detection section 68
The signals from 1, 68-3 are added by the fifth adder 84, and the second and fourth detection regions 66- of the first detection unit 66 are added.
The signals from 2, 66-4 and the signals from the second and fourth detection areas 68-2, 68-4 of the second detector 68 are added by the sixth adder 86. Fifth and sixth adder 8
The addition signals from 4, 86 are supplied to the subtraction circuit 88,
A focus error signal is generated from this subtraction circuit.

The objective lens is moved in the optical axis direction according to the focus error signal, the objective lens is always kept in focus, and the objective lens is moved in the direction perpendicular to the optical axis according to the track error signal. , The tracking guide is accurately tracked by the focused laser beam. as a result,
The record information is read from the optical disc, and the preformatted signal and the record information are reproduced. Similarly, when recording,
The information is written correctly, and the recorded information is surely erased at the time of erasing.

Next, another embodiment of the optical system of the present invention employing the knife edge method will be described with reference to FIGS. 5A to 5C.
In the drawings, the same reference numerals as those shown in FIGS. 1 and 4 indicate the same parts or the same parts, and the description thereof will be omitted. In the optical system shown in FIG. 5A, a wedge prism 56 having one surface formed as a polarization surface 56A and the other surface formed as a light reflection surface 56B is used as a polarization beam splitter, and the S component and P The deformable lens 90 is used as a lens for projecting the component laser beam onto the detector 64. This deformable lens 90 is
As shown in the figure, the flat portion of the plano-convex lens is formed by cutting so as to have the surfaces 90B and 90C inclined with respect to the flat surface 90A of the plano-convex lens. One surface of the deformable lens 90 is separated by a boundary line portion 92-1 where the inclined surfaces 90B and 90C are in contact with each other and a boundary line portion 92-2 and 92-3 where the flat surface 90A of the plano-convex lens is in contact with each other. ing. The detector 64 includes the first, second and third detectors 94,
96, 98, the first detection unit 94 has a boundary line portion 92.
First and second photodetection regions 94 separated in the −1 direction
−1 and 94-2, and the third detection unit 98 has the first and second photodetection regions 98− separated at intervals in the direction perpendicular to the direction of the boundary line portion 92-1. 1,98-
It consists of two.

The P-component and S-component laser beams emitted from the polarization beam splitter form lenses 90 at slightly different angles.
The S component laser beam is emitted from the inclined surfaces 90B and 90C and the flat surface 90A, and the P component laser beam is emitted only from the flat surface 90A. The P component convergent laser beam emitted from the flat surface 90A is directed to the first and second photodetection regions 96-1 and 96-2 of the second detection unit 96, and the inclined surfaces 90B and 90C.
And the converged laser beam of the S component emitted from the flat surface 90A is directed to the separated first and second photodetection regions 98-1 and 98-2 of the third detection unit 98, respectively, and the flat surface 90A The focused S component laser beam emitted from the pickup is
It is directed to the first detector 94.

The tracking error signal is generated by obtaining the difference between the signals generated from the separated first and second photodetection regions 98-1 and 98-2 of the third detector 98 by the subtractor, The signals from the first photodetection areas 94-1 and 96-1 of the second detectors 94 and 96 are added by an adder, and the second photodetection of the first and second detectors 94 and 96 is performed. A focus signal is generated by adding the signals from regions 94-2 and 96-2 with an adder and obtaining the difference between the added signals with a subtractor. Furthermore, the signals from the first and second photodetection regions 94-1, 94-2 and 96-1, 96-2 of the first and second detectors 94 and 96 are added by an adder, and the third signal is added. The information is reproduced by obtaining the difference from the signal generated from the detection unit 98 of 1 by the subtractor. In addition, the adder adds the signals from the first and second photodetection regions 94-1 and 94-2 of the first detector 94 and the second detector 96 and the signal generated from the third detector 98. Is added to reproduce the preformatted signal.

The polarization beam splitter as shown in FIG. 6 may be used instead of the polarization beam splitter as shown in FIG. 1 and FIG. 5A. That is, the polarization beam splitter 56 shown in FIG. 6 is made by bonding a rhombus prism to a right-angled prism, and the bonding surface of both is formed on the polarization surface 56A and a rhombus prism facing the bonding surface is formed. The surface may be formed as the reflecting surface 56B.

[Effects of the Invention] According to the present invention, in the optical system of the optical head, a single photodetector can obtain a focus error signal, a tracking error signal, a preformatted signal, and a signal for converting into recording information. Moreover, since the laser beam reflected from the optical disc is an optical system directed to the photodetector, the optical system becomes compact and the device itself can be made compact. Further, since the path of the laser beam is not divided into a plurality of parts, adjustment and assembly of the optical system are facilitated, the number of parts can be reduced, and the device can be manufactured at low cost. Further, in the optical system of the present invention, since the parallel laser beam is incident on the polarization beam splitter, light rays are incident on the polarization plane at the same incident angle, and as a result, the P component and the S component are incident. It is possible to prevent a situation in which the separation ratio of s is not constant.

[Brief description of drawings]

FIG. 1 shows an erasable optical head optical system adopting an astigmatism method according to an embodiment of the present invention, and FIGS. 2 and 3 show an erasable optical head adopting an astigmatism method. FIG. 4 shows a conventional example of an optical system of an optical head, FIG. 4 is a block diagram showing the photodetector shown in FIG. 1 and its peripheral circuits, and FIG.
FIG. 5B shows an optical system of an erasable optical head adopting the knife edge method according to another embodiment of the present invention, and FIGS. 5B and 5C are schematic plan views of the lens and the detector shown in FIG. 5A. , And FIG. 6 show an optical system of an erasable optical head according to still another embodiment of the present invention. 42 ... Laser semiconductors 42, 44 ... Beam splitter, 4
6 ... Objective lens, 48 ... Optical disk 48, 50 ... Magnet device 50, 54 ... 1/2 wavelength plate, 56 ... Polarization beam splitter 56, 58 ... Convex lens, 60 ... Cylindrical lens, 64 ... Photodetector 64, 90 ... Deformation lens.

Claims (1)

[Claims] 1. An objective lens for converging a light beam on an erasable information storage medium, and a polarization beam splitter for separating a light beam returned from the information storage medium via the objective lens into two polarization components. In the erasable optical head described above, the polarization beam splitter includes a polarization plane that reflects the first polarization component and a reflection plane that reflects the second polarization component that has passed through the polarization plane. Reflect the first and second polarization components in non-parallel and slightly different directions, respectively, and the first and second polarization components are coupled to a single photodetector via a single lens system. An erasable optical head characterized by being installed on a container.