JPH0652576B2 - Optical head - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head for optically recording / reproducing or erasing information on a disk medium used in an optical disk player such as a magneto-optical disk player / DRAW disk player.

2. Description of the Related Art Conventionally, for example, an optical head used in a magneto-optical disk player is composed of an optical system as shown in FIG.

In this conventional optical head, a semiconductor laser 1 and a collimator lens 2 having a large numerical aperture for converting an anisotropically emitted light emitted from the semiconductor laser 1 into a parallel beam.
And a pair of anamorphic biprisms 3 and 3 for shaping the elliptical parallel beam converted through the collimator lens 2 into a circular parallel beam, and the objective lens 4 supported by an objective lens actuator (not shown). A forward path of the optical system from the disk to the disk D is formed. A beam splitter 5 is arranged on the optical path from the anamorphic biprism 3 that shapes the emitted light from the semiconductor laser 1 into a parallel beam and the objective lens 4. The return light from the disc D is deflected by 90 degrees through the beam splitter 5 and is guided to the optical system that constitutes the photodetection system. A half-wave plate 6, a focusing lens 7, and a polarization beam splitter 8 are arranged in a direction deflected by 90 degrees through the beam splitter 5. Then, a two-division optical sensor 9 is arranged in a direction in which the return light goes straight through the polarization beam splitter 8, and a tracking error detection by a push-pull method is performed by a signal from this sensor. on the other hand,
In the direction in which the return light is deflected by 90 degrees through the polarization beam splitter 8, the concave lens 10, the cylindrical lenses 11 and 4,
A split sensor 12 is provided, and a focus error is detected by the astigmatism method by the detection output of the split sensor 12. The tracking and focus error signals detected by the optical sensors 9 and 12 serve to servo the objective lens actuator. As a result, the beam spot focused through the objective lens 4 is accurately controlled to follow the track of the disc D.

However, according to the conventional head configuration as described above, the beam splitter 5 arranged on the optical path that guides the light emitted from the semiconductor laser 1 onto the disk D and the reflection from the disk D are reflected. The beam splitter 8 arranged on the optical path that guides the return light to the error detection system has a cubic block shape and requires a large space by itself, and the emitted light from the semiconductor laser 1 is shaped into an elliptical beam into a circular parallel beam. Therefore, a pair of triangular prisms called anamorphic biprisms are used, and a cylindrical lens 11 is used to perform focus error detection by the astigmatism method. Therefore, the following problems occur. It was

According to the conventional structure, the number of optical components constituting the optical system increases, and a space is required for each component, so that the optical path length of the entire optical system becomes long and a large space is required as a whole. Therefore, there has been a problem that the optical head cannot be made compact.
Further, there is a problem that the cost becomes high due to the difficulty of molding parts such as a triangular prism.

The present invention has been made in view of the above points, and an object of the present invention is to achieve a compact optical head as described above and to achieve cost reduction.

Means for Solving the Problems The present invention relates to a first beam splitter that guides laser light from a laser light source onto a recording medium through an objective lens and deflects the reflected return light in a direction different from that of the laser light source. A second beam splitter that splits the return light deflected by the first beam splitter into two directions of a focus error detection system and a radial error detection system, and the first beam splitter is formed by two flat plate transparent bodies. Then, while tilting at a predetermined angle with respect to the optical path of the laser light from the laser light source, one of them is extended to the optical path of the focus error detection system branched by the second beam splitter, and the return light is Astigmatism is achieved by passing through the converging convex lens that constitutes the focus error detection and radial error detection system and the extended portion of the flat plate transparent body. In addition, the second beam splitter is constituted by a parallel plate and is arranged at a predetermined angle with respect to the optical path.

Operation In the above-described configuration, the laser light emitted from the laser light source is incident on the first beam splitter at a predetermined angle, whereby the elliptic beam is shaped into a circular beam. The reflected return light of the laser light focused on the recording medium by the objective lens is converted into 90 by the first beam splitter.
After being deflected, the beam is split into a tracking error detection system and a focus error detection system by the second beam splitter. The return light, which is deflected by 90 degrees to this focus error detection system, causes astigmatism by passing through the extended portion of the first beam splitter, and this error detection system causes focus error of the astigmatism method. Detection is done. Therefore, a prism for beam shaping and a cylindrical lens for producing astigmatism in the returning light, which are conventionally required, are not necessary.

Embodiments Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of an optical system when the present invention is applied to a magneto-optical type optical head, in which 20 is a semiconductor laser,
Reference numeral 21 is an objective lens, D is a magneto-optical disk, and 30 is a magnet for applying a bias magnetic field for applying a vertical magnetic field when recording / erasing information on the disk D.

In the optical system shown in FIG. 1, a semiconductor laser 20, a collimator lens 22 having a large numerical aperture, a first beam splitter 23 formed by stacking two flat plate transparent bodies 231, 232, and a biaxial actuator (not shown). A forward path from the semiconductor laser 20 to the objective lens 21 is formed by the objective lens 21 supported by. The first beam splitter 23 is arranged so as to be inclined by 45 degrees with respect to the optical axis F guided vertically to the objective lens 21. The semiconductor laser 20 and the collimator lens 22 are
Its optical axis F0 is 45 with respect to the first beam splitter 23.
It is arranged so that the laser light is incident at an angle smaller than a degree. Irradiation light that is emitted from the semiconductor laser 20 and spreads anisotropically is converted into a substantially elliptical parallel beam through a collimator lens 22, and a parallel plane formed by the short axes of the elliptical parallel beam is perpendicular to the surface of the first beam splitter 23. Is made incident. As a result, the plane of polarization of the light is polarized by 90 degrees, and the elliptical parallel beam incident on the first beam splitter 23 is shaped into a circular parallel beam having the major axis as its diameter. This circular parallel beam is focused on the disk D through the objective lens 21.

Therefore, the first beam splitter 23 not only functions as a polarization beam splitter, but also serves as a beam shaping prism.

On the other hand, the return light reflected by the disk D is deflected by 90 degrees by the first beam splitter 23 which is provided with an inclination of 45 degrees. In this deflection direction, the half-wave plate 24, the converging convex lens 25, and the flat plate-shaped second polarization beam splitter 2 are arranged.
6. A first photodetector 27 is provided. The second beam splitter 26 is arranged in parallel with the first beam splitter 23. The return light is split into two by the second beam splitter in a straight traveling direction and a direction deflected by 90 degrees. A second photodetector 28 is arranged in the 90 ° deflection direction of the second beam splitter 26. This second
Of the two flat plate transparent bodies 231, 232 forming the first beam splitter 23, one end of one flat plate transparent body 231 obliquely intersects and extends on the optical axis reaching the photodetector 28. Has been done.

As described above, the first beam splitter 23 has a long flat plate transparent body 231 having a length long enough to intersect the optical axis guided to the second photodetector 28, and has a length shorter than that. The flat plate transparent body 232 is integrally formed by stacking and adhering.

Next, a tracking and focus error detection operation by the optical head configured as described above will be described.

The first photodetector 27 performs tracking error detection, and a two-division photosensor is used for this. When the beam spot applied to the disc D via the objective lens 21 illuminates the step of the track groove of the disc D, the return light has a wavelength shift in the radial direction of the disc D. The return light is returned to the first beam splitter 23 through the objective lens 21 and is deflected by 90 degrees. The return light deflected by 90 degrees goes straight through the second beam splitter 26 and is received by the light receiving portion of the two-division optical sensor. A tracking error is detected by detecting the difference in the amount of incident light received. That is, the tracking error is detected by the push-pull method.

On the other hand, the second photodetector 28 performs focus error detection, and a 4-division photosensor is used for this. The second photodetector 28 includes a converging convex lens 25, a second beam splitter 26, and a flat plate transparent body 23 when the beam spot irradiated on the disk D is just in focus.
It is arranged at a position where the return light that enters through 1 is imaged in a perfect circle. Then, the objective lens 21 and the disc D
When the distance between and becomes shorter or shorter, the image incident and imaged on the photodetector 28 through the flat plate transparent body 231 changes into a vertically long or horizontally long ellipse. A focus error is detected by detecting this change with a four-division optical sensor. The focus error is detected by the so-called astigmatism method.

When performing this error detection, the second beam splitter 2
The return light deflected by 90 degrees at 6 causes astigmatism by passing through the extended portion of the flat plate transparent body 231 of the first beam splitter 23, and the return light having this astigmatism is generated. The light is detected by the second photodetector 28.

By detecting the focus and tracking error signals, the objective lens 21 is servoed via the biaxial actuator, and the beam spot is controlled so as to accurately follow the track groove of the disk D.

Next, recording / erasing and reproduction of information on the magneto-optical disk will be described.

Information is recorded on the magneto-optical disk by Curie point writing performed by irradiating a laser beam while applying a magnetic field to the disk D. Information is erased by continuously tracing a recording track with a beam spot with a constant laser output while applying a magnetic field.

On the other hand, the reproduction of information is performed by detecting the Kerr rotation angle. In this case, the returning light from the disc D is 1 /
After passing through the two-wave plate 24, the polarization plane angle is rotated by 45 degrees. The return light whose polarization plane is rotated by 45 degrees is split into two directions through the second polarization beam splitter 26,
The light is detected by the second photodetectors 27 and 28. At that time, the Kerr effect is generated depending on the direction of the vertical magnetic field applied to the information track of the disk D, and the polarization plane angle of the returning light is returned to the Kerr-rotated state at one angle. Therefore, the amount of return light split through the polarization beam splitter 26 differs between the straight traveling direction and the 90-degree deflecting direction depending on the Kerr rotation direction. Therefore, the first
And a difference occurs in the incident light amount of the return light received by the second photodetectors 27 and 28, and a reproduction RF signal can be obtained by extracting this difference.

FIG. 2 shows another embodiment in which the present invention is applied to an optical head used for recording / reproducing on a DRAW disk. In this figure, the difference from FIG. 1 is that the half-wave plate 24 is removed and the objective lens 2 is removed from the semiconductor laser 20.
That is, the quarter-wave plate 29 is interposed between the first beam splitter 23 and the objective lens 21 on the optical path reaching 1. This constitutes an optical head compatible with the DRAW disk. In the case of this embodiment, the first beam splitter 23 and the second beam splitter 26 are not formed in the rectangular flat plate shape as described above, but are formed in a parallelogram cross section in which both ends in the longitudinal direction are cut in parallel. Has been done. In this way, the first and second beam splitters 23,
The plate 26 may have a flat plate shape, and various shapes can be adopted.

In this configuration as well, the first beam splitter 23 is a polarization beam splitter and, at the same time, has a function as a beam shaping prism, so that the laser light passing through the first beam splitter 23 changes from an elliptical parallel beam to a circular parallel beam. Shaped into a beam. Therefore, a beam shaping prism such as an anamorphic biprism is not necessary.

Furthermore, the second beam splitter 26 that constitutes the photodetection system
Is composed of parallel flat plates, and is thinner and more compact than a cube. In addition, the flat plate transparent body 231 of the second beam splitter 23 extending on the optical path between the beam splitter 26 and the second photodetector 28 does not convert the return light received by the photodetector 28. Point aberration is produced. Therefore, the first beam splitter 23 also has the function of generating astigmatism in the return light in the focus error detection system based on the astigmatism method. Therefore, a cylindrical lens or the like for generating astigmatism becomes unnecessary.

EFFECTS OF THE INVENTION As described above, according to the present invention, the number of optical components forming the optical system of the optical head can be reduced, the head structure can be simplified, and the size and cost can be reduced.

[Brief description of drawings]

FIG. 1 is a block diagram showing an example of an optical head according to the present invention, FIG. 2 is a block diagram showing another example of an optical head according to the present invention, and FIG. 3 is a block showing an optical head having a conventional structure. It is a block diagram. 20 ... Semiconductor laser, 21 ... Objective lens, 22 ... Collimator lens, 23 ... First beam splitter, 231, 232 ... Parallel transparent plate, 24 ... 1/2 wavelength plate, 25 ... Convergent convex lens , 26 ... second beam splitter, 27 ... first photodetector, 28 ... second photodetector, D ... disk medium.

Claims (2)

[Claims] 1. Information is recorded on a recording medium by using a laser beam.
In an optical head for reproducing or erasing, a first beam splitter that guides laser light from a laser light source onto the medium through an objective lens and changes its reflected return light in a direction different from that of the light source, and the first beam splitter. A second beam splitter for branching the return light deflected by the beam splitter into two directions of a focus error detection system and a radial error detection system, wherein the first beam splitter is formed of two flat plate transparent bodies. Then, while arranging at an angle to the optical path of the laser light from the laser light source at a predetermined angle, one of them is extended on the optical path of the focus error detection system branched by the second beam splitter, Converging convex lens forming the focus error detection system and the radial error detection system by the return light and the extended portion by the flat plate transparent body Allowed rise to astigmatism by passing through a further second beam splitter constituted by parallel plate optical head being characterized in that disposed inclined at a predetermined angle with respect to the optical path. 2. The light emitted from the laser light source, which has been converted into an elliptical parallel beam through a collimator lens, is incident on the first beam splitter at an angle smaller than 45 degrees. The optical head according to the first item of the range.