JPS6314330A - Optical head for optical disk - Google Patents

Abstract

PURPOSE:To easily assemble the optical head by adjusting the strength of a magnetic field fed to a Faraday rotator so as to match a Faraday rotating angle to 45 deg. thereby detecting a reflected light from an optical disk efficiently without requiring accurate optical alignment. CONSTITUTION:A polarized beam splitter 3 transmitting only the P polarized component in the laser light projected from a semiconductor laser 2 and reflecting the S polarized component orthogonal thereto and the Faraday rotator 4 rotating the polarized plane of the laser light from the polarized beam splitter 3 by 45 deg. in a prescribed direction and transmitting it are provided on an optical path between the optical disk 1 and the semiconductor laser 2. The quantity of the rotating angle of the Faraday rotator 4, that is, the Faraday rotating angle depends on the size of material and the magnetic field applied to the material and when the size of the material is decided once, the Faraday rotating angle is adjusted to 45 deg. depending on the strength of the applied magnetic field. Thus, the optical head is assembled simply without tight adjustment of the position on the optical path.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to an optical head for an optical disc that projects light onto an optical disc and reads or records information stored on the optical disc from the reflected light. It's something.

[Prior Art] Conventionally, this type of reading device detects the reflected light from the optical disc and eliminates the returning light to the light emitting element, which can cause malfunctions. A polarizing beam splitter and a quarter wavelength plate are provided to reflect the reflected light in a predetermined direction.

When such a reading device is used, the projection light projected from the light emitting element becomes linearly polarized light by the dark beam splitter, and then becomes circularly polarized light by passing through the quarter-wave plate and is projected onto the optical disk.

Then, the reflected light reflected by the optical disk becomes circularly polarized light in the opposite direction, and when it passes through the quarter-wave plate again, it becomes linearly polarized light, but it is 90 degrees different from the linearly polarized light in the direction of projection Q = I. Since the reflected light has a polarization plane, it is reflected in a predetermined direction by the polarizing film of the polarizing beam splitter and then received by the photodetector.

[Problems to be Solved by the Invention] As described above, the conventional reading device is good because it can efficiently detect the reflected light from the optical disc without converting it into the return light of the light emitting element. When positioning the optical head on the optical path, its position adjustment requires setting its optical axis at exactly 45 degrees with respect to the P polarization plane of the polarizing beam splitter, which is time-consuming and requires a lot of work when assembling the optical head. The problem was that it was bad.

SUMMARY OF THE INVENTION Therefore, the present invention has been made with the object of providing an optical head for an optical disc that does not require accurate optical position adjustment, can efficiently detect reflected light from an optical disc, and can be easily assembled. be.

[Means for Solving the Problems] That is, the present invention for achieving the above object includes: a projection means for projecting light onto an optical disc; a projection means provided on an optical path between the projection means and the optical disc; A Faraday rotator is provided on the optical path between the 7197 rotator and the projection means, and a Faraday rotator rotates the light by 45 degrees and transmits the light. a polarizing beam splitter that allows light to pass through the optical disc and reflects in a predetermined direction the reflected light from the optical disk whose polarization plane has been rotated by 90 degrees by reciprocating through the Noah Arab rotator; and detecting the light reflected by the polarizing beam splitter. The present invention is characterized by comprising a detection means for detecting.

The above-mentioned optical discs are intended for those in which information is stored so that the reflectance of the projected light changes, and there are those in which pits are formed on the disc surface, and those in which the disc surface changes from crystal to crystal by projecting laser light. Examples include those that reversibly change to amorphous state.

[Operation] The light projected from the projection means toward the optical disk passes through the polarizing beam splitter and becomes linearly polarized light.The plane of polarization is further rotated by 45 degrees by a Faraday rotator, and then is projected onto the optical disk.

When the reflected light from the optical disk passes through the Faraday rotator again, the plane of polarization further changes to 4 due to the difference in the direction of incidence.
The light is rotated by 5 degrees to become linearly polarized light having a plane of polarization that is 90 degrees different from that at the time of projection, is deflected in a predetermined direction by the polarizing beam splitter, and is detected by the detection means.

[Example] An example of the present invention will be described below with reference to FIGS. 1 and 2. Incidentally, FIG. 1 is a general configuration diagram showing the overall configuration of the optical disc reading device of this embodiment, and FIG.
FIG. 2 is an explanatory diagram showing changes in the plane of polarization of light indicated by arrows a to f in FIG. 1;

As shown in the figure, in the optical disc reading device of this embodiment, on the optical path between the optical disc 1 and the semiconductor laser 2,
A polarizing beam splitter 3 that passes only the P-polarized light component of the laser light emitted from the semiconductor laser 2 and reflects the S-polarized light component that is perpendicular thereto; A Faraday rotator 4 is provided that rotates the surface by 45 degrees in a predetermined direction and passes the surface.

This polarizing beam splitter 3 is manufactured by coating the slope of a right-angle prism made of optical glass such as BK7 with multiple dielectric thin films, and then bonding another uncoated right-angle prism thereon.

Further, the Faraday rotator 4 is a hollow cylindrical coil that houses a magnetic garnet crystal such as YIG (yttrium iron garnet), and is set so that the Faraday rotation angle is 45 degrees by applying a constant magnetic field in the optical path direction. .

Further, in front of the semiconductor laser 2 on this optical path, a collimating lens 5 is provided to make the laser beam from the semiconductor laser 2 parallel, and further, in front of the optical disk 1, an objective lens is provided to converge the laser beam onto the optical disk 1. 6 is provided.

In the optical head of this embodiment, the semiconductor ray
After the laser beam projected from 2 is made into a parallel laser beam by the collimating lens 5, it enters the polarizing beam splitter 3 so that its polarization plane is parallel to the P polarization plane of the polarizing beam splitter 3 ( Figure 2a). Therefore, the incident laser beam passes through the polarizing beam splitter 3 without being reflected (Fig.
(C), the light is focused by the objective lens 6 and projected onto the surface of the optical disc 1. The reflected laser beam reflected by the optical disk 1 is made parallel again by the objective lens 6 (FIG. 2 d), and the plane of polarization is roughly rotated by 45 degrees by the Faraday rotator 4, so that it is directed toward the polarizing beam splitter 3. It becomes a polarized reflected laser beam (FIG. 2e), and is entirely reflected in a 90 degree direction by the polarizing film surface of the polarizing beam splitter 3 (FIG. 2f).

Next, the reflected light 11 is converged by a converging lens 7, and is split into two parts, a transmitted light and a reflected light, through a beam splitter 8 made of a half mirror. Then, one of them is detected as a data signal by the cylindrical lens 9 and the four-division photodiode 10, and also as a focus control signal by the astigmatism method, and the other is detected by the two-division photodiode 11 as a tracking signal by the push-pull method. .

In this way, in the optical head for optical disks of this embodiment, the Faraday rotator 4 is used as a means for rotating the polarization plane of the laser beam emitted from the semiconductor laser 2 by 90 degrees in a round trip. In other words, the Faraday rotation angle is determined by the dimensions of the material and the magnetic field applied to the material. Once the material dimensions are determined, the Faraday rotation angle can be adjusted to 45 degrees by the strength of the applied magnetic field, and the position on the optical path can be adjusted. The optical head can be easily assembled without making severe adjustments.

The material t1 placed in the magnetic field of the Faraday rotator is not limited to magnetic garnet crystals such as YIG (yttrium iron garnet), but any material with a large Verdet constant may be used, such as FR-5 (trade name). : Manufactured by HOYA)
A thin film of magnetic garnet formed on a paramagnetic Farade glass or a substrate may also be used.

In addition, as a material for the polarizing beam splitter, the above BK
The prism is not limited to optical glass such as No. 7, and a Glan type prism made of crystal exhibiting birefringence such as calcite may also be used.

[Effects of the Invention] As described above, according to the optical head for an optical disk of the present invention, the Faraday rotation angle can be easily adjusted to 45 degrees by adjusting the strength of the magnetic field applied to the Faraday rotator. The reflected light from the optical disk can be detected efficiently without the need for target position adjustment, and the optical head can be easily assembled.

[Brief explanation of drawings]

FIG. 1 shows the overall configuration of the optical head for an optical disk according to this embodiment. ! FIG. 2 is an explanatory diagram showing changes in the polarization plane of laser light at each point on the optical path shown in FIG. 1. 1... Optical disk 2... Semiconductor laser 3... Polarizing beam splitter 4... Faraday rotator 8... Beam splitter 9... Cylindrical lens

Claims (1)

[Scope of Claims] 1. Projection means for projecting light onto an optical disk; a Faraday rotator that is provided on the optical path between the projection means and the optical disk and rotates the incident light by 45 degrees and passes it; Provided on the optical path between the Faraday rotator and the projection means, the light having a specific polarization plane among the light projected from the projection means passes through the Faraday rotator and the polarization plane becomes 90. An optical head for an optical disc, comprising: a polarizing beam splitter that reflects light reflected from the optical disc that has been rotated in a predetermined direction; and a detection means that detects the light reflected by the polarizing beam splitter. 2. Claim 1, wherein the optical disc stores information such that the reflectance of light changes.
An optical head for an optical disk as described in .