JPS6353739A - Magneto-optical reproducer - Google Patents

Abstract

PURPOSE:To enable detection of signals without deterioration by disposing a 1st cylindrical lens and 2nd cylindrical lens on the same optical axis, and disposing the respective focal positions so as to be brought on the same photodetector. CONSTITUTION:Polarization direction 30 of the incident parallel light on a half wavelength plate 12 is rotated 45 deg. by the plate 12 to a deflection direction 31 which is parallel with the axial direction of the 1st cylindrical lens 28. The 1st cylindrical lens has curvature only in the direction perpendicular to the polarization direction; therefore, the polarization to the elliptical polarized light by a difference in the refractive index between P polarized light and S polarized light by the material quality of the lens is obviated and the light is focused perpendicularly to the plane of polarization on the photodetectors 16, 17. The 2nd cylindrical lens 29 is so formed that the axial direction is perpendicular to the polarization direction 31 and the focal position thereof rides on the same phase plane of the incident beam on the axis on the photodetector. The polarization of the exit light to the elliptical polarized light by a difference in the refractive index between the P polarized light and the S polarized light is, therefore, obviated and said light is focused in the polarization direction on the photodetectors 16, 17. The good reproduction signal is thereby obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is a method for converting analog information such as digital information, audio, and images recorded on a magneto-optical recording medium as changes in the direction of magnetization using coherent light such as a laser beam. The present invention relates to a magneto-optical reproducing device that focuses light on the surface of a recording medium and reads out information as the rotation of the polarization plane of transmitted or reflected light.

2. Description of the Related Art In recent years, information storage devices such as magneto-optical disks using magneto-optical recording media have been developed as rewritable high-density information recording media. This uses the magneto-optical effect of the medium to record information by changing the direction of magnetization in minute areas of the medium using sufficiently focused light and the direction of an externally applied magnetic field. In this case, focused light is applied to the surface of the medium, and information is extracted by detecting the rotation of the plane of polarization due to the Faraday effect of the transmitted light or the Kerr effect of the reflected light. DESCRIPTION OF THE PREFERRED EMBODIMENTS An example of a conventional magneto-optical recording/reproducing device will be described below with reference to the drawings, taking a magneto-optical disk as an example.

FIG. 3 shows a conventional recording and reproducing optical system for a magneto-optical disk, in which 1 is a laser diode, 2 is a collimating lens, 3 is a beam shaping prism, 4 is a half mirror, 16 is an objective lens, 6 is a magneto-optical disk, 7 is a polarizing beam splitter, 8, 9.10 is a lens, 11 is a knife, 1
2 is a half-wave plate, 13 is a polarizing beam splitter, 14.1
5 is a multi-element photodetector, and 18.17 is a photodetector. 0 The linearly polarized light exiting the laser diode 1 is shaped into parallel light by a collimating lens 2 and a beam shaping prism 3.
After passing through the half mirror 4, the light is focused onto the magneto-optical disk by the objective lens 5. The plane of polarization of the light reflected here is rotated by the Kerr effect depending on the direction of magnetization of the magneto-optical recording medium. This reflected light is returned to the objective lens 5.
After passing through, it is reflected by the half mirror 4 and enters the polarizing beam splitter 7. Here, while the transmitted light is focused by the lens 8, a portion is reflected upward by the knife 11, enters the multi-element photodetector 14, and is extracted as a far-field tracking servo signal. The light that has passed through the knife 11 is focused by a lens 9 onto a multi-element photodetector 16, and is extracted as a signal for a knife focus servo. In addition, the linearly polarized light incident on the polarizing beam splitter 7 is P-polarized light if there is no Kerr effect on the magneto-optical disk. The plane of polarization is rotated by a rotation angle of , and an S-polarized component is generated. This S-polarized light component is the signal to be extracted.

For example, the polarizing beam splitter 7 reflects ao% of the P-polarized light,
S-polarized light is made to include all signal components in the reflected light, such as 10% reflection.

The light reflected by the polarizing beam splitter 7 passes through the half-wave plate 12
The plane of polarization is rotated by 46°. This light edge/zu 1
After passing through 0, the light enters a polarizing beam splitter 13 that totally transmits the P-polarized light and totally reflects the S-polarized light. The polarization direction of the light here is as shown in FIG. The light intensity when there is no rotation angle of the polarization plane due to the Kerr effect is 18+L, the light intensity at rotation angle 〇 and -Ok is 18b and 18c, respectively, and these P polarized light components are 191L, 19b, and 1, respectively.
The 9Q S polarization components are 20a, 20b, and 2oc, respectively.
It is decomposed into Of these, the P-polarized light component is transmitted through the polarization beam splitter 13 and focused on the photodetector 16, and the S-polarized light component is reflected by the polarization beam splitter 13 and focused on the photodetector 17. The light incident on these two photodetectors has the same DC component, and changes in light amount due to the Kerr rotation angle ±Ok have opposite phases. Therefore, by taking the difference in the electrical outputs of these photodetectors 16 and 17, noise caused by changes in the light amount, such as noise caused by fluctuations in the output light amount of the laser 1 and noise caused by uneven reflectance of the disk, can be reduced. , changes in light amount due to Kerr rotation angle are added and information is extracted.

Problems to be Solved by the Invention However, in the magneto-optical reproducing device with the above configuration,
The light must remain linearly polarized until it enters the final polarizing beam splitter 13. When trying to focus light on a photodetector 16 or 17 using a normal convex lens 10, a straight line drawn from the center of the lens in the polarization direction 22 of the incident light 21 as shown in Fig. 5 in the peripheral part of the light. 23 and a straight line 24 drawn from the lens center perpendicular to the polarization direction, the vector 25 of the polarization direction projected onto the tangential plane of the lens at the incident point ζ The incident point and the lens center point are It can be decomposed into a component 26 parallel to the connecting straight line and a component 2a perpendicular to it.

However, since the material forming the lens usually has a different refractive index for S-polarized light and P-polarized light, a phase difference occurs between the two components mentioned above, and when the beam exits the lens, a portion of the beam becomes elliptically polarized light. Put it away. This elliptically polarized light has a large ellipticity in the portions away from the straight line 23 drawn from the lens center in the polarization direction of the incident light and the straight line 24 drawn from the lens center perpendicular to the polarization direction of the incident light. Become. Due to such causes, in the magneto-optical reproducing apparatus as shown in FIG. 3, the reproduced signal deteriorates.

Means for Solving the Problems In order to solve the above problems, the magneto-optical reproducing apparatus of the present invention has the following features:
As a means for condensing transmitted light or reflected light from a magneto-optical recording medium onto a photodetector via an analyzer, the axial direction is parallel to the polarization direction when there is no rotation of the polarization plane by the recording medium,
and a first cylindrical lens whose surface on which the light enters is shaped to have the same shape as the same phase plane of the light on that surface, and whose axial direction is perpendicular to the direction of polarization when the plane of polarization is not rotated by the recording medium. and a second cylindrical lens whose surface on which the light enters has the same shape as the same phase plane of the light on that surface,
The first cylindrical lens and the second cylindrical lens are arranged on the same original axis, and the focal position of the first cylindrical lens and the focal position of the second cylindrical lens are both on the same photodetector. It is equipped with a configuration in which it is placed.

action The present invention provides a lens according to the above configuration.

With both the second lens, the vector of the incident light is not split into two directions at the entrance surface of the lens, so the light after exiting the lens does not become partially elliptically polarized light, but instead becomes a straight line. The polarization is accurately preserved, and it enters a polarizing beam splitter where it is split into transmitted and reflected light and focused onto an analyzer. This makes it possible to detect signals without deterioration.

EXAMPLE Hereinafter, a magneto-optical reproducing apparatus according to an example of the present invention will be described with reference to the drawings.

FIG. 1 shows a signal detection optical system from a half-wave plate to a photodetector of a magneto-optical reproducing apparatus according to a first embodiment of the present invention. In FIG. 1, 12 is a half-wave plate, 28 is a first cylindrical lens, 29 is a second cylindrical lens, 13 is a polarizing beam splitter that completely transmits P-polarized light and totally reflects S-polarized light, 16.
17 is a photodetector.

The polarization direction 30 of the parallel light incident on the half-wave plate 12 is
The polarization direction is rotated by 46 degrees by the half-wave plate 12.
1 direction. The axial direction of the first cylindrical lens 28 is parallel to the polarization direction 31. In addition, the focal position of this first cylindrical lens is 0 on the photodetector 16.17.Since the curvature of this first cylindrical lens is only in the direction perpendicular to the polarization direction, P-polarized light is generated due to the material of the lens. Due to the difference in refractive index between
17 in a direction perpendicular to the plane of polarization.

The second cylindrical lens 29 is shaped so that its axial direction is perpendicular to the polarization direction 31, its focal point is on the photodetector, and its axis is on the same phase plane of the beam incident on this lens. has been done. Therefore, this lens has the effect of focusing light only in the polarization direction.The angle between the light and the lens surface exists only in the direction of polarization, and there is no angle in the direction perpendicular to the direction of polarization. . This prevents the emitted light from becoming elliptically polarized light due to the difference in refractive index between P-polarized light and S-polarized light due to the material of the lens, and can be focused on the photodetector 16.degree. 17 in the polarization direction.

As described above, according to this embodiment, the axis of the first cylindrical lens 29 is parallel to the polarization direction 31, and the axis of the second cylindrical lens 29 is parallel to the polarization direction 31.
are arranged so that their axes are perpendicular to the polarization direction 31 and on the same phase plane of the light, and the focal positions of the first cylindrical lens 28 and the second cylindrical lens 29 are aligned with the photodetector 16. ，
By setting it above 1γ, the incident linearly polarized light becomes
The light can be focused on a photodetector through a polarizing beam splitter without being made into elliptically polarized light.

A second embodiment of the present invention will be described below with reference to the drawings.

FIG. 2 shows a signal detection optical system from the optical wavelength plate 12 to the photodetector 34 of a magneto-optical reproducing device showing a second embodiment of the present invention, and a shows the optical system and the half-wave plate 12. 12 is a half-wave plate, 32 is a first
, 33 is a second cylindrical lens, 13 is a polarizing beam splitter that completely transmits P-polarized light and totally reflects S-polarized light, and 31 indicates the polarization direction of the light after passing through the half-wave plate 12. The configuration is similar to that shown in FIG. The difference from FIG. 1 is that the focal lengths of the first cylindrical lens 32 and the second cylindrical lens 33 are the same, and that the photodetector 34 is placed at the center of the focal positions of both lenses. The point is that the photodetector 34 is composed of a four-divided photodetection element. In both figures a and b, a photodetector for detecting the reflected light from the polarizing beam splitter is omitted. C shows the focused light spot, the polarization direction, and the positional relationship of the four elements of the photodetector.

The operation of the magneto-optical reproducing apparatus configured as described above will be explained below.

The optical path indicated by a solid line in FIG. 2 is the parallel light that returns when the disk is at the just focus point.

In a, parallel light is focused by the second cylindrical lens 33 in the polarization direction of the light slightly after the photodetector 34, and in b, the parallel light is detected by the first cylindrical lens 32. The light is focused slightly in front of the vessel 34 in a direction perpendicular to the polarization direction of the light. At this time, a circular beam spot is formed on the photodetector as shown by C. Next, when the disk moves away a little, the light becomes like a broken line at &,b, and the focusing position of the light by the first cylindrical lens and the second cylindrical lens moves toward the polarizing beam splitter side. At this time, a beam spot as shown by the broken line C appears on the photodetector. On the other hand, if the disk comes a little closer, a beam spot will appear on the photodetector as shown by a dashed line. Therefore, by arranging the four elements of the photodetector 34 as shown in C, calculating the sum of the outputs of element 1 and element m, the sum of the outputs of element II and element iv, and taking the difference between the results, the focus error can be detected. I can get a signal. Furthermore, a reproduced signal can be obtained by summing up each element.

As described above, the focal lengths of the first cylindrical lens and the second cylindrical lens are made the same, and the photodetector is composed of a four-split photodetecting element. By placing them at the center of each focus position, it is possible to detect the playback signal and extract the focus error signal at the same time. This eliminates the need for the focus servo optical system, which was conventionally configured as a separate optical path, and reduces the number of parts. Space saving becomes possible.

Effects of the Invention As described above, the present invention is a means for condensing light transmitted or reflected by a magneto-optical recording medium onto a photodetector via an analyzer, in which the axial direction is free from rotation of the plane of polarization by the recording medium. a first cylindrical lens shaped so that it is parallel to the polarization direction of the transmitted light or reflected light and whose axis overlaps the same phase plane of the light;
a second cylindrical lens shaped so that its axial direction is perpendicular to the polarization direction of transmitted light or reflected light when there is no rotation of the polarization plane by the recording medium, and its axis overlaps with the same phase plane of the light; By arranging the first cylindrical lens and the second cylindrical lens on the same optical axis, it is possible to prevent the polarized light from becoming partially elliptically polarized light due to the lenses, and the original polarized light can be transferred to the analyzer while preserving the linearly polarized light. It is possible to make the light incident on the photodetector and focus it on the photodetector, and it is possible to obtain a better reproduced signal than in the past.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a signal detection optical system of a magneto-optical reproducing device according to a first embodiment of the present invention, and FIG. 2 is a side view of the signal detecting optical system of a magneto-optical reproducing device according to a second embodiment of the present invention. Figure 3 is a 4+li diagram of the entire optical system of a conventional magneto-optical reproducing device, and Figure 4 is a front view showing the shape of the light spot.
FIG. 6 is a schematic diagram of the polarization direction of the light after passing through the half-wave plate, and FIG. 6 is a schematic diagram of the polarization direction on the lens 10 of FIG. 3. 1...Laser diode, 6...Magneto-optical disk, 10...Lens, 12...
...Half-wave plate, 13...Polarizing beam splitter, 16.17...Photodetector, 28...
First cylindrical lens, 29... second cylindrical lens, 34... 4-split photodetector. Name of agent: Patent attorney Toshio Nakao and 1 other person34
-4-split optical ridge number (a-)

Claims (2)

[Claims] (1) Information recorded on a magneto-optical recording medium as a change in the direction of magnetization is obtained by focusing linearly polarized light on the surface of the recording medium, and converting the recorded information into a rotation of the polarization plane of transmitted light or reflected light. The magneto-optical reproducing device reads out the transmitted light or the reflected light as a means for condensing the transmitted light or reflected light onto a photodetector via an analyzer, in which the axial direction is the transmitted light when the polarization plane is not rotated by the recording medium. or parallel to the polarization direction of the reflected light,
and a first cylindrical lens shaped so that its axis overlaps the same phase plane of light, and whose axis direction is perpendicular to the polarization direction of transmitted light or reflected light when the polarization plane is not rotated by the recording medium,
and a second cylindrical lens shaped so that its axis overlaps with the same phase plane of light, the first cylindrical lens and the second cylindrical lens are arranged on the same optical axis, and the first cylindrical lens is arranged on the same optical axis. A magneto-optical reproducing device characterized in that the focal position of the cylindrical lens and the focal position of the second cylindrical lens are both arranged on the photodetector. (2) The focal length of the second cylindrical lens is the same as the focal length of the first cylindrical lens, and two orthogonal dividing lines
The first cylindrical lens, the second cylindrical lens, and the intersection of the dividing line of the photodetector are arranged on the same optical axis, and the first cylindrical lens is arranged on the same optical axis. Claim 1, characterized in that the focal position of the cylindrical lens and the focal position of the second cylindrical lens are arranged at equal distances across the photodetector. The magneto-optical reproducing device described in Section 1.