JPS6214342A - Optical head - Google Patents

Abstract

PURPOSE:To enable the reproducing of recording information by providing a driving device that makes retire a plate of 1/4 wave length from an optical path between a recording medium and a polarization beam splitter when a beam reflected by the recording medium is detected. CONSTITUTION:A beam 13 transmitted a polarization beam splitter 14 transmits an objective lens 15 and if focused on the recording surface of a disk 15. However, when the disk is an optical one, a plate of 1/4 wave length 17 is positioned in the optical path between the polarization beam splitter 14 and the objective lens 15 by the driving device. When the disk 16 is a photomagnetic disk, the plate of 1/4 wave length is made retire from the optical path by the driving device. The beam 13 that is reflected by the disk 16 and transmits the objective lens 15 and farther, reflected by the polarization beam splitter 14 transmits a plate of 1/2 wave length and is made incident on a polarization beam splitter 22. Thereby, it is possible to reproduce the recording information from both of the recording media of reflectance of changeable type and of plane of polarization of rotatable type.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical head that obtains a tracking error signal based on the detection output of each of a plurality of beams reflected by a recording medium.

[Summary of the invention]

In the optical head as described above, the present invention uses a quarter-wave plate when detecting a beam reflected by a recording medium of a variable reflectance type, and also detects a beam reflected by a recording medium of a rotating polarization plane type. By using a quarter-wave plate when detecting the transmitted beam, a reproduced signal with a good S/N ratio can be obtained from either of the above two types of recording media while performing accurate tracking control. It was made so that it could be done.

[Conventional technology]

A three-spot method is widely known as a method for obtaining a tracking error signal based on the detection output of each of a plurality of beams reflected by a recording medium.

In addition, as shown in Japanese Patent Application No. 59-215860, for example, a tracking/knocking error signal can be obtained from the difference between the push-pull signals of a beam incident on a recording track and a beam incident between recording tracks. There is also a way to obtain .

However, in this method of obtaining tracking error signals from a plurality of beams, for example, Japanese Patent Application No. 59-145767
As shown in the above publication, direct current fluctuations occur in the tracking error signal due to skew of the recording medium in the direction along the recording trunk, so-called tangential skew.

If the coupling efficiency between a light source such as a laser diode and a collimator lens is η, the i3 pass rate of the beam splitter is T, and the reflectance of the recording medium is R, then the proportion of the beam that returns to the light source among the beams emitted from the light source is FB is expressed as FB-η・T2・R. Then, when the tracking error (modulation degree of No. 8 is P), the direct current variation rate ε of the trunking error signal is expressed as ε FB/P.

Assuming η-50% and T-90%, if the recording medium is a recordable and reproducible magneto-optical disk, R# is 50%, so FB=20%. On the other hand, in a read-only optical disc using an A1 film, R=100%, so FB# is 40%.

Moreover, the modulation degree P of a read-only optical disk is less than half that of a magneto-optical disk in which a DC group is formed.

Therefore, read-only optical disks have a DC fluctuation rate ε that is four times or more higher than that of magneto-optical disks.

In other words, even if the DC fluctuation rate ε of the magneto-optical disk is suppressed to 10% or less, the DC fluctuation rate ε of the read-only optical disk becomes 40% or more, and the tracking of the read-only optical disk is unstable.

Therefore, in order to reproduce recorded information from a read-only optical disc, it is necessary to lower the FB using a quarter-wave plate and a polarizing beam splitter.

[Problem that the invention seeks to solve]

However, when a quarter-wave plate is used, the beam incident on the recording medium becomes circularly polarized light. Therefore, although a quarter-wave plate can be used to play back a variable-reflectance type optical disc, a quarter-wave plate can be used to play back a magneto-optical disc that has a rotating polarization plane and requires the use of a linearly polarized beam. It is not possible to use a half-wave plate.

Therefore, with the conventional optical head, it has not been possible to reproduce recorded information from either a magneto-optical disk with a rotating polarization plane or an optical disk with a variable reflectance type.

Means for Solving Problem C] The optical head according to the present invention is arranged in the optical path between the light source 11 and the recording medium 16, and the polarization plane of the beam I3 when emitted from the light source 11 is A polarizing beam splinter 14 that reflects at least 50% of a beam having orthogonal polarization planes, and a case where the recording medium 16 is a recording medium 16 of a variable reflectance type and the beam 13 reflected by this recording medium 16 is detected. This recording medium 16
A quarter-wave plate 17 is positioned in the optical path between the polarizing beam splitter 14 and the recording medium 16.
is a recording medium 16 with a rotating polarization plane, and when the beam 13 reflected by this recording medium 16 is to be detected, the quarter beam is detected from the optical path between this recording medium 16 and the polarized beam splinter 14. A driving device 35 for retracting the wave plate 17 is provided.

[Effect]

In the optical head according to the invention, when detecting the beam 13 reflected by the variable-reflectance recording medium 16, at most only 50% of this reflected beam 13 returns to the light source 11.

Furthermore, when detecting the beam 13 reflected by the recording medium 16 with a rotating polarization plane, the quarter-wave plate 17 is not used.

〔Example〕

Hereinafter, an embodiment of the present invention applied to an optical head of a device capable of recording and reproducing information on both a magneto-optical disk and an optical disk will be described with reference to FIGS. 1 to 8. do.

FIG. 1 shows the optical path in the optical head of this embodiment. A linearly polarized beam 13 emitted from a light source 11 such as a laser diode and incident on a collimator lens 12 is
It becomes a parallel beam and enters the polarizing beam splitter 14.

This polarizing beam splitter 14 has a transmittance of 5 for the linearly polarized beam 13 emitted from the light source 11.
0-90%, the reflectance is 50-10%, and the beam 13
The transmittance for a beam having a polarization plane perpendicular to the polarization plane is 0%, and the reflectance is 100%. Note that it is desirable that the reflectance shown last is 100%, but it may be at least 50%.

The beam 13 that has passed through the polarizing beam splitter 14 passes through an objective lens 15 and is focused on the recording surface of a disk 16 . However, if the disk 16 is an optical disk, a quarter-wave plate 17 is installed in the optical path between the polarizing beam splitter 14 and the objective lens 15 by a driving device to be described later.
is located, and if the disk 16 is a magneto-optical disk, the quarter-wave plate 17 is moved out of the optical path by the drive device.

The beam 13 reflected by the disk 16, transmitted through the objective lens 15, and further reflected by the polarizing beam splitter 14 is
The light passes through the half-wave plate 21 and enters the polarizing beam splitter 22 .

Half-wave plate 21 is angled about the optical axis to rotate the plane of polarization of the incident beam by 45°.

The polarizing beam subrifter 22 has a transmittance of 100% and a reflectance of 0% for a beam having a plane of polarization in the same direction as the beam 13 when it is emitted from the light source 11, and conversely, the plane of polarization of the beam 13 is The transmittance for beams with orthogonal polarization planes is 0% and the reflectance is 100%.

The beam 13 that has passed through the polarizing beam splitter 22 passes through a converging lens 23 and a semicircular cylindrical lens 24 and enters a photodetector 25 . Furthermore, the beam 13 reflected by the polarizing beam splitter 22 is incident on the photodetector 26 .

By the way, as mentioned above (if the disk 16 is an optical disk, the quarter-wave plate 17 is located in the optical path, so that the beam 13 reflected by the disk 16 and incident on the polarizing beam splitter 14 is The plane of polarization is orthogonal to the plane of polarization of the beam 13 at the time it exits from the light source.

For this purpose, the beam 13 reflected by the disk 16 is
All of the light does not return to the light source 11, but enters the half-wave plate 21. Since the polarization plane of the beam 13 is rotated by 45° by the half-wave plate 21, the beam 13 is divided into three equal parts by the polarizing beam splitter 22.

Therefore, a reproduction signal is obtained from the sum of the detection outputs of the photodetectors 25 and 26, and a focus error signal is obtained from the detection output of the photodetector 25 by the astigmatism method using the cylindrical lens 24. There is.

Although not shown, a diffraction grating is arranged between the light source 11 and the collimator lens 12, and the beams 13 split by the diffraction grating and incident on the photodetector 25 are as described above. The tracking error signal is obtained by the following method.

On the other hand, when the disk 16 is a magneto-optical disk, the quarter-wave plate 17 is not located in the optical path, so the plane of polarization of the beam 13 reflected by the disk 16 and incident on the polarizing beam splitter 14 is Compared to the polarization plane of the beam 13 at the time it is emitted from the light source, it is rotated by an amount modulated by the recorded information.

Of the beam 13 incident on the polarizing beam splitter 14, 100% of the component with a rotated polarization plane is incident on the half-wave plate 21, but the remaining component with an unrotated polarization plane is also 50 to 10%. % is incident on the half-wave plate 21. For this reason, even if the degree of modulation due to recorded information is not very high, the beam 13 incident on the half-wave plate 21
The overall light intensity will not be very low.

If the degree of modulation by the recorded information is not very high, the beam 13 will be divided into approximately two equal parts by the polarizing beam splitter 22, but in the case of a magneto-optical disk, the difference in the detection outputs of the photodetectors 25 and 26 will cause the beam 13 to be divided into two equal parts. A reproduced signal can be obtained. Note that the focus error signal and tracking error signal are obtained in the same manner as in the case of an optical disc.

Note that since recordable and reproducible optical discs generally have a low reflectance R of 10 to 20%, there is little problem that the DC fluctuation rate ε of the reproduced signal becomes large.

However, in the polarizing beam splitter 14, the reflectance for a beam having a polarization plane in the same direction as the linearly polarized beam 13 emitted from the light source 11 is only 50 to 10%, whereas the polarization plane of the beam 13 is Since the reflectance for a beam having a plane of polarization perpendicular to
The amount of incident light becomes 2 to 10 times greater.

Therefore, when the reflectance R is low and there is an upper limit to the intensity of the beam 13 incident on the recording medium during reproduction due to thermal problems, as in the case of the above-mentioned recordable and reproducible optical disc, the quarter-wave plate 17 By using this, the S/N of the reproduced signal is improved.

Next, a mechanism for positioning the quarter-wave plate 17 in the optical path and removing it from the optical path as described above will be explained.

2 and 3 show the optical system. In this optical system 27, the objective lens 15 is attached to a bobbin 28, and the bobbin 28 is attached so as to be slidable and rotatable about the shaft 31 so as to enable focusing and tracking operations.

This shaft 31 is attached to a main base 32. A sub-base 33 faces the main base 32, and a sliding plate 34 made of a temporary spring material is held between the main base 32 and the sub-base 33 so as to be linearly slidable. .

The quarter-wave plate 17 is adhered to the sliding plate 34, and a protrusion 34a having a chevron-shaped cross section is formed at one end. On the other hand, the sub-base 33 has two holes 33a lined up in the sliding direction of the sliding plate 34.

33b is formed.

When the protrusion 34a is in click engagement with the hole 33a, the quarter-wave plate 17 is located in the optical path of the beam 13. Further, when the sliding plate 34 slides and the protrusion 34a is elastically deformed, the engagement between the protrusion 34a and the hole 33a is released, and then the protrusion 34a is click-engaged with the hole 33b. The half-wave plate 17 has been removed from the optical path of the beam 13.

4 to 6 show a drive device 35 for sliding the sliding plate 34 as described above. The drive device 35 includes a control motor 36 and a spur gear 37a rotated by the control motor 36.

It consists of 37b and 37C. Note that the control motor 36
A plunger or the like may be used instead.

One bottle 41 is planted on the surface of the spur gear 37c. On the other hand, the other end of the sliding plate 34 is formed with a U-shaped cutout 34b having a 45° entrance. Then, with the pin 41 and the cutout portion 34b engaged, the spur gear 37
As C rotates back and forth, the sliding plate 34 linearly slides back and forth.

Note that one bottle 42 is also planted on the back surface of the spur gear 37C. Also, a control motor 3 is located near the spur gear 37c.
Two limit switches 43 and 44 are arranged to control 6. When the spur gear 37C rotates and the pin 42 operates the limit switch 43 or 44, the reciprocating sliding stroke of the sliding plate 44 is controlled.

FIGS. 7 and 8 show a recording/reproducing apparatus having an optical head as described above. In this recording/reproducing apparatus, a disk 16 is rotationally driven by a spindle motor 45.
A guide shaft 46 is provided to guide the optical system 27 in the radial direction.

On the other hand, the driving device 35 shown in FIGS. 4 to 6 is fixed near the spindle motor 45 in the main body of the recording/reproducing apparatus, and does not move like the optical system 27. Only when the optical system 27 is located at a position corresponding to the innermost circumference of the disk 16, the cutout 34b of the sliding plate 34 and the spur gear 37
The pin 41 of c is engaged.

By the way, the recording/reproducing apparatus of this embodiment always returns to the position corresponding to the innermost circumference of the disk when loading or ejecting the disk. On the other hand, the disks 16 on which recording and playback are performed in this recording/playback device are housed in disk cartridges of the same size and shape. A detected portion such as a cutout portion indicating whether it is a surface rotation type is provided.

That is, at the same time as the disc 16 is loaded into the recording/reproducing device, the detection target section -1- is automatically detected, but at that time, the optical system 27 is located at a position corresponding to the innermost circumference of the disc 16. There is. In other words, since the cutout portion 34b and the bottle 41 are engaged, the sliding portion of the sliding plate 34 and the quarter-wave plate 7 are moved based on the detection result of the detected portion described above. be exposed.

Since the drive device 35 is not fixed to the optical system 27 in this way, the optical system 27 can be moved easily and the optical system 27 can be moved at high speed.

In this embodiment, the quarter-wave plate 17 is moved only depending on the type of disc 16, but at least when reproducing a recording medium of variable reflectance type, the quarter-wave plate 17 is moved in the optical path. It suffices if the quarter-wave plate 17 is located in the optical path and removed from the optical path at least when reproducing a recording medium of a rotating polarization plane type.

〔Effect of the invention〕

In the optical head according to the invention, when detecting a beam reflected by a variable reflectance recording medium, at most only 50% of this reflected beam returns to the light source.

Therefore, DC fluctuations in the tracking error signal caused by skew of the recording medium in the direction along the recording track can be suppressed, and accurate tracking control can be performed.

Furthermore, as mentioned above, when detecting a beam reflected by a recording medium with variable reflectance, only 50% of the reflected beam returns to the light source, so it cannot be detected by a photodetector. The amount of light that can be produced is large, and the S/N of the reproduced signal is good.

Furthermore, when detecting a beam reflected by a recording medium with a rotating polarization plane, a quarter-wave plate is not used. Recorded information can also be reproduced from recording media.

[Brief explanation of the drawing]

The drawings show an embodiment of the present invention; FIG. 1 is a side view showing the optical path in the optical head, FIG. 2 is a side sectional view of the optical system, and FIG. 3 is a cross section taken along line mm in FIG. 2. 4 is a plan view of the drive device, FIGS. 5 and 6 are side and bottom views of the main parts of the drive device, and FIGS. 7 and 8 are a plan view and side view of the recording and reproducing device, respectively. It is a diagram. In addition, in the symbols used in the drawings, 11-----1111--Light source 13-...-----Beam 14-11---------- Polarizing beam splitter 16-----------Disc 17--
-----------1---- Quarter wavelength plate 35---
----------------It is a driving device.

Claims (1)

[Scope of Claims] A method in which a beam emitted from a light source is divided into a plurality of beams, which are incident on a recording medium, and a tracking error signal is obtained based on the detection output of each of the plurality of beams reflected by the recording medium. The optical head is arranged in an optical path between the light source and the recording medium, and reflects at least 50% of the beam having a polarization plane perpendicular to the polarization plane of the beam at the time it is emitted from the light source. a polarizing beam splitter, and when the recording medium is a recording medium of variable reflectance type and a beam reflected by this recording medium is to be detected, a polarizing beam splitter having a polarizing beam splitter that In addition to positioning a half-wavelength plate, if the recording medium is a polarization plane rotation type recording medium and a beam reflected by this recording medium is to be detected, the polarization beam splitter is positioned between the recording medium and the polarization beam splitter. and a drive device for moving the quarter-wave plate out of the optical path.