JPS61198456A - Optical pickup - Google Patents

Abstract

PURPOSE:To attain compact constitution and to simplify optical adjustment by constituting an optical pickup of a means for dividing a luminous flux into two and leading them to difference components of a photodetector and a means for detecting differentially an information signal from output of each component of the photodetector. CONSTITUTION:The luminous flux irradiated from an LD 15 is collimated by a collimator lens 16, reflected in the 1st beam splitter 17 and focused on a recording medium 19 as a minute spot by an objective lens 18. The reflected luminous flux from the recording medium 19 passes again through the objective lens 18, the direction of polarized plane is rotated by 45 deg. through the 1st beam splitter 17 and a 1/4 wavelength plate 23 and the result is made incident on the 2nd beam splitter 20 and separated into two optical fluxes. The 1st split luminous flux 21 is condensed toward a light sensor 24 by a condenser lens 25. The 2nd split luminous flux 26 is reflected in the reflecting section 27 at the bottom of the beam splitter 20 and condensed on the light sensor 24 by the condenser lens 25. Thus, the optical pickup is constituted compactly and the S/N of the signal detection is improved by differential detection.

Description

[Detailed Description of the Invention] [Technical Field] The present invention relates to a recording medium on which information is recorded magnetically [Prior art] In recent years, so-called magneto-optical recording, in which information is recorded and reproduced optically and is rewritable, has been developed. Research and development of media and optical pickups for reproducing information from such media are actively being conducted. Signal reproduction from the above-mentioned magneto-optical recording medium is usually performed using magneto-optic effects called the Kerr effect or the Faraday effect. In other words, the light irradiated onto the medium is
The plane of polarization is rotated according to the recorded information and reflected or transmitted, and the rotational component is converted into intensity modulation using an element such as a polarizing plate for signal detection. Furthermore, the rotation angle of this plane of polarization is approximately 1°, and therefore the signal component obtained through the polarizing element is minute, and several improvements have been made to light pickup for signal detection.

FIG. 8 is a schematic diagram showing an example of the configuration of a conventional optical pickup.
The luminous flux emitted from 1 is.

The collimator lens 2 converts the light into a parallel beam.

The parallel light flux then passes through the beam splitter 3 and is focused by the objective lens 4 onto a minute spot of approximately φ14 m on the recording medium 5. The light beam reflected from the recording medium 5 undergoes polarization plane modulation due to the Kerr effect and Faraday effect, passes through the objective lens 4 again, and is sent to the beam splitter 3.
The 0-separated luminous flux separated from the incident luminous flux by the second
It is partially reflected by the beam splitter 6 of the lens system 7.
The light enters the optical sensor 8 through the. The lens system 7 is composed of a well-known system, such as an astigmatism system, a Naifetsu system, or a Two-two prism system, and collects information on the distance between the recording medium 5 and the objective lens 4, that is, autofocus (hereinafter referred to as AF) error. A signal is obtained from an optical sensor.

Also, this and the well-known method such as push-pull method cause a discrepancy with the information track, that is, auto tracking (hereinafter referred to as AT).
An error signal is obtained. These error signals are fed back to a drive system (actuator) of the objective lens (not shown) to perform accurate track tracing at an accurate focus position and record/reproduce the signals.

The remaining light flux passing through the second beam splitter 6 is 1
The light passes through a 72 wavelength plate 9 and is split into two in two directions by a polarizing beam splitter 10.

When the optical crystal axis of the 1/2 wavelength plate 9 is inclined by 22.5 degrees with respect to the polarization axis of the incident light beam, the amount of light split into two by the polarizing beam splitter 10 is equal, and the polarizing plate is used to separate each light beam. This is equivalent to having transmission axes of 45° and -45°. The two divided luminous fluxes are each focused on a signal detection sensor 13.14 by a sensor condenser lens 11.12. Information on the recording medium can be detected by subtracting the electrical signals from the signal detection sensors 13 and 14 (differential detection).

The advantages of this differential detection method will be explained below. Figure 9 (A)
, (B) show the signal amplitude components that are split by the 172-wave plate 9 and the polarizing beam splitter 10 and reach the sensor 14, 13, respectively. The light beam reflected from the recording medium 5 whose vertical axis is the polarization direction of the incident light beam has a polarization plane of θK depending on the direction of the axis of the magneto-optical pattern (upward or downward).
Or rotate to =θ. The combination of the 1/2 wavelength plate 9 and the polarizing beam splitter 10 is equivalent to a system in which polarizing plates are arranged with the transmission axis tilted at 45°.
The difference between the projection components S1 and S'l on the axis of the tilted broken line becomes the signal amplitude component. θ changes over time due to the magneto-optical pattern, so the signal strength changes are shown in Figure 1O (A
) and (B), the phases of the divided light beams are shifted by 180°.

The light is received by these optical signal sensors 13 and 14. Although the phases of the magneto-optical signals are reversed as described above, noise components (noise from the recording medium, fluctuation noise of the LD light) are usually superimposed on these signals, and these noise components become in phase.

Therefore, when the signals obtained from the sensors 13 and 14 are differentiated, the signal components strengthen each other and the noise components are reduced. If the optical system is arranged correctly, the signal components S1 and S'1 obtained from each sensor are equal and the noise amplitudes are also equal, so the signal is doubled and the noise is zero. In this way, the differential detection method shown in FIG. 8 has the advantage of being able to detect signals with a good S/N ratio.

However, the conventional optical pickup as described above has many parts, which is disadvantageous in terms of miniaturization and cost reduction. Furthermore, it is necessary to accurately position a plurality of photodetectors, and adjustment is also complicated.

[Summary of the invention]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical pickup which eliminates the above-mentioned drawbacks of the conventional example, can be constructed compactly, and allows easy optical adjustment.

The above-mentioned object of the present invention is to provide a single photodetector having a light-receiving surface divided into a plurality of parts, and a light beam from a recording medium on which information is magnetically recorded, which is subjected to intensity modulation of opposite phase according to the information. An optical pickup may be constructed from means for dividing the light beam into two light beams and guiding the divided light beams to different parts of the photodetector, and means for differentially detecting information signals from the outputs of the respective parts of the photodetector. achieved by.

〔Example〕

FIG. 1 shows an example of the configuration of the optical pickup of the present invention.
The luminous flux emitted from the collimator lens 16
The light becomes a parallel beam of light, is reflected by the first beam splitter 17, and is focused by the objective lens 18 onto a recording medium 19 as a minute spot. The reflected light flux from the recording medium 19 passes through the objective lens 18 again and passes through the first beam splitter 1.
7, passes through the 1/2 wavelength plate 23, rotates its polarization plane direction by 45 degrees, enters the second beam splitter 20, and is separated into two beams. Second beam splitter-2
The first divided light beam 21 reflected at 0 passes through the focusing lens 25
The light is focused toward the optical sensor 24. The other second divided light beam 26 is reflected by a reflecting portion 27 on the bottom surface of the beam splitter 20 and is focused by a condensing lens 25 toward the optical sensor 24 .

In the optical sensor 24, the divided light beams 21 and 26 are connected to a converging lens 25.
It is placed at a position away from the focal plane where the light is focused. For example, as shown in FIG. 2(A), the optical sensor 24 has a light receiving section divided into 12 on its light receiving surface.

The principle by which detection of AF and AT error signals and differential detection of information signals can be performed with the configuration shown in FIG. 1 will be explained below.

FIG. 3 shows the principle of AF multiplied signal detection. 1st
Only the configuration necessary for AF detection in the figure is shown in FIG. When the recording medium 19 is located at the focal plane of the objective lens 18, the light ray moves away from the black surface as shown by the solid line, and the light ray converges in front of the optical axis of the focusing lens 25 as shown by the broken line. When the recording medium approaches the focal plane of the objective lens 18, the light beam is focused beyond the optical axis of the focusing lens 25. Therefore, when the optical sensor 24 is placed offset from point F, the distribution of each divided light beam on the optical sensor 24 surface becomes smaller or larger depending on the position of the recording medium.

First, a case where the light receiving section of the optical sensor 24 is configured into 12 sections as shown in FIG. 2(A) will be explained. The shaded area in the figure shows the part of the light beam from the optical sensor when the objective lens 18 and the recording medium 19 are in focus.The outputs from each light receiving section are now labeled IA, IB, IC, and ID, respectively. , I.E., I.
F, IG, IH.

If II, IJ, IK, IL, then AF error signal IA
F is IAF= (IA+IB+IE+IF+IG+I
H+IK+IL)-(IC+ID+II+IJ).

In addition, in the case of an optical sensor divided into concentric light receiving sections as shown in Fig. 2 (B), the respective outputs are IA, IB, and IA.
Let them be C, ID, IE, IF, IG, and IH.

The AF error signal IAF is IAF = (IA+IB+IE+IF) (IC+
ID+IG+IH).

Note that the AF error signal can be obtained by calculating only the light beam on one side. In the example of Figure 2 (A), IAF=(
It can also be obtained as IA+IB+IE+IF)-(IC+ID).

Next, the AT double signal detection principle will be explained.

A groove with a depth of approximately 1/8 wavelength is usually provided near the recording medium surface of the recording medium 19. Signals are recorded and reproduced using this groove as a guide, and the light beam reflected from this groove is returned to the objective lens. As is well known, the far field pattern formed through the light beam 18 changes depending on the positional relationship between the light spot and the groove.

The situation is shown in FIG. 4. The upper part of FIG. 4 shows the positional relationship between the groove and the light spot, and the lower part shows the intensity distribution of the far field pattern.

Therefore, in the case of the split sensor shown in FIG. 2(A), T −T'
By matching the direction of the groove to the direction in which the groove runs (signal track direction), the AT error signal IAT is obtained as follows.

Also, in the case of the split sensor shown in Fig. 2 (B).

IAT= (IA+IC+IE+IG) -(IB+I
D+IF+IH).

Even in the AT multiplied signal, an error signal can be obtained by calculating the output of a light receiving section that receives only one beam.

Next, the reason why magneto-optical information signals can be differentially detected will be explained.

In FIG. 1, the reflected light from the recording medium 19 enters the half-wave plate 23 with its polarization plane rotated by θK or −θ due to the magnetic pattern (upward or downward direction of the magnetic domain) due to the magneto-optic effect. The half-wave plate is arranged with its crystal axis at an angle of 22.5° with respect to the incident plane of the beam splitter 20. When the beam splitter 20 transmits and reflects two beams, each of the transmitted and reflected beams can be detected by an optical sensor as a signal whose phase is reversed, as shown in FIG.

In FIG. 5, the X axis is the reflection of the polarizing beam splitter, and the Y axis is the axis showing the transmitted component.

A light flux that has been rotated by θK or −0 from the polarization plane of the light incident on the medium is rotated by 45° in the polarization plane by a half-wave plate (the dashed line in FIG. 5 indicates the polarization plane).

After that, it is reflected by the polarizing beam splitter 20.

Since it is divided into two by transmission, its fluctuation amplitude (S2', S2)
are equal, resulting in two beams with opposite phases. Therefore, by detecting these two beams with the optical sensor 24, the information recorded on the recording medium can be read out. That is, Fig. 2 (A
), the magneto-optical information signal IS is I s= (IA+IB+4C+ID+IE+IF)
-10 (I G+ I H+ I engineering + I actually + IL) Figure 2 (
For the optical sensor in B), Is= (IA+IB+IC+ID)-(IE+IF+
You can get it with IG+IH).

Furthermore, by integrating each optical element in Fig. 1, optical adjustment during assembly becomes easier, axis misalignment during use is also prevented, and highly reliable optical pickup can be achieved.For example, Fig. 6 shows Beam splitter 17, l/2 wavelength plate 2
3. This is an example in which the beam splitter 20 is integrated with adhesive or the like.

In addition, in FIG. 7, the 1/2 wavelength plate 23 is connected to the beam splitter 1.
It is placed immediately after the reflective surface of 7.

The same parallelogram blocks 30 and 31 can be used, and element processing can be simplified.

In addition, in order to detect AF, AT error signals, and information signals, it is necessary to calculate the signals obtained from each light receiving section, but if the optical sensor includes a differential AMP for calculation in addition to the light receiving section, disturbances may occur. Signal detection that is resistant to noise interference is possible.

〔Effect of the invention〕

As explained above, in the present invention, the light beam from the recording medium is divided and differential detection is performed using a single photodetector with a divided light receiving surface.1. The optical pickup can be configured compactly.

2. Improve the S/N ratio of signal detection by differential detection.

3. The number of parts can be reduced and costs can be lowered.

4. By integrating parts, a more reliable pickup can be realized.

5. Since there is only one photodetector, there is less need to consider variations in characteristics between detectors, and position adjustment is easy.

6. By creating arithmetic circuits for each signal on the same chip as the photodetector, signal detection that is resistant to disturbance noise is possible.

Effects such as this can be obtained.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing a configuration example of an optical pickup according to the present invention, FIG. 2 is a diagram showing a light receiving surface of a photodetector used in the present invention,
Fig. 3 is a diagram explaining the principle of detecting the AF multiplied signal in the present invention, Fig. 4 is a diagram explaining the principle of detecting the AAT signal in the present invention, and Fig. 5 is a diagram explaining the principle of detecting the information signal in the present invention. FIGS. 6 and 7 are schematic diagrams showing modified examples of the optical pickup shown in FIG. 1, FIG. 8 is a schematic diagram showing an example of the configuration of a conventional optical pickup, and FIGS. FIG. 3 is a diagram illustrating the principle of differential detection. 15--------1 semiconductor laser 16-1--
-------- Collimator lens 17.20 ----- Beam splitter 18 -------- One objective lens 19 ----- One -- Recording medium 23 ----- --------1/2 wavelength plate 24---
-----------One light sensor 25------------One focusing lens 27---One reflection part Fig. 2゛(A>(B)/7'(A ) (B)
(C) Figure 7 Figure 70 ゛

Claims (1)

[Claims] (1) A single photodetector with a light-receiving surface divided into multiple parts and a light beam from a recording medium on which information is magnetically recorded are divided into two light beams that are intensity-modulated in opposite phases according to the information. an optical pickup comprising means for guiding the divided light beams to different parts of the photodetector, and means for differentially detecting information signals from the outputs of the respective parts of the photodetector.