JPS60237648A - Optical pickup device - Google Patents

Abstract

PURPOSE:To improve noise characteristics by controlling the output of a semiconductor laser so that the detection output of a photodetector which detects the quantity of transmitted light of a variable optical attenuating element installed in an optical system is a specific value. CONSTITUTION:The projection light of the semiconductor laser 1 passes through a collimator lens 2 and the variable optical attenuating element 14 such as a TN type liquid crystal element and is reflected by a beam splitter 3 to enter the photodetector 15. Its output is inputted to a laser output control circuit 16, which controls the driving current of the laser 1 so that the output of the photodetector 6 is constant. Consequently, constant light power is obtained on a recording medium 6, so the S/N ratio during reproducing operation is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical pickup device that uses a semiconductor laser and is suitable for an information recording device that optically writes and reproduces information.

(Prior Art and its Problems) As a high-density recording device, an optical recording device that performs recording and reading by condensing a minute spot onto a recording medium such as a metal thin film or an organic thin film has been put into practical use.

FIG. 1 shows the basic configuration of an optical pickup used in such optical recording. The emitted light of the semiconductor laser 1 is converted into a scollimated light by a lens 2, passed through a polarizing beam splitter 3 and an wavelength plate 4, and is then converted into a siliform light by a lens 5.
The light is focused as a spot. The reflected light from the medium is passed through the lens 5. It passes through the wavelength plate 4 again, but since it passes through the wavelength plate twice and the polarization direction has been rotated by 90', it is reflected by the polarization beam splitter 3 and enters the detector 7. Adjust the output of the semiconductor laser according to the characteristics of the recording medium so that the light intensity is about 10 to 15 mW on the medium when writing to the recording medium, and about 05 to 3 mW when reproducing. do.

In this configuration, the polarizing beam splitter and the wavelength plate prevent reflected light from the medium from returning to the semiconductor laser. However, the surface of a recording medium is generally covered with a polymeric resin such as acrylic or polycarbonate, and such polymeric resins have slight birefringence due to stress during manufacturing. As a result, the separation of reflected light may not be sufficient.
A slight return light is generated in the semiconductor laser. This light returned to the laser increases the noise of the output light.

FIG. 2 shows an example of the dependence of the noise characteristics of a semiconductor laser on the amount of returned light. The amount of returning light represents the ratio of the amount of light returning to the laser end face to the total amount of light emitted from the front end face of the laser. The S/N is a value at a bandwidth of IQKHz. Reproduction of analog signals such as image information requires an S/N of 95 dB or more.

In the configuration shown in Figure 1, the amount of light returned to the semiconductor laser is 0.
．． Approximately 1 to 1% is sufficient, and is sufficient for analog signal playback.
/N is difficult to secure.

Return to the semiconductor laser) As a method of reducing the amount of light, it is conceivable to insert an optical attenuator with a fixed amount of optical attenuation between the lens 2 and the polarizing beam splitter 3.

For example, if you use an attenuator with a transmittance of 1/1o → return the light amount to 10
0 and less than 0.01% From Figure 2, S/N 105
dB. However, in this case, the semiconductor laser needs to have 10 times the output, and an output of 100 mW or more is required when writing information, and it is difficult to realize a laser with such high output and high reliability.

(Objective of the Invention) An object of the present invention is to eliminate the drawbacks of the prior art and provide an optical pickup device for recording and reproducing with good noise characteristics.

(Structure of the Invention) The present invention includes a light source including a semiconductor laser, a photodetector, and an optical system that focuses light from the light source and guides it to a recording medium, and guides reflected light from the recording medium to the photodetector. an optical pickup device comprising: a variable optical attenuation element installed in the optical system; a photodetector for detecting the amount of transmitted light of the variable optical attenuation element; and a control circuit that controls the output of the semiconductor laser.

(Detailed Description of Configuration) In an optical pickup device that performs self-recording and reproduction on a recording medium, since the semiconductor laser is driven by a square wave during the self-recording operation on the medium, there is sufficient tolerance against noise. The required S/N during reproduction operation differs depending on whether the reproduction signal is digital or analog. For digital signals, it is sufficient to have an S/N of the same level as during a write operation, but a high S/N is required during an analog signal reproduction operation.

As a method of increasing the S/N during reproduction operation, the present invention uses the collimating lens 2 and the polarizing beam splitter 3 shown in FIG.
A variable optical attenuation element whose transmittance changes is inserted between the two. The amount of returned light can be reduced by increasing the transmittance during a recording operation and lowering the transmittance during a reproducing operation.

As shown in FIG. 3, the variable optical attenuation element has an optical path 11
The simplest configuration is to take the optical filter 12 in and out.

FIG. 4 shows a method in which a wavelength plate 13 is inserted into the optical path 11 and the amount of light is changed by rotating the wavelength plate.

The light emitted from a semiconductor laser is generally linearly polarized light, and the direction of polarization can be rotated by an arbitrary angle depending on the set angle of the optical axis and the wavelength plate. When the polarization direction is rotated, only the component in the polarization direction before rotation is transmitted by the next polarizing beam splitter, so the amount of transmitted light is attenuated. The optical wavelength plate similarly applies polarization rotation to the returned light. Since a semiconductor laser is affected by the polarization perpendicular to the emitted light and only by the component in the polarization direction before rotation, the amount of returned light decreases to k. In order to ensure operation as an attenuation element, an analyzer that transmits only polarized light components in one direction may be inserted between the collimating lens and the wavelength plate. In this method, in order to achieve a transmittance of 10%, it is sufficient to rotate the polarization direction by 840 degrees, and for this purpose, it is sufficient to rotate the wavelength plate by 42 degrees. The wavelength plate is set such that the rotation angle is 00'' during the recording operation and the rotation angle is 42'' during the reproduction operation.

A function similar to the rotational operation of the wavelength plate can also be realized by a TN type liquid crystal element. As shown in FIG. 5, the TN type liquid crystal element 14 exhibits polarization rotation in transmitted light when no voltage is applied, and no longer exhibits polarization rotation when a voltage is applied. Therefore, it can be used as a variable optical attenuation element.

The problem here is that the switching time required to change the transmittance of these variable attenuation elements is long. For example, a TN type liquid crystal element requires a switching time of 10 to 30 milliseconds.

It is possible to generate polarization rotation using a crystal that exhibits an electro-optic effect such as lithium niobate, which has a fast switching time, but it requires a high voltage of several hundred volts or more to generate polarization rotation of nearly 900 degrees. Not practical.

Problems caused by a long switching time are that the optical power becomes unstable on the recording medium during the switching operation, and that the S/N ratio decreases due to an increase in the amount of returned light due to an increase in transmittance. These problems are solved as follows.

In order to stabilize the optical power, the amount of light transmitted through the variable attenuation element may be detected and the output of the semiconductor laser may be controlled so that the amount of light is always constant on the recording medium.

FIG. 6 shows changes in transmittance of the variable optical attenuation element, changes in laser output, and changes over time in optical power on the surface of the recording medium. Figure 6B
The transmittance changes with a certain switching time width as shown in FIG. Even during the switching operation, the laser output is controlled according to the change in transmittance as shown in FIG. 6C so that the optical power remains constant on the surface of the recording medium. As a result, the same optical power can be obtained on the recording medium surface during the switching operation as shown in FIG. 6A as during the reproducing operation.

Optical recording generally involves rotating a disk-shaped recording medium to record and reproduce information on concentric or spiral tracks. The rotation speed is 600 to 1800 revolutions per minute. In order to perform a reproduction operation and a recording operation, the optical pickup must be moved to a desired track position and the address information indicating the track position recorded on the disc must be confirmed in advance.

To confirm the address information, the disk needs to rotate more than once, and even with a disk rotating at 1800 revolutions per minute, the disk must rotate more than once.
It takes more than 3 milliseconds. If the address information is recorded digitally, a high S/N ratio is not required, so switching can be completed during the address information confirmation operation. During the switching operation from recording operation to playback operation, playback with a poor S/N ratio is performed for one rotation, but
In analog image playback, only the first screen is low S/
Since it is only N, there is no problem in visually checking the recorded information of images reproduced at 30 screens per second.

(Example) As a detector for detecting the transmittance of a variable optical attenuation element, the first
By sharing the photodetector 7 shown in the figure for signal detection, the transmittance can be obtained from the amount of light reflected from the recording medium. However, with this method, it is difficult to obtain accurate transmittance because the reflectance varies depending on the presence or absence of recording on the recording medium.

In a TN type liquid crystal element shown in FIG. 5 which uses polarization rotation, a polarization beam splitter 3 reflects the attenuated amount of light. Therefore, by detecting this reflected light with a photodetector 15 as shown in FIG. 7, the transmittance of the variable light attenuation element can be obtained. The transmittance control circuit 16 controls the drive current of the semiconductor laser to obtain a constant optical power on the recording medium.

FIG. 8 shows an example of the configuration of the control circuit 16. How the photodetector in the package of the semiconductor laser 1 comes out and the photodetector 15
A comparator 17 compares the outputs of the two to calculate the power on the disk surface. A comparator 18 compares the output of 17 with a reference signal 21 to obtain an error signal, and a drive circuit 19 performs feedback control.

The above operations work during regeneration and switching operations. During the recording operation, the switch 2o is switched to drive the semiconductor laser using the recording signal 22.

(Effects of the Invention) As explained above, the present invention improves the S/N during playback operation.
It is possible to obtain an optical pickup device for recording and reproducing which has good performance.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the basic configuration of an optical pickup, Figure 2 is a diagram showing the dependence of semiconductor laser noise on the amount of reflected light, Figures 3 to 5 are diagrams showing examples of the configuration of a variable optical attenuation element, FIG. 6 is a diagram showing the operation of the present invention, FIG. 7 is a diagram showing an embodiment of the present invention, and FIG. 8 is a diagram showing an example of the configuration of a control circuit. In the figure, l...semiconductor laser, 2... collimating lens,
3... Polarizing beam splitter, 4... Wavelength plate, 5
- Lens, 6 Recording medium, 7 Photodetector,
11... Optical path, 12... Optical filter, 13...
Wing wavelength plate, 14... TN type liquid crystal element, 15... Photodetector, 16... Laser output control circuit, 17.18-.
- Comparator, 19...drive circuit, 20...switch. r (byakko, a well-versed patent attorney, admiration and enemy 1 mouth!!; 2 back to figure's amount of light)

Claims (1)

[Claims] An optical pickup comprising a light source made of a semiconductor laser, a photodetector, and an optical system that focuses light from the light source and guides it to a recording medium, and guides reflected light from the recording medium to the photodetector. In the apparatus, a variable optical attenuation element installed in the optical system, a photodetector for detecting the amount of transmitted light of the variable optical attenuation element, and a photodetector for detecting the amount of transmitted light of the semiconductor laser so that the amount of transmitted light becomes a predetermined value. An optical pickup device comprising: a control circuit for controlling output.