JPH0799589B2 - Optical pickup device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical pickup device suitable for an information recording device that optically writes and reproduces information by using a semiconductor laser.

(Prior Art and Its Problems) As a high-density recording device, an optical recording device for recording and reading by converging a minute spot on a recording medium of a metal thin film or an organic thin film has been put into practical use.

The basic structure of an optical pickup used in such optical recording is shown in FIG. The light emitted from the semiconductor laser 1 is made into collimated light by the lens 2, passed through the polarization beam splitter 3 and the 1/4 wavelength plate 4, and condensed by the lens 5 as a spot on the recording medium 6. The light reflected from the medium passes through the lens 5 and the 1/4 wavelength plate 4 again, but passes through the 1/4 wavelength plate twice and is reflected by the polarization beam splitter 3 because the polarization direction is rotated by 90 °. It is incident on the detector 7. When writing on the recording medium, make the light intensity of about 10 to 15 mW on the medium,
Also, the output of the semiconductor laser is adjusted according to the characteristics of the recording medium so that the light intensity is about 0.5 to 3 mW during reproduction.

In this configuration, the polarization beam splitter and the quarter-wave plate prevent the reflected light from the medium from returning to the semiconductor laser. However, the surface of the recording medium is generally covered with a polymer resin such as acrylic or polycarbonate, and such a polymer resin has a slight birefringence due to stress during manufacturing. Therefore, the reflected light is not sufficiently separated, and a slight return light is generated in the semiconductor laser. The return light to this laser increases the noise of the output light.

FIG. 2 shows an example of the returning light amount dependency of the noise characteristics of the semiconductor laser. The amount of returned light represents the ratio of the amount of light returned to the laser end face to the total amount of light emitted from the front facet of the laser. S / N is the value at a bandwidth of 10 KHz. S / N of 95 dB or higher is required to reproduce analog signals such as image information.

In the configuration shown in FIG. 1, the amount of light returned to the semiconductor laser is 0.
It is about 1 to 1%, and it is difficult to secure a sufficient S / N during analog signal reproduction.

As a method of reducing the amount of light returning to the semiconductor laser, it is conceivable to insert an optical attenuator having a fixed optical attenuation amount between the lens 2 and the polarization beam splitter 3. For example, transmittance 1/10
With the attenuator, the amount of returned light is 1/100, which is 0.01% or less, which is S / N 105 dB from Fig. 2. However, at this time, the semiconductor laser needs to output 10 times more power,
Since an output of 0 mW or more is required, it is difficult to realize such a high output and highly reliable laser.

(Object 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 / reproducing with good noise characteristics.

(Structure of the Invention) According to the present invention, a light source including a semiconductor laser, a first photodetector, and light from the light source are narrowed down and guided to a recording medium,
An optical pickup device including an optical system that guides reflected light from the recording medium to the first photodetector, the optical pickup device being installed in the optical system and increasing a transmittance during a recording operation on the recording medium. A variable light attenuating element that increases the amount of transmitted light and reduces the amount of light returning to the light source during reproduction operation; a second photodetector that detects the amount of transmitted light of the variable light attenuating element; And a control circuit for controlling the output of the semiconductor laser so that the amount of transmitted light becomes a predetermined value.

(Detailed description of configuration) In an optical pickup device for recording / reproducing on / from a recording medium, since the semiconductor laser is driven by a square wave during recording operation on the medium, the noise tolerance is large and the S / N of optical output is large. Is about 60 to 70 dB. The required S / N at the time of reproduction operation differs depending on whether the reproduction signal is digital or analog. Although the S / N ratio of the digital signal is the same as that of the writing operation, a high S / N ratio is required in the reproducing operation of the analog signal.

According to the present invention, the first method is to increase the S / N ratio during the reproducing operation.
A variable optical attenuator whose transmittance changes is inserted between the collimating lens 2 and the polarization beam splitter 3 in the figure.
The amount of return light may be reduced by increasing the transmittance during the recording operation and decreasing the transmittance during the reproducing operation.

As the variable optical attenuating element, as shown in FIG. 3, the simplest structure is to put an optical filter 12 in and out of the optical path 11.

FIG. 4 shows a method in which the half-wave plate 13 is inserted in the optical path 11 and the wavelength plate is rotated to change the light amount. The emitted light of the semiconductor laser is generally linearly polarized light, and the polarization direction can be rotated by an arbitrary angle depending on the set angle between the optical axis and the half-wave plate. When the polarization direction is rotated, only the component of the polarization direction before rotation is transmitted by the next polarization beam splitter, so that the transmitted light amount is attenuated. The 1/2 wave plate similarly imparts polarization rotation to the returning light. The semiconductor laser is less likely to be affected by the polarization orthogonal to the emitted light, and is affected only by the component in the polarization direction before rotation, so that the amount of returned light is reduced. In order to ensure the operation as the attenuator, an analyzer that transmits only the polarized component in one direction may be inserted between the collimating lens and the half-wave plate. In this method, in order to obtain a transmittance of 10%, the polarization direction should be rotated by 84 °, and for this purpose, the 1/2 wavelength plate should be rotated by 42 °. The half-wave plate is set so that the rotation angle is 0 ° during the recording operation and the rotation angle is 42 ° during the reproducing operation.

The same operation as the rotation operation of the half-wave plate can be realized by the TN type liquid crystal element. As shown in FIG. 5, the TN type liquid crystal element 14 shows that transmitted light shows polarization rotation when no voltage is applied,
When the voltage is applied, it does not show polarization rotation. Therefore, it can be used as a variable optical attenuator.

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.

As a fast switching time, it is possible to cause polarization rotation by using a crystal that exhibits an electro-optical effect such as lithium niobate, but a high voltage of several hundred volts or more is required to cause polarization rotation near 90 °. Therefore, it is not practical.

The problem with the long switching time is that the optical power becomes unstable on the recording medium during the switching operation and the S / N decreases due to the increase in the amount of returning light due to the increase in the transmittance. These problems are solved as follows.

In order to stabilize the optical power, the amount of light transmitted through the variable attenuator 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 the transmittance and laser output of the variable optical attenuator and changes in the optical power on the surface of the recording medium with time. Fig. 6B
As described above, the transmittance changes with a certain switching time width. The laser output is controlled according to the change in transmittance as shown in FIG. 6C so that the optical power is constant on the surface of the recording medium even during the switching operation. As a result, on the surface of the recording medium, the same optical power as in the reproducing operation can be obtained during the switching operation as shown in FIG. 6A.

In optical recording, generally, a disk-shaped recording medium is rotated to perform recording / reproduction on a concentric circle or spiral track. The rotation speed is 600 to 1800 rpm. In order to perform the recording operation from the playback operation, to the desired track position,
By moving the optical pickup, the address increase indicating the track position previously recorded on the disc is confirmed. It is necessary to rotate the disc once or more to confirm the address information, and it takes 33 milliseconds or more even for a disc of 1800 revolutions per minute. Since the high S / N is not required if the address information is recorded digitally, it is possible to complete the switching during the confirmation operation of the address information. During the switching operation from the recording operation to the playback operation, the S / N is reproduced poorly for one rotation, but in the case of analog image reproduction, only the first one screen has a low S / N, so 30 screens per second. There is no problem in visually checking the recorded information of the reproduced image.

(Example) As a detector for detecting the transmittance of a variable optical attenuator,
The photodetector 7 for signal detection in the figure can be shared, and the transmittance can be obtained from the amount of reflected light from the recording medium. However, with this method, it is difficult to obtain an accurate transmittance because the reflectance differs depending on the presence or absence of recording on the recording medium.

In the TN type liquid crystal device shown in FIG. 5, which uses polarization rotation, the polarization beam splitter 3 reflects the attenuated light amount. Therefore, this reflected light is converted into a photodetector as shown in FIG.
By detecting at 15, the transmittance of the variable optical attenuator can be obtained. Based on this transmittance, the control circuit 16 controls the driving current of the semiconductor laser to obtain a constant optical power on the recording medium.

FIG. 8 shows a configuration example of the control circuit 16. Semiconductor laser 1
The output of the photodetector and the output of the photodetector 15 in the package (1) are compared by the comparator 17 to obtain the power on the disk surface.
The output of 17 and the reference signal 21 are compared by the comparator 18 to obtain an error signal, and the drive circuit 19 performs feedback control.

The above operation works during the reproduction and switching operations. During the recording operation, the switch 20 is switched to drive the semiconductor laser by the recording signal 22.

(Effects of the Invention) As described above, according to the present invention, it is possible to obtain the recording / reproducing optical pickup device having a good S / N ratio during the reproducing operation.

[Brief description of drawings]

FIG. 1 is a diagram showing the basic configuration of an optical pickup, FIG. 2 is a diagram showing the return light amount dependency of noise of a semiconductor laser, FIGS. 3 to 5 are diagrams showing a configuration example of a variable optical attenuating element, and FIG. FIG. 7 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 a configuration example of a control circuit. In the figure, 1 ... Semiconductor laser, 2 ... Collimating lens, 3 ...
… Polarizing beam splitter, 4 …… 1/4 wavelength plate, 5 …… Lens, 6 …… Recording medium, 7 …… Photodetector, 11 …… Optical path,
12 …… Optical filter, 13 …… 1/2 wave plate, 14 …… TN type liquid crystal element, 15 …… Photodetector, 16 …… Laser output control circuit,
17,18 …… Comparator, 19 …… Drive circuit, 20 …… Switch.

Claims (1)

[Claims] 1. A light source comprising a semiconductor laser, a first photodetector, and an optical for narrowing down the light from the light source and guiding it to a recording medium, and guiding reflected light from the recording medium to the first photodetector. In an optical pickup device including a system,
A variable optical attenuator installed in the optical system for increasing the transmittance to increase the amount of transmitted light during a recording operation on the recording medium and decreasing the transmittance to decrease the amount of light returning to the light source during a reproducing operation. And a second photodetector for detecting the amount of transmitted light of the variable optical attenuator, and a control circuit for controlling the output of the semiconductor laser so that the amount of transmitted light becomes a predetermined value. Optical pickup device.