JPS61294645A - Information recording and reproducing device - Google Patents

Abstract

PURPOSE:To reduce laser noises during reproduction by increasing the transmissivity of an optical element when a laser light source emits high output light, i.e. in recording, etc., and decreasing the transmissivity of the optical element when the laser light source emits low output light, i.e. during reproduction, etc. CONSTITUTION:Quantization noises normally increase in the low output range of the semiconductor laser, so when reproducing power is too low, the S/N ratio of a signal decreases. For the purpose, a transmissivity converting element 3 is inserted into an optical head to increase its transmissivity when the laser output is low, i.e. during reproduction, etc., thereby increasing the laser output by the extent. For example, when the transmissivity is 50%, the laser output is made double. When the output is high, i.e. during recording or erasure of a photomagnetic disk, etc., a nearly normal laser output is generated with transmissivity of, for example, -100%, thereby reducing laser noises during the reproduction.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an information recording/reproducing device for recording/reproducing or erasing information, and particularly to an information recording/reproducing device suitable for reducing laser noise by a semiconductor laser during reproduction. Regarding equipment.

[Background of the invention]

Conventionally, in information recording and reproducing devices using a laser as a light source, for example, Hitachi Hyoron VOL, 65N, 10 (1983-
10) As described in P.20, the laser output during recording is 2
The laser output at one time is about 0 mW, and the reproduction power is selected to be about 4 mW, taking into account the characteristics of the electric circuit that detects the signal and the influence of the reproduction power on the recording order, and the ratio is 5. 6 In the future, as the sensitivity of recording films improves, it will be necessary to set the reproducing power lower due to the above-mentioned ratio relationship. However, when using a semiconductor laser as a light source, quantization noise usually increases in the low output range of the semiconductor laser, so if the reproduction power is too low, the signal-to-noise ratio of the signal will decrease and normal signal reproduction will not be possible. The problem is that it disappears.

In addition, in magneto-optical disks in which information is recorded, reproduced, and erased by the upward and downward magnetization of the perpendicularly magnetized film,
Since the detected signal amount is two to three orders of magnitude smaller than that of write-once optical discs, which record and reproduce information by forming holes, laser noise becomes a major problem even in the output of conventional semiconductor lasers during playback. ing.

[Purpose of the invention]

An object of the present invention is to install an optical element that can change optical transmittance at high speed, and to increase the transmittance of the optical element when a semiconductor laser emits high output light during recording and erasing.
To provide an information recording and reproducing device capable of reducing laser noise by lowering the transmittance of an optical element when a semiconductor laser emits low output power during reproduction and effectively increasing the output of the semiconductor laser during reproduction. It is in.

[Summary of the invention]

When a semiconductor laser emits low-output light during playback, the quantization noise of the laser itself becomes a problem, so by effectively lowering the transmittance of the optical system during playback and effectively increasing the semiconductor laser output, Efforts are being made to reduce laser noise and ensure signal-to-noise ratio.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described in accordance with Section 1. This is the case with write-once optical discs in which information is recorded by forming holes in the recording film, and the light emitted from the semiconductor laser 1 that is the light source is converted into linear light by the coupling lens 2 and transmitted through the transmittance conversion element 3. The light passes through the prism 4 and the λ/4 plate 5 and is narrowed down to a spot of about 1 μm on the recording medium disk 8 by the focusing lens 7 attached to the two-dimensional actuator 6. A guide groove 9 is formed in advance on the disk 8 for track tracking, and the guide groove 9
ID signals including track addresses and sector addresses are formed in the form of uneven pits at equal intervals so that they can be accessed. The light reflected by the disk 8 is passed through the aperture lens 7, λ
After passing through the /4 plate 5, the optical path is separated by a prism 4, and after passing through a convex lens 10, it is separated into two beams by a prism 11. The transmitted component enters a photodetector 12 for track tracking, and the reflected component is automatically After passing through a knife lens 13 for focusing, the light enters a photodetector 14. The photodetector 12 is divided into two parts, and the output from each photodetector is input to a differential amplifier 5, where a tracking error signal is detected, and this signal is sent to a compensation circuit prepared for track tracking. After passing through a tracking control circuit 16 including a drive circuit, the light spot tracks the track by driving the track tracking coil 17 of the two-dimensional actuator 6. Photodetector 1
When the outputs of 2 are added by an adder 18, more information including signals and recording signals is obtained, and after being demodulated by a signal processing circuit 19, the outputs are sent to a photodetector 14 for defocus detection, which is sent to a controller 20. It is also divided into two parts, and when each output is differentially outputted by a differential amplifier 21, a defocus signal is detected, and this signal is sent to an autofocus control circuit 22 prepared for autofocus.
After passing through, the automatic focusing coil 23 of the two-dimensional actuator 6 is driven to constantly position the light spot on the disk surface.

To record the information, the modulated information signal from the controller 20 is guided to the laser drive circuit 24.
This is done by driving the semiconductor laser 1 in pulses to emit high-output light, and in response to the pulses, the recording film formed on the disk 8 is melted to form holes. Recording will be specifically explained using FIG. 2. Figure 2, 1st
The quadrant represents the relationship between the laser drive current and the laser output P, and the origin is the current Ith that starts laser emission.
If you choose , the relationship will be almost linear. The second quadrant represents the laser output P and the reproduction signal amount (modulation degree) M of the recorded information. When P exceeds the threshold value Pth, recording starts rapidly, and the modulation degree M increases according to P. Above a certain P, it does not change much.

Normally, the reproduction power PR is determined by the characteristics of the electric circuit of the signal detection system and the sensitivity of the recording film, and is selected to be about 1/2 to 3 of Pth, for example, about 1 to 1.5 mW. Further, the recording power Pw is determined by the number of disk rotations, etc., but is selected to be a power at which the degree of modulation M does not change, and the recording power of the recording film is, for example, 7 to 10 mW. The laser drive waveform during recording takes the form of a recording pulse superimposed on the current waveform 8 during reproduction.

Now, as shown in the fourth quadrant of the laser noise N with respect to the laser drive current I, the quantization noise due to the spontaneous emission component is large near Ith, and as the laser emission component increases, the laser noise rapidly decreases. Therefore, even when the recording sensitivity increases and the playback power must be further lowered, or when the playback signal light intensity is two to three orders of magnitude smaller than that of a write-once optical disk, as in the case of magneto-optical disks, the Laser noise becomes a problem.

Therefore, in the present invention, a transmittance conversion element 3 is inserted into the optical head, and when the laser output is low during playback, the transmittance is lowered and the laser output is increased accordingly (for example, the transmittance is increased by 50%).
If it is set to 0%, the laser output is doubled), and when the laser output is high, such as during recording or erasing on magneto-optical disks, the transmittance is increased (for example, ~100%), which is almost normal. By emitting light with laser output, the problem caused by laser noise during playback can be solved. Note that by lowering the light utilization efficiency of the entire optical head, the same effect can be obtained during playback, but this becomes a problem because the laser power increases during recording, which requires a high-output laser. Become.

An example of the transmittance conversion element 3 will be explained using FIG. 3. Basically, an electro-optic crystal 33 such as PLZT or LiNbO3 having electro-optic properties is arranged between a pair of polarizers 31 and analyzers 32 whose polarization directions are diagonal to each other for an incident beam having linearly polarized light. do. electro-optic crystal 3
A parallel electrode 34 is provided on the surface of the electro-optic crystal 33, and when a voltage is applied to this electrode from the crystal drive circuit 25, the electro-optic crystal 33
Birefringence occurs and the amount of emitted light changes. Incident light amount E1 and output light amount E. The intensity ratio of is given by. ■ is the applied voltage, and ■ is the voltage required to give a phase difference of ππ. Eo/Ei represents the transmittance, and when a graph is drawn with the horizontal axis representing the applied voltage and the horizontal axis representing the transmittance, it becomes as shown in FIG. Therefore, any desired transmittance can be obtained by selecting the applied voltage. Also, the third
In the figure, by inserting a quarter-wave plate giving a phase difference of π/2 between the electro-optic crystal 33 and the analyzer 32,
Since a bias of 7□ is applied, the applied voltage can also be reduced. When PLZT is used for the electro-optic crystal 33, the maximum applied voltage is about 100 to 300V.

Semiconductor laser drive waveform in the present invention using FIG.
The electro-optic crystal driving waveform will be explained.

First, in the playback mode, the semiconductor laser drive current is
C, and this is set to about 2 digits of the usual 8 (the dotted line shows the conventional case). At this time, the electro-optic crystal driving voltage is approximately 'v, c7□, reducing the transmittance to 50%.
It is said that By doing this, the reproduction power on the film surface can be made the same as the normal one, and furthermore, the laser noise can be reduced by increasing the laser current during reproduction by a factor of 2, as shown in the fourth quadrant of Figure 2. Laser noise 10-20
It was confirmed that it was possible to reduce the noise by about dB. In recording mode, recording pulses are emitted sector by sector, but at this time the ID
The peak value of the pulse current superimposed on C is the same as in normal processing, the electro-optic crystal driving voltage is V, and the transmittance is 100%.By doing this, the recording power on the film surface is is the same as the normal one, and two identical lasers can be used.

Although the case of a write-once optical disc has been described above, similar effects can be obtained with a magneto-optical disc on which information can be rewritten. Further, although the case where an electro-optic crystal is used as the transmittance conversion element has been described, the same effect can be obtained by using an element whose transmittance can be varied using another electric field or magnetic field.

〔Effect of the invention〕

According to the present invention, since laser noise during reproduction can be reduced by changing the transmittance of the optical head during recording and reproduction, it is possible to realize an information recording and reproducing apparatus that can perform stable signal reproduction.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention, and FIGS. 2 to 5 are supplementary diagrams for explaining the present invention in detail. 1...Semiconductor laser, 3...Transmittance conversion element ,6・
... Two-dimensional actuator, 19... Signal processing circuit,
20... Controller, 24... Laser drive circuit,
25... Electro-optic crystal drive circuit.

Claims (1)

[Claims] 1. A laser light source, an optical system that guides the light beam emitted from the laser light source onto a recording medium, and an analysis system provided for detecting information recorded on the recording medium. What is claimed is: 1. An information recording and reproducing apparatus comprising: an optical recording optical system including an optical element capable of changing transmittance at high speed; 2. In the information recording and reproducing apparatus according to claim 1, when the laser light source emits high output light during recording, etc., the transmittance of the optical element is increased, and when the laser light source emits low output power during playback, etc. An information recording/reproducing device characterized in that the transmittance of the optical element is lowered when emitting light.