JPS63276720A - Optical disk device - Google Patents

Abstract

PURPOSE:To reduce a laser noise by making variable a high frequency superposing current according to the reproducing power of an optical disk. CONSTITUTION:An HF module 17 overlaps a high frequency current with a laser driving current from a laser driving circuit 16 through a capacitor 175. By a PIN diode control current from an ON/OFF circuit 18, the ON/OFF action of the high frequency current and a current value variable action are executed. In the circuit 18, the PIN diode control current is determined by the base current of a TR2 as a constant current source and changed by making variable a resistance string 181 and with a WRGATE signal which is a control signal at the time of recording/erasing from a controller 14, a transistor TR1 is switched and the PIN diode control current comes to be ON/OFF. A resistance string 163 and the resistance train 181 in the circuit 18, by data from the controller 14, make variable a resistance value and switch a power high frequency current. Thus, a modulation factor can be optimized and the reduction of a laser noise can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical disc device that uses a semiconductor laser as a light source and employs a high frequency superimposed current method to reduce laser noise generated by the semiconductor laser.

In particular, the present invention relates to an optical disc device that can always obtain the optimum modulation degree for noise reduction even if the disc reproduction power changes.

[Prior art]

Mode hop noise generated by semiconductor lasers.

As described in Japanese Patent Publication No. 59-9086, a method for reducing various types of laser noise such as backlight noise is to superimpose a high-frequency current on the DC drive current of a semiconductor laser to cause pulse oscillation, thereby converting the laser's longitudinal oscillation mode into a single mode. There is a way to change from to multiple modes.

[Problem that the invention seeks to solve]

The above conventional technology describes a method of superimposing a high-frequency current on a direct current to cause a semiconductor laser to oscillate in pulses, but the oscillation characteristics of the semiconductor laser may vary depending on the temperature, etc., or when optical discs with different playback powers are played back using the same device. No consideration was given to the case where the drive current changes, such as when the drive current changes, and there was a frequent problem that the optimum modulation degree for reducing laser noise could not be achieved. Here, the degree of modulation refers to the ratio of the DC output of the laser when the high-frequency current is not N multiplied to the DC output when the high-frequency current is superimposed.

An object of the present invention is to always achieve an optimal modulation degree even if external conditions such as reproduction power and temperature change.

[Means for solving problems]

The above object is achieved by varying the high frequency superimposed current in accordance with the reproduction power of the optical disc.

[Effect]

As a method for varying the high frequency superimposed current, (1) the superimposed current is manually varied for each reproduction power based on the pre-measured relationship between the reproduction power and the superimposed current; (2) Detect the reproduction power using the rear monitor of the semiconductor laser or the front monitor installed in the optical system, calculate the modulation degree from the output ratio of each monitor output when the high-frequency current is ON/OFF, and find the optimal modulation degree. There are devices that automatically vary the high frequency current so that In each method, the optimal modulation degree can be obtained for the reproduction power, so the laser noise reduction ability is not degraded.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. The light emitted from the semiconductor laser 1 is converted into parallel light by the coupling lens 2, and then enters the prism 3. The light reflected by the prism 3 enters a photodetector that directly receives the light emitted from the semiconductor laser 1. This photodetector is called the front monitor 4 because it can observe the power of the semiconductor laser. The light passing through the prism 3 passes through a galvanometer mirror 5 and is focused by a focusing lens 6 onto a disk-shaped recording medium 7 as a spot of about 1 μm. The light reflected by the recording medium 7 passes through the aperture lens 6 and the galvano mirror 5 again, and is separated into optical paths by the prism 3.

The separated light is autofocus (AF), ) rack tracking (r
R) and a signal detection optical system for detecting the information signal. Although various optical systems have been devised as a servo optical system and a signal detection optical system, they are not the main parts of the present invention, so the lens 8 and the photodetector 9 will be explained as representatives.

The AF and TR error signals detected by the photodetector 9 are calculated and phase compensated by a servo circuit 10, and then passed through a drive circuit 11 to drive a voice coil 12 for vertically moving the AF multiplier signal focusing lens 6.

The T signal drives the galvanometer mirror 5 by 1 in order to track the spot on the track eccentricity of the recording medium as it rotates.

Similarly, the information signal received by the photodetector 9 is amplified by the signal processing circuit 13 and then pulsed by the controller 1.
Leads to 4.

As a power detection means for automatic power control (APC) to keep the output power of the semiconductor laser 1 constant, the output of the photodetector 4 for the front monitor or the output of the photodetector 4 for the rear monitor housed in the same baggage of the semiconductor laser 1 is used. The output of the photodetector 15 is used. These outputs are led to the semiconductor laser drive circuit 16, and one of them is used for APC operation, and the other is used, for example, for power abnormality detection. Furthermore, in addition to the APC operation, a laser driving section is included, which adds the DC driving current of the semiconductor laser 1 and the pulse current during recording/erasing to drive the semiconductor laser 1.

Semiconductor lasers generally suffer from mode hop noise and induced noise due to returned light, resulting in a reduction in the signal-to-cone ratio of the information signal. An effective measure to solve this problem is to superimpose a high-frequency current of 500 MHz or more on the DC drive current of a semiconductor laser to generate pulse oscillation, as described in Japanese Patent Publication No. 59-9086, thereby converting the longitudinal oscillation mode of the laser from a single mode to multiple modes. There are ways to change it. In order to realize this method, a high frequency superimposition module (referred to as an HF module) 17 equipped with a high frequency oscillator is provided.

HF to the laser drive current from the semiconductor laser drive circuit 16
After superimposing high frequency current in module 17.

By driving the semiconductor laser 1, laser noise can be completely reduced. Assuming that the modulation degree is the ratio of the DC power output and the DC average power output when high frequency is superimposed as the current value superimposed by this) fF module 17, the laser noise reduction effect is good when the modulation degree is about 130 to 150%. From that.

For example, the modulation degree is set to 151. Assuming that the high frequency current is a sine wave and expressing it in a general formula, the modulation degree is M, and the ratio of the peak value of the DC current exceeding the L threshold current and the superimposed current is m.
Then, the following formula is obtained.

ω3−・ (1)+V〈−2t π Therefore, if the modulation degree M=150%, the flow ratio m′=i3
becomes. For example, the differential Doji efficiency of a semiconductor laser to, 5
If mW/mA and emission power are 3 mW, the DC current exceeding the threshold value will be 6 mA, so the peak value of the high frequency current will be 18 mA. If a pulse is further superimposed during recording/erasing in this state, the peak value of the information pulse may exceed the allowable maximum output of the laser because a high-frequency current is superimposed on the pulse, causing a shortened lifetime power ratio. Therefore, when recording/erasing, the superimposition of high-frequency current is stopped.
An N/OFF circuit 18 is provided to protect the laser. Further, this ON/OFF circuit 18 is designed to be able to vary the peak value of the superimposed high frequency current.

It can deal with variations in the characteristics of semiconductor lasers, variations in oscillation properties of HF modules, and even when changing the reproduction power, and can always obtain the optimum modulation degree for laser noise reduction. A specific example will be described below.

In Figure 2, the high frequency current is varied for each reproduction power.

A first embodiment for optimizing the degree of modulation is shown, in particular, a semiconductor laser drive circuit 16, which is the main part of the present invention, (F module 17) and a high frequency current ON/OFF circuit 1.
8 will be specifically shown. The laser drive circuit 16 includes a laser drive section 161 that adds a laser DC drive current and a pulse current during recording/erasing, and an APC servo circuit 162.

Although the APC operation is illustrated using the rear monitor 15, the front monitor 4t may also be used. The rear monitor 15 sets a target power value using a resistor array 163.

This is because the APC operation works to keep the monitor output constant with respect to the reference voltage in the APC servo circuit 162, so if you increase the value of the resistor string.

Since the rear monitor output becomes small, the reproduction power becomes small. Conversely, when the resistance value is made small, the rear monitor output becomes large and the reproduction power becomes large. The output of the front monitor 4 is guided to an abnormality detection circuit 164 in the laser drive circuit 16 to detect a laser abnormality.

The HF module 17 includes, for example, a high frequency oscillator 171 (oscillation frequency 500 to 800 MHz) using a transistor or the like.
), PIN diode 172. Inductor/
S173. The high-frequency current is superimposed on the laser drive current from the laser drive circuit 16 through the capacitor 175 as a thick film IC composed of capacitors 174 and 175. The PIN diode 172 has a high frequency +7 = good arity in the high frequency band, so the PIN diode 1
When the control current flowing through 72 is varied, it functions as an attenuator.

Using this feature, the PIN from the ON/OFF circuit 18
The diode control current performs high frequency current ON/OFF operation and current value variable operation.

In the ON/OFF circuit 18, the PIN diode control current is determined by the base voltage of the transistor TR2 as a constant current source, and if the emitter resistance is Ro, then the base voltage is divided by R. Base voltage is resistor string 1
WRG is a control signal for recording/erasing from the controller 14.
The transistor TRI is switched by the ATE signal.

Turns on/off the PIN diode control current.

The resistance string 163 for laser power setting and the resistance string 181 for high frequency current control in the ON 10FF circuit are 2°R12'.
R, .
182't- is provided, and the resistance value is varied according to n-bit data from the controller 14, and the power and high frequency current are switched. These data are measured for each head and stored in a RAM area in the controller in advance, and the data is sent to the analog switch array when the device is started up. By doing this, the modulation degree t can be optimized.

Laser noise reduction can be achieved.

FIG. 3 shows a method using a constant current source in the ON/OFF circuit 18 using a D/A converter 183 and an operational amplifier 184 instead of a resistor string. The n-pit data from the controller 14 is converted into an analog voltage by D/A conversion, and a current corresponding to this voltage is passed through TR2, and the same effect as when using the resistor string shown in Figure 2 can be obtained. It will be done.

FIG. 4 shows a second embodiment in which the degree of modulation is automatically kept constant according to the reproduction power. It is assumed that the laser power has already been set in the laser drive circuit 16, and the high frequency current is adjusted so that the degree of modulation is constant using the time when the device is not selected by the host controller, such as when the device is started up.

A pulse oscillation circuit 19 for generating TEST GATE signals is provided for modulation measurement.1 Normal WRGATE and TEST
p, TE by the analog switch 20 that changes GATE.
Select STGATE and turn this signal ON10 F
It leads to the F circuit 18 and the APC servo circuit 162 in the laser drive circuit 16. High frequency current is turned off by TESTGATE.
When it is N/OFF, a waveform corresponding to ON/OFF is obtained in the rear monitor output or the front monitor output. Since the ratio of the level when ON and the level when OFF is the modulation degree, these monitor outputs are led to sample and hold circuits 21 and 22.

The ON level and OFF level of TEST ()ATE are held through timing circuits 23 and 24. The output of the sample and hold circuits 21 and 22 is the divider 2.
5, and an output proportional to the modulation degree is obtained. After passing through the level correction circuit 26, this output is sent to the controller 14 as n-bit digital data by the A/Di converter 27 and stored therein. Furthermore, these digital data are sent from the controller 14 to the ON/OFF circuit 18,
By adopting the same ON/OFF circuit configuration as shown in FIG. 3, it is possible to control the high frequency current using the output of the D/converter 183 and obtain an optimum modulation degree. By generating digital data regarding the modulation degree using the divider 25 in this way, the relationship between the reproduction power and the high-frequency current can be automatically calculated without having to measure the relationship between the reproduction power and high-frequency current for each head as in the first embodiment. % resemblance obtained.

As explained above, since the value of the superimposed high-frequency current can be varied according to the reproduction power and the degree of modulation can be kept constant, laser noise can be reduced.

〔Effect of the invention〕

According to the present invention, since the value of the modulation degree with respect to the reproduction power can be kept constant, the effect of laser noise reduction can be sufficiently exhibited.

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining one embodiment of the present invention, FIG. 2 is a diagram for explaining the first embodiment of the present invention, FIG. 3 is a diagram for supplementary explanation of the first embodiment, and FIG. FIG. 4 is a diagram explaining the second embodiment. 1... Semiconductor laser, 4... Front monitor, 15...
- Rear monitor, 16... Laser drive circuit, 17...
HF module, 18...ON/OFF circuit, 19.
...Pulse oscillation circuit, 21.22...Sample and hold circuit. 23.24...Timing circuit, 25...Divider. 26...Level correction circuit, 27...A/D converter.

Claims (1)

[Scope of Claims] 1. A light source, an optical system that guides the light emitted from the light source to an information recording medium, and an information detection circuit that separates the light reflected from the information recording medium from the optical system and performs photoelectric conversion. The optical disc device is characterized in that a semiconductor laser is used as the light source, a high frequency superimposition circuit is provided for superimposing a high frequency current on the semiconductor laser, and the high frequency current is varied in accordance with the reproduction power of the information recording medium. optical disk device. 2. The optical disc device according to claim 1, further comprising an output detection circuit for detecting the reproduction power, and the high frequency current is controlled by the output of the output detection circuit. 3. The optical disc device according to claim 1, wherein the high frequency superimposing circuit has a high frequency current ON/OFF function. 4. Claims characterized in that a rear monitor housed in the same package as the semiconductor laser or a front monitor that detects a light beam directly separated from the optical system is used as the detection means of the output detection circuit. The optical disc device according to item 1.