JPH06132592A - Semiconductor laser driving device - Google Patents

Abstract

PURPOSE:To simply avoid the overcurrent of a semiconductor laser without fail by a method wherein an integrating circuit comprising a resistor and a capacitor is provided in the input of a driving circuit while the time constant of this integrating circuit is set up larger than that of a feedback loop. CONSTITUTION:The time constant of an integrating circuit comprising a resistor 8 and a capacitor 9 is made satisfactorily larger than that of a negative feedback loop. Thus, a detection system, etc., is restored to normal state earlier than the current value of a driving circuit 5 is restored to proper level thereby enabling any overshooting to be avoided. Accordingly, the voltage of the capacitor 9 is to be raised gradually at the time constant of the integrating circuit thus gradually, raising the laser driving current also so that the current of a semiconductor laser 1, when restored from a discon-nection, may easily kick off out of a low current without fail thereby enabling any overcurrent of the semiconductor laser l during the dis-connection time to be avoided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving device for a semiconductor laser used as a light source for recording or reproducing in an optical information recording / reproducing device or the like.

[0002]

2. Description of the Related Art FIG. 7 is a circuit diagram showing a general semiconductor laser driving device, 1 is a semiconductor laser to be driven, and 2 is a semiconductor laser to be driven.
Is a photodiode for monitoring the light quantity of the semiconductor laser 1. The light quantity of the semiconductor laser 1 is constantly monitored by the photodiode 2, and the detection signal thereof is converted into a voltage signal by the current-voltage conversion circuit (hereinafter, IV conversion circuit) 3 and then sent to the control circuit 4. . Control circuit 4
Then, the voltage signal is compared with the reference voltage to generate a control signal corresponding to the difference and output to the drive circuit 5. The drive circuit 5 is a constant current circuit that supplies a current to the semiconductor laser 1, and is composed of a transistor 6 and a resistor 7 connected to its emitter. Here, when the emission power of the semiconductor laser 1 increases due to a change in ambient temperature or the like, the detection value of the photodiode 2 increases, so that the control circuit 4 lowers the control signal as a result of comparing the detection value with the reference voltage. Control. That is, since the base voltage of the transistor 6 of the drive circuit 5 is reduced and the current of the collector of the transistor 6 is reduced, the supply current of the semiconductor laser 1 is also reduced and control is performed so as to reduce the light emission amount of the semiconductor laser 1. . As a result, the light emission amount of the semiconductor laser 1 returns to a predetermined value and is maintained at a constant light emission amount. Further, when the light emission amount of the semiconductor laser 1 is lowered for some reason, control is performed so that the supply current of the semiconductor laser 1 is increased contrary to the above, and the light emission amount of the semiconductor laser 1 is kept constant.

[0003]

By the way, conventionally, the semiconductor laser and the current source may be disconnected from each other due to some accident during the operation of the semiconductor laser driving device. In particular, a semiconductor laser is often connected to an external circuit by using a connector for replacement, and a disconnection may occur due to a connector connection error or the like during a device test.
In such a case, in the above-described conventional drive device, when a disconnection state occurs during operation, control works so as to increase the current of the semiconductor laser, so that the output voltage of the control circuit increases and the output voltage increases if the disconnection state continues for a long time. It will be the maximum. Then, when the disconnection is restored in this state, since the output voltage of the control circuit is the maximum voltage, the semiconductor laser is driven by the overcurrent,
The characteristics may deteriorate. Therefore, conventionally, in order to avoid such a situation, the following solutions have been taken. (1) The upper limit of the output voltage of the control circuit 4 is regulated to the extent that an overcurrent of the semiconductor laser does not occur. (2) By taking a large time constant of the feedback loop of the drive device,
Make sure the control does not work for very short breaks. (3) Conversely, the time constant of the feedback loop is made as small as possible to speed up the response characteristic of control, and the laser current is controlled to an appropriate value by the current control operation before the semiconductor laser reaches overcurrent. (4) The control circuit 4 is provided with a circuit for detecting disconnection.
The output voltage of is controlled to prevent overcurrent.

However, in the above method (1), the maximum current of the semiconductor laser changes due to variations in the characteristics of the semiconductor laser, temperature changes, etc. Therefore, adjustment is necessary to regulate the output voltage of the control circuit 4. . Further, if the upper limit of the voltage is set to a low value without adjustment, the maximum emission output of the semiconductor laser cannot be obtained, and there is a possibility that recording failure may occur. Further, in the method (2), if the disconnection time becomes long, there is no solution, and in the method (3), the deterioration of the semiconductor laser occurs even when the overcurrent is applied for a very short time. It wasn't enough when I thought about it. Furthermore,
The method of (4) requires a detection circuit,
There is a problem that the structure of the driving device becomes complicated and the cost becomes high.

The present invention has been made in order to solve the above problems, and an object thereof is to provide a semiconductor laser driving device capable of easily and surely preventing an overcurrent of a semiconductor laser. .

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to drive a sensor for detecting the light quantity of a semiconductor laser, a transistor for supplying a current to the semiconductor laser, and an emitter resistor connected to its emitter. In a semiconductor laser drive device comprising a circuit and a control circuit for controlling the drive circuit based on a detection value of the sensor, and controlling a light amount of the semiconductor laser to a predetermined value, a resistor and an input are provided to the drive circuit. In addition to providing an integrating circuit composed of a capacitor, the time constant of the integrating circuit is set to be larger than the time constant of the feedback loop for current control, and when the semiconductor laser is not connected to the drive circuit, the capacitor is provided with the control circuit. When a voltage obtained by dividing the output of the circuit by the resistance of the integrating circuit and the emitter resistance is applied and the semiconductor laser is connected to the drive circuit It is accomplished by a semiconductor laser driving apparatus characterized by gradually increasing the input voltage of the driver circuit from the divided initial voltage of the capacitor.

[0007]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of a semiconductor laser driving device of the present invention. In FIG. 1, the same parts as those in the conventional device shown in FIG. 7 are designated by the same reference numerals, and detailed description thereof will be omitted in the present embodiment. In this embodiment, an integrating circuit composed of a resistor 8 and a capacitor 9 is provided between the control circuit 4 and the driving circuit 5. The time constant of this integrator circuit is the time constant of the negative feedback loop of the drive device, that is, the time constant of the control loop that leads to the photodiode 2, the IV conversion circuit 3, the control circuit 4, the drive circuit 5, and the semiconductor laser 1. It is set larger than the constant. Further, the resistance value of the resistor 8 of the integrating circuit is set sufficiently larger than the resistance value of the resistor 7 of the drive circuit 5. The rest of the configuration is the same as in FIG. 7.

Next, the operation will be described. First, in normal operation, the input resistance seen from the base of the transistor 6 becomes h _FE (current amplification factor of the transistor 6) times the resistance value of the resistor 7 connected to the emitter, and this input resistance is Since the resistance value of the resistor 8 is set to be sufficiently larger than the resistance value of the resistor 8, the existence of the base resistor 8 can be ignored. Further, since the time constants of the base resistor 8 and the capacitor 9 are set to be sufficiently smaller than the speed of light output fluctuation due to the disturbance of the semiconductor laser 1, the normal operation is exactly the same as the conventional operation. Here, when the connection between the semiconductor laser 1 and the drive circuit 5 is cut off, the base and emitter of the transistor 6 may be regarded as a simple diode, and the voltage across the capacitor 9 is approximately the output voltage of the control circuit 4 as a resistor. It is the value divided by 7 and 8. Even if the output voltage of the control circuit 4 is the maximum voltage, the resistance value of the base resistor 8 is sufficiently larger than the resistance value of the emitter resistor 7, so that the voltage across the capacitor 9 becomes sufficiently small.

FIG. 2 shows the signal waveform of each part at the time of disconnection. FIG. 2 (a) shows the output voltage of the IV conversion circuit 3, and FIG.
(B) is the output voltage of the control circuit 4. When the disconnection occurs, the semiconductor laser 1 is turned off, so that the photocurrent of the photodiode 2 becomes 0 and the output voltage of the IV conversion circuit 3 also becomes 0 as shown in FIG. Further, the output voltage of the control circuit 4 increases as shown in FIG. 2B because the control works to increase the current of the semiconductor laser 1, and increases to the maximum voltage. On the other hand, since the voltage across the capacitor 9 is divided by the resistors 7 and 8 as described above, it becomes sufficiently small as shown in FIG. 2C, and the current of the semiconductor laser 1 at this time is as shown in FIG. Naturally 0 as shown in d)
Becomes When the disconnection state is released and the semiconductor laser 1 and the drive circuit 5 are connected again, the charging voltage of the capacitor 9 is given to the base of the transistor 6 as an initial value, and the current of the transistor 6 restarts from a small current. . In this case, by setting the time constant of the integrating circuit composed of the resistor 8 and the capacitor 9 sufficiently larger than the time constant of the negative feedback loop, the current value of the drive circuit 5 returns to an appropriate value and the detection system or the like operates normally. Then, the current of the semiconductor laser 1 does not overshoot. Therefore,
The voltage of the capacitor 9 gradually rises with the time constant of the integrating circuit as shown in FIG. 2C, and the laser drive current also gradually rises as shown in FIG. 2D. When the semiconductor laser 1 recovers from the disconnection, the current of the semiconductor laser 1 inevitably rises from a low current, which can surely prevent the overcurrent of the semiconductor laser 1 at the time of disconnection.

In the above embodiment, the input resistance of the transistor 6 is sufficiently larger than the resistance value of the resistor 8 for simplification of description, but it may be about the same. It suffices that the control circuit 4 can output a voltage that can sufficiently compensate for the voltage drop of No. 8. Further, the time constant of the integrating circuit at this time must be larger than the time constant of the feedback loop.

3 to 6 are circuit diagrams showing other embodiments of the present invention. The embodiment of FIG. 3 is an example in which a PNP transistor is used as the transistor 6, and the embodiment of FIG. 4 is an example in which when the control circuit 4 has an output impedance, that impedance is used instead of the resistor 8. Is. Further, the embodiment of FIG. 5 is an example in which the transistors 6A and 6B in Darlington connection are used. Since it is in Darlington connection here, the resistor 10 is connected in parallel with the capacitor 9 so that the voltage of the capacitor 9 is lowered. I am doing it. Further, the embodiment of FIG. 6 is an example in which the semiconductor laser 1 is driven by a differential amplifier circuit, and the voltage value of the resistor 7 of the emitter is fed back to control the current.

[0012]

As described above, according to the present invention, when the semiconductor laser is in a disconnection state, the overcurrent of the semiconductor laser can be surely prevented by simply adding a simple circuit, thereby deteriorating the characteristics of the semiconductor laser. This has the effect of preventing this and improving reliability.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a semiconductor laser driving device of the present invention.

FIG. 2 is a time chart showing signal waveforms of various parts of the embodiment of FIG.

FIG. 3 is a circuit diagram showing another embodiment of the present invention.

FIG. 4 is a circuit diagram showing another embodiment of the present invention.

FIG. 5 is a circuit diagram showing another embodiment of the present invention.

FIG. 6 is a circuit diagram showing another embodiment of the present invention.

FIG. 7 is a circuit diagram showing a conventional semiconductor laser driving device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Photodiode 3 Current-voltage conversion circuit (IV conversion circuit) 4 Control circuit 5 Drive circuit 6 Transistor 7,8 Resistor 9 Capacitor

Claims (2)

[Claims] 1. A sensor for detecting a light quantity of a semiconductor laser, a drive circuit including a transistor for supplying a current to the semiconductor laser and an emitter resistor connected to an emitter thereof, and a detection value of the sensor. In the semiconductor laser drive device including a control circuit for controlling the drive circuit by controlling the drive circuit and controlling the light quantity of the semiconductor laser to a predetermined value, an input circuit of the drive circuit is provided with an integrating circuit including a resistor and a capacitor, and When the time constant of the integrating circuit is set to be larger than the time constant of the feedback loop of the current control and the semiconductor laser is not connected to the drive circuit, the output of the control circuit is connected to the resistance of the integrating circuit and the resistance of the integrating circuit. When a voltage divided by the emitter resistance is applied and the semiconductor laser is connected to the drive circuit, the input voltage of the drive circuit is A semiconductor laser driving device characterized in that the voltage gradually increases from the divided initial voltage of the capacitor. 2. The semiconductor laser driving device according to claim 1, wherein the resistance value of the resistance of the integrating circuit is larger than the resistance value of the emitter resistance.