JPS59147476A - Protective device for semiconductor laser - Google Patents

Abstract

PURPOSE:To prevent the deterioration or breakdown of a semiconductor laser by bypassing currents flowing through the semiconductor laser when overcurrents flow through the semiconductor laser and an optical output from the semiconductor laser intends to exceed a fixed value. CONSTITUTION:When overcurrents intend to flow through a semiconductor laser 1, an optical output from the laser 1 increases. The impedance of a photodiode 2 drops at the same time, and input voltage to a minus terminal for a comparison amplifier 5 drops. When the input voltage drops to a value lower than the threshold voltage of the amplifier 5, an output from the amplifier reaches an ''H'' level, and an output from an AND circuit 7 is brought to an ''L'' level. An inverter 8 receives the signal of the ''L'' level, an output from the inverter is brought to an ''H'' level, and a thyristor 4 is turned ON. Consequently, most of currents flowing the laser 1 by that time are bypassed to the thyristor 4 side, and currents hardly flow through the laser 1. Accordingly, the laser 1 can be protected from breakdown or deterioration.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a protection device for a semiconductor laser used in a semiconductor laser printer or the like.

A semiconductor laser requires a predetermined forward current to flow in order to obtain a predetermined optical output. The major difference between semiconductor lasers and conventional laser oscillations is that the time required to start laser oscillation is significantly shorter. Therefore, high-speed optical modulation is possible by turning on and off the current flowing through the semiconductor laser as necessary. Normally, a direct current power source is used to control a semiconductor laser, and a transistor is used to control the on/off state and to keep the optical output constant. However, if the voltage of the DC power supply rises abnormally, if the control transistor breaks down, or if the optical output control circuit of the semiconductor laser malfunctions, an excessive current will flow through the semiconductor laser. 9゜An object of the present invention is to provide a protection device in case an excessive current attempts to flow through the semiconductor laser and causes the semiconductor laser to deteriorate or be destroyed. .

The present invention detects the optical output of a semiconductor laser, and when an excessive current flows through the semiconductor laser and the optical output of the semiconductor laser attempts to exceed a predetermined value, the current flowing through the semiconductor laser is stopped. This prevents damage and deterioration of the laser.

An embodiment of the present invention will be described above with reference to the drawings.

In the figure, l is a semiconductor laser, 3 is a transistor that controls the semiconductor laser l, 10 is a variable resistor that limits the current flowing through the semiconductor laser fl, 2 is a photodiode that responds to the optical output of the semiconductor laser l, and 13 is a photodiode 2. A fixed resistor connected in series with the photodiode 2, which acts together with the photodiode 2 and changes the voltage level at the connection point between the photodiode 2 and the fixed resistor 13 in accordance with the optical output from the semiconductor laser 1. It's peaceful.

14 is a transistor 30 base resistor. 5 detects the voltage level at the contact point between fixed resistor B and photodiode 2;
It is a comparison amplifier that amplifies. 17 is a fixed resistor for setting the amplification factor of the comparison amplifier 5 to a certain value, 9 is a diode provided on the output side of the comparison amplifier, and 7 is the comparison amplifier 5.
"PWRRDY" shows the output of the power supply and the rise of the power supply.
Performs logical product with J signal; AND circuit; 8 is AN
An inverter that inverts and amplifies the output of the D circuit 7, 4 a thyristor inserted in parallel with the semiconductor laser 1 and the transistor 3, 6 a constant voltage diode for supplying the reference voltage of the comparison amplifier 50, and b a constant voltage diode. This is an okiya resistor that allows the necessary current to flow to the voltage diode 6. ■ and L are voltage dividers that divide the voltage of the constant voltage diode, and ■ is a variable resistor in order to be able to control the voltage division ratio. , Hill uses a fixed resistor.

The operation of the circuit of this embodiment will be briefly explained. First, the resistance value of the variable resistor is maximized, and the LD0ONT signal is output to turn on the transistor 3. While measuring the optical output of the semiconductor laser l with a laser power meter etc., adjust the variable resistor p.
The resistance value of is decreased, and the current flowing through the semiconductor laser I is increased. It is fixed at the time when the optical output of the semiconductor laser l increases to the value to be set before it exceeds the maximum rated output. In this state, the next step is to vary the variable resistor 11 to change the threshold voltage of the comparator amplifier.
The output is fixed at the point where it changes from low level to high level.

If the above-mentioned adjustment is carried out, if the optical output of the semiconductor laser is about to exceed a predetermined value, the comparator amplifier 5 can detect it.

For example, if the transistor 3 that controls the semiconductor laser 1 breaks down due to conduction, or if an abnormality occurs in the control circuit that generates the control signal LD C0NT signal of the transistor 3 and an excessive current attempts to flow to the semiconductor laser,
The advance of the semiconductor laser 1 increases. At the same time, the impedance of the photodiode 2 receiving the optical output decreases, and the input terminal to the comparison amplifier e terminal decreases. When the voltage drops below the previously set threshold voltage of the comparison amplifier 5, the output of the comparison amplifier 5 changes from low level to high level.
The output of the AND circuit 7 is changed from high level to low level. Inverter 8 receives this signal, sets its output to high level, and turns on thyristor 4. The forward voltage of the thyristor 4 is usually smaller than the j@direction current of the semiconductor laser l, and the current t that has previously flowed through the semiconductor laser is
It is bypassed to the thyristor 4 side, and almost no current flows through the semiconductor laser l. Therefore, the semiconductor laser can be protected from destruction or deterioration.

In the figure, the PWRRDY signal is an input signal to the AND circuit 70, but this is a signal to prevent the signal that turns on the thyristor 4 from being output when the power supply is uncertain, such as when a current is turned on.The LDPWRALM signal is This is a signal whose voltage level changes from a high level to a low level when an excessive current attempts to flow through the semiconductor laser 1 for some reason and the comparator amplifier operates. This signal can serve to notify the main body of the device of an abnormality in the semiconductor laser control circuit or drive circuit. Needless to say, the main body can receive this signal and stop the device or display an appropriate display.

One embodiment of the present invention is shown in the figure. In this embodiment, a thyristor 4 is connected in parallel with the semiconductor laser l and the transistor 3. However, in order to achieve the object of the present invention, it is necessary to connect the thyristor 4 in parallel with the semiconductor laser l and the transistor 3. Any element may be used as long as it has a forward voltage lower than the one-way voltage of the semiconductor laser IoJ. For example, it goes without saying that the same purpose can be achieved by using a transistor or a relay contact instead of a thyristor. Further, the thyristor and the transistor may be connected directly in parallel to the semiconductor laser l, other than in the embodiment.

According to the present invention, it is possible to prevent a semiconductor laser drive circuit from becoming abnormal, causing an excessive current to flow through the semiconductor laser, and causing the semiconductor laser to deteriorate or be destroyed.

[Brief explanation of the drawing]

The figure is a circuit diagram showing an embodiment of a semiconductor laser protection device according to the present invention. In the figure, l is a semiconductor laser, 2
is a photodiode, 3 is a transistor, 4 is a thyristor,
5 is a comparator amplifier, 6 is a constant voltage diode, 7 is an AND circuit, 8 is an inverter, 9 is a diode, ■, ■ are variable resistors, and 12 and the other are fixed resistors.

Claims (1)

[Claims] A semiconductor laser driving device having a semiconductor laser and a control circuit for driving the semiconductor laser, comprising: means for detecting the optical output of the semiconductor laser; A detection circuit that outputs a signal and an output from the detection circuit turn on a current bypass circuit inserted between the semiconductor laser and the diagonal array, thereby bypassing the current flowing through the semiconductor laser to the bypass circuit. A protection device f for a semiconductor laser that prevents the laser from emitting an optical output higher than the allowable optical output.