JPS6331413Y2 - - Google Patents

Description

[Detailed Description of the Invention] A. Field of Industrial Application The present invention relates to a protection circuit for a semiconductor laser. B. Prior Art There is a semiconductor laser drive circuit shown in FIG. 1, for example, which is configured so that the oscillation output of the semiconductor laser is constant regardless of changes in ambient temperature, deterioration of the laser itself, etc. In the figure, 1 is a semiconductor laser, 2 is a monitor photodiode, and both are housed in the same cap. As the laser output increases, the current flowing through the monitor photodiode 2 increases, and this current flows through the amplifier circuit 3.
It is converted to voltage at This is applied to the comparison circuit 4 as a comparison voltage. On the other hand, a reference voltage set by resistors R ₁ , R ₂ , variable resistor R ₃ , Zener diode 5 and the like is applied to the other input terminal of the comparator circuit 4 . Transistor 6 is controlled based on the output of comparison circuit 4, and thus the output of semiconductor laser 1 is controlled to be constant. In addition, power supply 7
has a built-in smoothing circuit to suppress the influence of the fluctuation. Now, in such a conventional circuit, if there is an instantaneous disconnection due to poor contact or the like in the output line of the monitor photodiode, the semiconductor laser 1 will be
A current exceeding the rated value would flow, and there was a risk that the semiconductor laser 1 would be completely destroyed. The conventional circuit also includes a current limiting resistor _R4 , but this alone does not provide sufficient protection. C. Purpose of the invention The present invention aims to protect a semiconductor laser using a simple circuit. D. Structure of the invention The present invention is provided with a switching circuit that opens the drive line of the semiconductor laser in response to a decrease in the output of the monitoring photodiode. E. Embodiment In this embodiment, a switching circuit 10 composed of switching transistors 8 and 9 is further provided in addition to the conventional circuit shown in FIG.
In the normal state, transistor 8 is on and transistor 9 is off. Now, when a momentary disconnection occurs in the output line of the monitor photodiode 2, the output voltage of the amplifier circuit 3 decreases significantly, and the transistor 8 is turned off. Then, the transistor 9 is turned on and the drive line of the semiconductor laser 1 is opened. That is, excessive current is prevented from flowing through the semiconductor laser 1. Furthermore, when the power is turned on, switching circuit 1
In order to prevent the transistor 0 from malfunctioning, the bias power supply for the transistor 9 is a delayed power supply 11 whose rise time is slower than that of the power supply 7. F. Effects of the invention According to the invention, a semiconductor laser can be protected from destruction due to instantaneous disconnection with a simple configuration. In addition, a device similar to the present invention is published in Utility Model Publication No. 56-23904.
It is also disclosed in the issue. In this prior art, the input side voltage of the semiconductor laser drive circuit is cut off during abnormal operation, so the response is slow due to the capacitors included in the circuit. In contrast, in the present invention, the control transistor connected in series with the semiconductor laser is set to the OFF state by the output of the switching circuit including the switching transistor, so the response is fast and the semiconductor laser can be turned off. It can be set to an inactive state immediately. Furthermore, according to the present invention, since the bias of the switching transistor included in the switching circuit is configured to rise with a delay when the power is turned on, it is possible to reliably prevent malfunctions when the power is turned on.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a conventional semiconductor laser drive circuit, and FIG. 2 is a diagram showing a semiconductor laser protection circuit according to the present invention. 1 is a semiconductor laser, 2 is a monitor photodiode, 4 is a comparison circuit, and 10 is a switching circuit.

Claims (1)

[Scope of utility model registration request] a semiconductor laser; a monitoring photodiode for monitoring the output of the semiconductor laser;
a control transistor connected in series to the semiconductor laser and controlling the drive current of the semiconductor laser in response to the output of the monitor photodiode; and a control transistor in response to a decrease in the output of the monitor photodiode. A semiconductor laser comprising: a switching circuit including a switching transistor that operates to set the semiconductor laser to an OFF state; and a delay power source that delays the rise of bias of the switching transistor of the switching circuit when the power is turned on. protection circuit.