Diode Sensor For Lasers

Laser-receiver circuits must bias their avalanche photo diodes (APD) to achieve optimal gain. Unfortunately, an APD
Diode Sensor For Lasers - schematic

`s gain depends on the operating temperature. The circuit controls the operating voltage of an APD over a large temperature range to maintain the gain at the optimal value. The circuit uses Dl as a temperature sensor, thermally matched with the APD. A voltage regulator, IC1, supplies the necessary reference voltage to the circuit. IC2A and Ql bias Dl at a constant current. IC2B, IC2C, IC3A/IC3B, and IC3C amplify Dl`s varying voltage and set Q2 to the optimal-gain corresponding value. Potentiometer Rl controls the amplification over a range of 5 to 15. R2 controls the voltage level, which corresponds to the optimal gain of the APD at 22°C (the temperature is specific to the type of APD). The circuit shown was tested with an RCA C 30954E APD. The tests covered -40 to + 70°C and used a semiconductor laser. The laser radiation was transmuted on the APD`s active surface in the climatic room via fiberoptic cable. The gain varied by, at most, ±0.2 dB over the entire temperature range.

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