Simple, small-sized, lock-in light detection system using a gated Si avalanche photodiode

We have constructed a simple, small-sized, lock-in light detection system using a gated Si avalanche photo-diode (APD). The gate mode operation of the APD was achieved by a transistor-transistor-logic (TTL) signal superimposed on a direct-current (dc) bias not exceeding the breakdown voltage of the A-PD. The attainable gain was thirty times larger than that obtained by the normal dc-biased APD. The APD is operated at a frequency of 2f (= 20 kHz) and its output signal is fed into a compact, laboratory-made lock-in amplifier that works in synchronous with the gated APD at a frequency f (= 10 kHz). The system is useful for detecting a weak signal light superimposed on a large background.