Construction of a pseudo-lock-in light-detection system using a gain-enhanced gated silicon avalanche photodiode

Abstract

We have constructed a simple pseudo-lock-in light-detection system incorporating a gated silicon (Si) avalanche photodiode (APD). The gate mode operation of the APD was achieved by superimposing a transistor–transistor-logic (TTL) signal pulse on the direct-current (dc) bias that was set slightly below the breakdown voltage Vb of the APD. When the incident light power of the signal and that of the background (BG) were low, depending on the level of the dc bias voltage Vs, the gain of the gated APD itself was enhanced about one hundred times at maximum in comparison with that of the dc-biased APD without the gate pulse. Although the gain advantage was decreased with high BG light, the gain was several-fold higher compared to the ungated-APD. The APD was operated at a frequency of 2f = 20 kHz and its output signal was fed into a phase sensitive detector (PSD) that worked in synchrony with the gated APD at a frequency f = 10 kHz. Thanks to the gate mode operation of the APD, the sensitivity of the pseudo-lock-in light-detection system was increased markedly for the low level of the incident BG light. The system was also useful for detecting weak signal light under the large BG light situation.