Abstract
Extremely weak optical signals were measured for the visible (λ=632.8 nm) and infrared (λ=1153.0nm) lights by the direct current method using avalanche photodiodes (Si-APD or Ge-APD) cooled at the liquid nitrogen temperature. Variations of the photocurrent and the signal-to-noise ratio vs. the optical input power are examined. A good linearity between the optical input power and the photocurrent is obtained in the range from about 10-11 W to 10-17 W. The minimum detectable optical input powers at S/N=1 are found to be 3×10-17 W and 6×10-15 W for a sample of Si-APD and that of Ge-APD, respectively. Experimental results show that one could use an APD as an analog optical sensor with a wide dynamic range. Theoretical analysis and discussion on the minimum detectable optical input power and the signal-to-noise ratio are given based on the noise multiplication factor and the noise equivalent power of the APD at the optimum bias voltage.
