Ultra-Sensitive Autocorrelation of 1.5 microns light with a Photon-Counting Silicon Avalanche Photodiode

Autocorrelation of 1.5 microns light using two-photon absorption in a photon-counting silicon avalanche photodiode (APD) is demonstrated. The large internal gain provide by the APD (~10 sup 6) and the use of photon counting detection offer unprecedented sensitivity in terms of the average and peak incident power. Two-photon absorption generated by single-mode CW laser dominates the signal output at average power levels above 100 microwatts. High signal-to-noise ratio autocorrelation trace of a 10 GHz repetition rate laser with 1.67 ps pulses can be obtained with only 7.2 microwatts of average power (220 microwatts peak power) incident onto the APD. This yields a measured peak power, times average power sensitivity of 1.5 x 10 sup -3 (mW) sup 2 , which to our knowledge is a record for two-photon absorption performed without the use of lock-in detection.