Ground detectors for optical communications from deep space

A variety of avalanche photodiodes (APD's) were tested with pulse position modulated (PPM) Q-switched laser pulses incident on the detector, with varying amounts of attenuation. The detector output was recorded and postprocessed in order to determine the signal and noise slot statistics, as well as, to estimate bit-error-rates (BER). The probability distribution functions predicted by a Webb+Gaussian model were compared to the measured slot statistics, as were theoretical BER curves. Allowing noise equivalent temperature to be a free fitting parameter yielded good fits between measurements and theory. All the measurements used 256-ary PPM and 10-25 ns slot widths, with a Q-switched Nd: YVO4 laser modulated at 50K-100K pulses per second. A 3 mm diameter, silicon (Si) APD with 80% quantum efficiency (QE) at 532 nm displayed a sensitivity of 11-12 photons/bit for a BER of 10(-2) in the absence of background light. For this detector the sensitivity deteriorated to 18 photons/bit in the presence of 100 photons per 25 ns slot of background light. A 0.8 mm diameter near infrared (NIR) enhanced Si APD with QE of 0.38 displayed sensitivities of 23 - 32 photons/bit for a BER of 10(-2) at 1064 nm in the absence of background light. Backgrounds of 400 photons per 25 ns slot degraded the sensitivity to similar to 58 photons/slot. Finally a 3 mm 0 diameter NIR enhanced Si APD yielded a sensitivity of similar to 100 photons/bit @ 1064 nm for a BER of 10(-2) with no background present.