Sub-band absorption at 1550 nm has been demonstrated and characterized on silicon Geiger mode detectors, which
normally would be expected to have no response at this wavelength. We compare responsivity measurements to single-photon
absorption for wavelengths slightly above the band gap wavelength of silicon (~1100 μm). One application for this low efficiency sub-band absorption is in deep space optical communication systems where it is desirable to track a 1030 nm uplink beacon on the same flight terminal detector array that monitors a 1550 nm downlink signal for pointing control. The currently observed absorption at 1550 nm provides 60-70 dB of isolation compared to the response at 1064
nm, which is desirable to avoid saturation of the detector by scattered light from the downlink laser.
A hardware prototype of a flight receiver for deep space optical communications has been developed where a single
detector array is used for acquisition, tracking, and high-speed data recovery. A counting algorithm accumulates pulses
on every pixel in a photon-counting array and extracts signal information encoded with a nested modulation scheme.