Characterization of photon-counting detector responsivity for nonlinear two-photon absorption process

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.