Depth imaging at kilometer range using time-correlated single-photon counting at wavelengths of 850 nm and 1560 nm

Active depth imaging approaches are being used in a number of emerging applications, for example in environmental sensing, manufacturing and defense. The high sensitivity and picosecond timing resolution of the time-correlated single-photon counting technique can provide distinct advantages in the trade-offs between required illumination power, range, depth resolution and data acquisition durations. These considerations must also address requirements for eye-safety, especially in applications requiring outdoor, kilometer range sensing. We present a scanning time-of-flight imager based on MHz repetition-rate pulsed illumination operating with sub-milliwatt average power. The use of a scanning mechanism permits operation with an individual, highperformance single-photon detector. The system has been used with a number of non-cooperative targets, in different weather conditions and various ambient light conditions. We consider a number of system issues, including the range ambiguity issue and scattering from multiple surfaces. The initial work was performed at wavelengths around 850 nm for convenient use with Si-based single photon avalanche diode detectors, however we will also discuss the performance at a wavelength of 1560 nm, made using superconducting nanowire single photon detectors. The use of the latter wavelength band allows access to a low-loss atmospheric window, as well as greatly reduced solar background contribution and less stringent eye safety considerations. We consider a range of optical design configurations and discuss the performance trade-offs and future directions in more detail.