Scanning of low-signature targets using time-correlated single-photon counting

This paper presents recent progress in the development of a scanning time-of-flight imaging system employing time-correlated single-photon counting (TCSPC) designed for the acquisition of depth information at kilometre ranges. The device is capable of acquiring information on non-cooperative target surfaces at eye-safe average optical power levels in the near-IR regime (<1 mW at 842 nm illumination wavelength). Target illumination is periodic or non-periodic at typical repetition frequencies in the MHz domain, utilising a sub-ns pulse-width laser diode. The system output is steered over the optical field of interest, and return photons from the target are routed towards a single-photon detector. Measurements are performed with a silicon single-photon avalanche diode (SPAD). Effective optical spatial and spectral filtering techniques permit operation in bright daylight conditions. Results in the form of depth images from a variety of targets, taken under various environmental conditions, are presented. Achieved improvements of this first-generation system are discussed in terms of parametric enhancement of quantities such as spatial and spectral filtering, internal optical attenuation and beam size. We detail progress in the design process both based on theoretical assumptions and actual measurements at distances between few 100's of metres and several km. The trade-offs between acquisition time, maximum range and excitation laser power levels are discussed and projections made for this and future depth imaging systems. State-of-the-art TCSPC hardware solutions facilitate the rapid transfer and storage of large quantities of raw data. This renders possible real-time analysis with speed-optimised algorithms such as fast Fourier transform-supported cross-correlation methods, as well as gathering additional information about the scene in post-processing steps, based on approaches such as reversible-jump Markov-chain Monte Carlo (RJMCMC). This algorithm dynamically adapts the number of degrees of freedom of a range measurement, resulting in multi-surface resolution and the possible identification of targets obscured by objects such as foliage. © 2009 SPIE.