Time-of-flight sensing using time-correlated single-photon counting

Time-correlated single-photon counting techniques using individual optimized detectors have been applied to time-of-flight ranging and depth imaging. This paper describes recent progress in photon-counting systems performing surface mapping of non-cooperative targets. This includes systems designed for short ranges of the order of 1-50 meters, and longer ranges of up to ten kilometers. The technique has also been applied to distributed surfaces. We describe the measurement approach, techniques used for scanning, as well as the signal analysis methodology and algorithm selection. The technique is fundamentally flexible: the trade-off between the integrated number of counts (or acquisition time) against range repeatability or depth resolution allows its application in a number of diverse fields. The inherent time gating of the technique, allied to the spatial filtering provided by small active area single-photon detectors, can lead to operation under high ambient light conditions even with low average optical power pulsed sources. We have demonstrated three-dimensional imaging of meter-dimensioned objects where reverse engineering methods using cooperative targets cannot be routinely employed: e.g. delicate objects, or objects with more than one reflective surface. Using more advanced signal processing algorithms, we have been able to improve the system performance significantly, as measured by the depth resolution at short and long ranges. Furthermore, the application of these methodologies has allowed us to characterize the positions and amplitudes of multiple returns. Hence, the approach can be used for characterization of distributed non-cooperative targets at kilometer ranges, even in environments where low-light level and and/or eye-safe operation is necessary. The technique has also been applied in conjunction with a rapid scanning approach, to acquire three-dimensional information of a target scene with frame times of approximately 1 second.