We describe a scanning time-of-flight system which uses the time-correlated single-photon counting technique to produce three-dimensional depth images of distant, noncooperative surfaces when these targets are illuminated by a kHz to MHz repetition rate pulsed laser source. The data for the scene are acquired using a scanning optical system and an individual single-photon detector. Depth images have been successfully acquired with centimeter xyz resolution, in daylight conditions, for low-signature targets in field trials at distances of up to 325 m using an output illumination with an average optical power of less than 50 microW.

This paper highlights a significant advance in time-of-flight depth imaging: by using a scanning transceiver which incorporated a freerunning, low noise superconducting nanowire single-photon detector, we were able to obtain centimeter resolution depth images of low-signature objects in daylight at stand-off distances of the order of one kilometer at the relatively eye-safe wavelength of 1560 nm. The detector used had an efficiency of 18% at 1 kHz dark count rate, and the overall system jitter was ~100 ps. The depth images were acquired by illuminating the scene with an optical output power level of less than 250 µW average, and using per-pixel dwell times in the millisecond regime.

We have used an InGaAs/InP single-photon avalanche diode detector module in conjunction with a time-of-flight depth imager operating at a wavelength of 1550 nm, to acquire centimeter resolution depth images of low signature objects at stand-off distances of up to one kilometer. The scenes of interest were scanned by the transceiver system using pulsed laser illumination with an average optical power of less than 600 µW and per-pixel acquisition times of between 0.5 ms and 20 ms. The fiber-pigtailed InGaAs/InP detector was Peltier-cooled and operated at a temperature of 230 K. This detector was used in electrically gated mode with a single-photon detection efficiency of about 26% at a dark count rate of 16 kilocounts per second. The system's overall instrumental temporal response was 144 ps full width at half maximum. Measurements made in daylight on a number of target types at ranges of 325 m, 910 m, and 4.5 km are presented, along with an analysis of the depth resolution achieved.

Time-correlated single-photon counting techniques have recently been used in ranging and depth imaging systems that are based on time-of-flight measurements. These systems transmit low average power pulsed laser signals and measure the scattered return photons. The use of periodic laser pulses means that absolute ranges can only be measured unambiguously at low repetition rates (typically <100 kHz for > 1 km) to ensure that only one pulse is in transit at any instant. We demonstrate the application of a pseudo-random pattern matching technique to a scanning rangefinder system using GHz base clock rates, permitting the acquisition of unambiguous, three-dimensional images at average pulse rates equivalent to >10 MHz. Depth images with centimeter distance uncertainty at ranges between 50 m and 4.4 km are presented.

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In recent years, time-correlated single-photon counting techniques have been applied to time-of-flight measurements for long-distance range-finding and depth imaging. Depth imaging has been performed by obtaining timing information from an individual single-photon detector and scanning the optical field to obtain a full depth image. Typically, the measurement is made by dwelling on each individual depth pixel for a pre-defined integration time and completing the data acquisition for that pixel before steering the beam to the adjacent spatial position. We present a novel photon-counting data acquisition mode where the time-of-flight histograms for each depth pixel are gradually populated. The system repeatedly scans the same spatial frame with short per-pixel dwell times, and sufficient photon statistics are built up over many frames by cumulating photon events from all acquired frames. The technique is used to compare the depth imaging performance of two single-photon avalanche diode detectors: a novel, resonant-cavity enhanced shallow-junction device; and a commercially available thick-junction device

The new generation of 3D imaging systems based on laser radar (ladar) offers significant advantages in defense and security applications. In particular, it is possible to retrieve 3D shape information directly from the scene and separate a target from background or foreground clutter by extracting a narrow depth range from the field of view by range gating, either in the sensor or by postprocessing. We discuss and demonstrate the applicability of full-waveform ladar to produce multilayer 3D imagery, in which each pixel produces a complex temporal response that describes the scene structure. Such complexity caused by multiple and distributed reflection arises in many relevant scenarios, for example in viewing partially occluded targets, through semitransparent materials (e.g., windows) and through distributed reflective media such as foliage. We demonstrate our methodology on 3D image data acquired by a scanning time-of-flight system, developed in our own laboratories, which uses the time-correlated single-photon counting technique.

Active, three-dimensional long-range imaging has varied applications in a number of disciplines, including manufacturing, environmental sensing and defence. Common constraints often include low average and peak illumination powers to ensure eye-safety, making the potentially high sensitivity of the single-photon counting technique a distinct advantage. We present a scanning time-of-flight imager based on high repetition-rate (>MHz) pulsed illumination and a silicon single-photon detector. In advanced photon-counting experiments, we recently employed the system for unambiguous range resolution at several kilometres target distance, multiple-surface resolution based on adaptive algorithms, and a cumulative data acquisition method that facilitates detector characterisation and evaluation. This article reviews these achievements and identifies multi-spectral imaging as a possible future application.