We investigate the reconstruction of depth and intensity profiles from data acquired using a custom-designed time-of-flight scanning transceiver based on the time-correlated single-photon counting technique. The system had an operational wavelength of 1550 nm and used a Peltier-cooled InGaAs/InP single-photon avalanche diode detector. Measurements were made of human figures, in plain view and obscured by camouflage netting, from a stand-off distance of 230 m in daylight using only submilliwatt average optical powers. These measurements were analyzed using a pixelwise cross correlation approach and compared to analysis using a bespoke algorithm designed for the restoration of multilayered three-dimensional light detection and ranging images. This algorithm is based on the optimization of a convex cost function composed of a data fidelity term and regularization terms, and the results obtained show that it achieves significant improvements in image quality for multidepth scenarios and for reduced acquisition times.
We investigate the potential of a depth imaging system for underwater environments. This system is based on the timeof- flight approach and the time correlated single-photon counting (TCSPC) technique. We report laboratory-based measurements and explore the potential of achieving sub-centimeter xyz resolution at 10’s meters stand-off distances. Initial laboratory-based experiments demonstrate depth imaging performed over distances of up to 1.8 meters and under a variety of scattering conditions. The system comprised a monostatic transceiver unit, a fiber-coupled supercontinuum laser with a wavelength tunable acousto-optic filter, and a fiber-coupled individual silicon single-photon avalanche diode (SPAD). The scanning in xy was performed using a pair of galvonometer mirrors directing both illumination and scattered returns via a coaxial optical configuration. Target objects were placed in a 110 liter capacity tank and depth images were acquired through approximately 1.7 meters of water containing different concentrations of scattering agent. Depth images were acquired in clear and highly scattering water using per-pixel acquisition times in the range 0.5-100 ms at average optical powers in the range 0.8 nW to 120 μW. Based on the laboratory measurements, estimations of potential performance, including maximum range possible, were performed with a model based on the LIDAR equation. These predictions will be presented for different levels of scattering agent concentration, optical powers, wavelengths and comparisons made with naturally occurring environments. The experimental and theoretical results indicate that the TCSPC technique has potential for highresolution underwater depth profile measurements.
Arrays of single-photon avalanche diode (SPAD) detectors were fabricated, using a 0.35 μm CMOS technology process, for use in applications such as time-of-flight 3D ranging and microscopy. Each 150 x 150 μm pixel comprises a 30 μm active area diameter SPAD and its associated circuitry for counting, timing and quenching, resulting in a fill-factor of 3.14%. This paper reports how a higher effective fill-factor was achieved as a result of integrating microlens arrays on top of the 32 x 32 SPAD arrays. Diffractive and refractive microlens arrays were designed to concentrate the incoming light onto the active area of each pixel. A telecentric imaging system was used to measure the improvement factor (IF) resulting from microlens integration, whilst varying the f-number of incident light from f/2 to f/22 in one-stop increments across a spectral range of 500-900 nm. These measurements have demonstrated an increasing IF with fnumber, and a maximum of ~16 at the peak wavelength, showing a good agreement with theoretical values. An IF of 16 represents the highest value reported in the literature for microlenses integrated onto a SPAD detector array. The results from statistical analysis indicated the variation of detector efficiency was between 3-10% across the whole f-number range, demonstrating excellent uniformity across the detector plane with and without microlenses.
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, high-performance
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
Active depth imaging approaches have numerous potential applications in a number of disciplines, including
environmental sensing, manufacturing and defense. The high sensitivity and picosecond timing resolution of the singlephoton
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 high repetition-rate (>MHz) pulsed illumination and a silicon single-photon detector. In advanced photon-counting
experiments, we have employed the system for unambiguous range resolution at several kilometer target distance,
multiple-surface resolution based on adaptive algorithms, and a cumulative data acquisition method that facilitates
detector characterization and evaluation. We consider a range of optical design configurations and discuss the
performance trade-offs in more detail. Much of this work has been performed at wavelengths around 850nm for
convenient use with Si-based single photon avalanche diode detectors, however we will also discuss the performance at
wavelengths around 1550 nm employing superconducting nanowire single photon detectors. The extension of this depth
profiling technique to longer wavelengths will lead to relaxed eye safety requirements, reduced solar background levels
and improvements in atmospheric transmission.