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.
Single-photon detectors play an increasing role in emerging application areas in quantum communication and low-light
level depth imaging. The single-photon detector characteristics have a telling impact in system performance, and this
presentation will examine the role of single-photon detectors in these important application areas. We will discuss the
experimental system performance of GHz-clocked quantum key distribution systems focusing on issues of quantum bit
error rate, net bit rate and transmission distance with different detector structures, concentrating on single-photon
avalanche diode detectors, but also examining superconducting nanowire-based structures. The quantum key
distribution system is designed to be environmentally robust and an examination of long-term system operation will be
presented. The role of detector performance in photon-counting time-of-flight three-dimensional imaging will also be
discussed. We will describe an existing experimental test bed system designed for kilometer ranging, and recent
experimental results from field trials. The presentation will investigate the key trade-offs in data acquisition time, optical
power levels and maximum range. In both examples, experimental demonstrations will be presented to explore future
perspectives and design goals.
Proc. SPIE. 7780, Detectors and Imaging Devices: Infrared, Focal Plane, Single Photon
KEYWORDS: Target detection, Signal to noise ratio, Detection and tracking algorithms, Imaging systems, Sensors, Data acquisition, Single photon detectors, Target acquisition, 3D image processing, Data analysis
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.
Single-photon detection technologies in conjunction with low laser illumination powers allow for the eye-safe
acquisition of time-of-flight range information on non-cooperative target surfaces. We previously presented a
photon-counting depth imaging system designed for the rapid acquisition of three-dimensional target models
by steering a single scanning pixel across the field angle of interest. To minimise the per-pixel dwelling times
required to obtain sufficient photon statistics for accurate distance resolution, periodic illumination at multi-
MHz repetition rates was applied. Modern time-correlated single-photon counting (TCSPC) hardware allowed
for depth measurements with sub-mm precision.
Resolving the absolute target range with a fast periodic signal is only possible at sufficiently short distances:
if the round-trip time towards an object is extended beyond the timespan between two trigger pulses, the return
signal cannot be assigned to an unambiguous range value. Whereas constructing a precise depth image based
on relative results may still be possible, problems emerge for large or unknown pixel-by-pixel separations or in
applications with a wide range of possible scene distances.
We introduce a technique to avoid range ambiguity effects in time-of-flight depth imaging systems at high average
pulse rates. A long pseudo-random bitstream is used to trigger the illuminating laser. A cyclic, fast-Fourier
supported analysis algorithm is used to search for the pattern within return photon events. We demonstrate this
approach at base clock rates of up to 2 GHz with varying pattern lengths, allowing for unambiguous distances
of several kilometers. Scans at long stand-off distances and of scenes with large pixel-to-pixel range differences
are presented. Numerical simulations are performed to investigate the relative merits of the technique.
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
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.
We describe a scanning time-of-flight system which uses the time-correlated single photon-counting technique to
produce three-dimensional depth images of scenes using low average laser power levels (ie <1mW). The technique is
fundamentally flexible: the trade-off between the integrated number of counts (or acquisition time) against depth
resolution permits use in a diverse range of applications. The inherent time gating of the technique, used in conjunction
with spatial and spectral filtering, permits operation under high ambient light conditions.
Our optical system uses a galvanometer mirror pair to scan the laser excitation over the scene and to direct the collected
scattered photon return to an individual silicon single-photon avalanche diode detector. The system uses a picosecond
pulsed diode laser at a wavelength of 850nm at MHz repetition rates. The source is directed to the target and the
scattered return is collected using a 200mm focal length camera lens. The optical system is housed in a compact customdesigned
slotted baseplate optomechanical platform. Currently, the system is capable of a spatial resolution and a depth
resolution of better than 10cm at 1km range. We present a series of measurements on a range of non-cooperative target