Advances in LIDAR-based methods have enabled the detection and reconstruction of images of static objects hidden from the direct line-of-sight [1, 2]. One of the drawbacks to the technology used in these demonstrations is the requirement for long acquisition times. More recently, Gariepy et al. have shown that it is possible to detect and track a moving hidden object, albeit with no information of the object’s form . Applications of this include, but are not limited to, search and rescue, and hazard detection.
We present a real-time tracking system that enables the detection of moving objects that are outside the direct line-of-sight. Our active imaging system is a single-pixel variant of the technology reported by Gariepy et al. It replaces the single-photon avalanche diode (SPAD) camera of 1024 pixels with a number of SPAD detectors to detect light back-scattered from the hidden object. The flexibility of the single-pixel detectors provides an increased field of view, allowing us to detect and simultaneously track with better precision with respect to a SPAD array. The use of single-pixel detectors also has the advantage of a high detection efficiency.
We perform two proof-of-concept experiments using three pixels and a single pulsed laser to interrogate a “room” for a hidden object. In the first experiment, we demonstrate that we can accurately locate the position of a hidden object. In the second experiment, we use the same system and demonstrate that we can accurately track the motion of a hidden object in real time.
The “room” is a purpose-built box measuring 102×102×77 cm. Optical access is provided by a 28×12 cm window. The target object is a 15×15 cm textured viewing screen that we move along a designated ground track outside the line-of- sight of our system. In our experiments, we send a train of light pulses through the window to the back of the room. The pulses scatter off the wall as a spherical wavefront that propagates in all directions. Some of this light reaches our hidden object and is scattered back again towards the rear wall where we image our three SPAD pixels. The SPAD detectors are capable of picosecond temporal resolutions. Our time-correlated single-photon counting system measures the photon arrival times (64 ps resolution) for the signal returning to each detector. A histogram is built up in one second of acquisition time over 80 million pulses. We use this temporal information in our target position retrieval of the hidden object.
We place the object at 11 positions in turn in a seven minute experiment, and localise its position. We then perform real-time tracking and move the object around the hidden scene for approximately one minute, processing the target position retrieval every 1.5 s.
The recent development of 2D arrays of single-photon avalanche diodes (SPAD) has driven the development of applications based on the ability to capture light in motion. Such arrays are composed typically of 32x32 SPAD detectors, each having the ability to detect single photons and measure their time of arrival with a resolution of about 100 ps. Thanks to the single-photon sensitivity and the high temporal resolution of these detectors, it is now possible to image light as it is travelling on a centimetre scale. This opens the door for the direct observation and study of dynamics evolving over picoseconds and nanoseconds timescales such as laser propagation in air, laser-induced plasma and laser propagation in optical fibres. Another interesting application enabled by the ability to image light in motion is the detection of objects hidden from view, based on the recording of scattered waves originating from objects hidden by an obstacle. Similarly to LIDAR systems, the temporal information acquired at every pixel of a SPAD array, combined with the spatial information it provides, allows to pinpoint the position of an object located outside the line-of-sight of the detector. A non-line-of-sight tracking can be a valuable asset in many scenarios, including for search and rescue mission and safer autonomous driving.
The ability to detect motion and to track a moving object that is hidden around a corner or behind a wall
provides a crucial advantage when physically going around the obstacle is impossible or dangerous. One recently
demonstrated approach to achieving this goal makes use of non-line-of-sight picosecond pulse laser ranging.
This approach has recently become interesting due to the availability of single-photon avalanche diode (SPAD)
receivers with picosecond time resolution. We present a time-resolved non-sequential ray-tracing model and its
application to indirect line-of-sight detection of moving targets. The model makes use of the Zemax optical
design programme's capabilities in stray light analysis where it traces large numbers of rays through multiple
random scattering events in a 3D non-sequential environment. Our model then reconstructs the generated
multi-segment ray paths and adds temporal analysis. Validation of this model against experimental results is
shown. We then exercise the model to explore the limits placed on system design by available laser sources and
detectors. In particular we detail the requirements on the laser's pulse energy, duration and repetition rate, and
on the receiver's temporal response and sensitivity. These are discussed in terms of the resulting implications
for achievable range, resolution and measurement time while retaining eye-safety with this technique. Finally,
the model is used to examine potential extensions to the experimental system that may allow for increased
localisation of the position of the detected moving object, such as the inclusion of multiple detectors and/or
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.
Using an electron multiplying CCD camera we observe both image plane (position) and far field (momentum) correlations between photon pairs produced from spontaneous parametric down-conversion when using a 201 x 201 bi-dimensional array of pixels and a flux of around 0.02 photons/pixel. After background subtraction we characterize the strength of signal and idler correlations in both transverse dimensions by applying entanglement and EPR criteria, showing good agreement with the theoretical predictions. The application of such devices in quantum optics could have a wide range, including quantum computation with spatial degrees of freedom of single photons.
We have developed a new approach to measuring the spatial position of a single photon. Using fibers of different
length, all connected to a single detector allows us to use the high timing precision of single photon avalanche diodes
(SPAD) to spatially locate the photon. We have built two 8-element detector arrays to measure the full-field quantum
correlations in position, momentum and intermediate bases for photon pairs produced in parametric down conversion.
The strength of the position-momentum correlations is found to be an order of magnitude below the classical limit.
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.
Single-photon sources and detectors are key enabling technologies for photonics in quantum information science and
technology (QIST). QIST applications place high-level demands on the performance of sources and detectors; it is
therefore essential that their properties can be characterized accurately. Superconducting nanowire single-photon
detectors (SNSPDs) have spectral sensitivity from visible to beyond 2 μm in wavelength, picosecond timing resolution
(Jitter <100 ps FWHM) and the capacity to operate ungated with low dark counts (<1 kHz). This facilitates data
acquisition at high rates with an excellent signal-to-noise ratio.
We report on the construction and characterization of a two-channel SNSPD system. The detectors are mounted in a
closed-cycle refrigerator, which eliminates reliance on liquid cryogens. Our specification was to deliver a system with
1% efficiency in both channels at a wavelength of 1310 nm with 1 kHz dark count rate. A full width at half maximum
timing jitter of less than 90 ps is achieved in both channels. The system will be used to detect individual photons
generated by quantum-optical sources at telecom wavelengths. Examples include single-photon sources based on
quantum dots (emitting at 1310 nm). The SNSPD system's spectral sensitivity and timing resolution make it suited to
characterization of such sources, and to wider QIST applications.