Nils J. Krichel

Long-range time-of-flight scanning sensor based on high-speed time-correlated single-photon counting

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.

Kilometre-range, high resolution depth imaging using 1560 nm wavelength single-photon detection

This paper highlights a significant advance in time-of-flight depth imaging: by using a scanning transceiver which incorporated a free-running, 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.

Kilometer-range depth imaging at 1550 nm wavelength using an InGaAs/InP single-photon avalanche diode detector

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 systems 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.

Resolving range ambiguity in a photon counting depth imager operating at kilometer distances.

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.

Full wave form analysis for long-range 3D imaging laser radar

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.

Analysis of detector performance in a gigahertz clock rate quantum key distribution system

We present a detailed analysis of a gigahertz clock rate environmentally robust phase-encoded quantum key distribution (QKD) system utilizing several different single-photon detectors, including the first implementation of an experimental resonant cavity thin-junction silicon single-photon avalanche diode. The system operates at a wavelength of 850 nm using standard telecommunications optical fibre. A general-purpose theoretical model for the performance of QKD systems is presented with reference to these experimental results before predictions are made about realistic detector developments in this system. We discuss, with reference to the theoretical model, how detector operating parameters can be further optimized to maximize key exchange rates.

Cumulative data acquisition in comparative photon-counting three-dimensional imaging

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.

Scanning of low-signature targets using time-correlated single-photon counting

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 objects.

Depth imaging at kilometer range using time-correlated single-photon counting at wavelengths of 850 nm and 1560 nm

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 detail.

Kilometer range depth imaging using time-correlated single-photon counting

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.