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We demonstrate a fully submerged underwater LiDAR transceiver system based on single-photon detection technologies. The LiDAR imaging system used a silicon single-photon avalanche diode (SPAD) detector array fabricated in complementary metal-oxide semiconductor (CMOS) technology to measure photon time-of-flight using picosecond resolution time-correlated single-photon counting. The SPAD detector array was directly interfaced to a Graphics Processing Unit (GPU) for real-time image reconstruction capability. Experiments were performed with the transceiver system and target objects immersed in a water tank at a depth of 1.8 meters, with the targets placed at a stand-off distance of approximately 3 meters. The transceiver used a picosecond pulsed laser source with a central wavelength of 532 nm, operating at a repetition rate of 20 MHz and average optical power of up to 52 mW, dependent on scattering conditions. Three-dimensional imaging was demonstrated by implementing a joint surface detection and distance estimation algorithm for real-time processing and visualization, which achieved images of stationary targets with up to 7.5 attenuation lengths between the transceiver and the target. The average processing time per frame was approximately 33 ms, allowing real-time three-dimensional video demonstrations of moving targets at ten frames per second at up to 5.5 attenuation lengths between transceiver and target.

Data security plays an increasingly important role in modern telecommunications. The advent of quantum computational processors presents a significant threat to today's widely employed public key encryption algorithms, necessitating the adoption of new approaches to data encryption. Whilst quantum key distribution guarantees unconditional security for cryptographic key exchange in optical communication networks, the data rate is slow (Mbit/s), especially when compared to conventional optical communication. Here we present a highly secure encryption approach in which the encryption key, generated by quantum key distribution at a rate of up to 2.9 Mbit/s, was used to seed physical layer encryption performed using time domain spectral phase encoding (TDSPE). This allowed us to demonstrate encrypted 40 Gbit/s quadrature phase shift keyed data communications over 52.3 km of installed optical fiber, which cannot be eavesdropped using brute force computational attacks. Any attempt to eavesdrop the encrypted signal in the physical layer is highly time-sensitive - the phase states must be measured and decrypted prior to optical signal attenuation, which means that the attack procedure typically needs to be completed within a few milliseconds. This work represents the first example of quantum-enhanced physical layer encryption at realistic optical data rates that is fully secure from brute force computational attacks and the first demonstration of TDSPE using continuous-wave laser source and quadrature phase shift key modulation.

We describe a technique whereby a coherent amplitude can be imprinted nonlocally on to a beam of light with thermal statistics that has no phase information on average. We have successfully performed the first experimental realisation. The technique could have applications in the sharing of quantum information and in covert quantum imaging scenarios.

We present and discuss the case for full waveform pixel and image acquisition and processing to enable LiDAR sensors to penetrate and reconstruct 3D surface maps through obscuring media. To that end, we review work on signal propagation, on scanning and arrayed sensors, on signal processing strategies for independent pixels and employing spatial context, on reducing complexity and accelerating processing by sensor design, algorithmic changes, compressed sensing, and parallel processing. We report several experimental studies on LiDAR imaging through complex media, and how these can inform the automotive LiDAR scenario. We conclude with a discussion of future development and potential for full waveform LiDAR (FWL).

The challenges of real world applications of the laser detection and ranging (Lidar) three-dimensional (3-D) imaging require specialized algorithms. In this paper, a new reconstruction algorithm for single-photon 3-D Lidar images is presented that can deal with multiple tasks. For example, when the return signal contains multiple peaks due to imaging semitransparent surfaces, or when imaging through obscurants such as scattering media. A generalization to the multidimensional case, including multispectral and multitemporal 3-D images, is also provided. The approach is based on the minimization of a cost function accounting for Poissonian observations of the single-photon data, the nonlocal spatial correlations between pixels and the small number of depth layers inside the observed range window. An alternating direction method of multipliers that offers good convergence properties is used to solve this minimization problem. The resulting algorithm is validated on synthetic and real data and in challenging realistic scenarios including sparse photon regimes for fast imaging, the presence of high background due to obscurants, and the joint processing of multispectral and/or multitemporal data.

Germanium-on-silicon (Ge-on-Si) based single-photon avalanche diodes (SPADs) have recently emerged as a promising detector candidate for ultra-sensitive and picosecond resolution timing measurement of short-wave infrared (SWIR) photons. Many applications benefit from operating in the SWIR spectral range, such as long distance light detection and ranging, however, there are few single-photon detectors exhibiting the high-performance levels obtained by all-silicon SPADs commonly used for single-photon detection at wavelengths µm. This paper first details the advantages of operating at SWIR wavelengths, the current technologies, and associated issues, and describes the potential of Ge-on-Si SPADs as a single-photon detector technology for this wavelength region. The working principles, fabrication and characterisation processes of such devices are subsequently detailed. We review the research in these single-photon detectors and detail the state-of-the-art performance. Finally, the challenges and future opportunities offered by Ge-on-Si SPAD detectors are discussed.

Single-photon avalanche diode (SPAD) detectors are of significant interest for numerous applications, including light detection and ranging (LIDAR), and quantum technologies such as quantum-key distribution and quantum information processing. Here we present a record low noise-equivalent-power (NEP) for Ge-on-Si SPADs using a pseudo-planar design, showing high detection efficiency in the short-wave infrared; a spectral region which is key for quantum technologies and hugely beneficial for LIDAR. These devices can leverage the benefits of Si avalanche layers, with lower afterpulsing compared to InGaAs/InP, and reduced cost due to Si foundry compatibility. By scaling the SPAD pixels down to 26μm diameter, a step change in performance has been demonstrated, with significantly reduced dark count rates (DCRs), and low jitter (134ps). Ge-on-Si SPADs were fabricated using photolithography techniques and characterised using time-correlated single-photon counting. The DCR reaches as low as kilocount/s at 100K for excess bias up to ~5%. This reduction in DCR enables higher temperature operation; e.g. the DCR of a 26μm diameter pixel at 150 K is approximately equivalent to a 100 μm diameter pixel at 77 K (100s of kilocounts/s). These low values of DCR, coupled with the relatively temperature independent single photon detection efficiencies (SPDE) of ~29% (at 1310nm wavelength) leads to a record low NEP of 7.7×10−17WHz−1/2. This is approximately 2 orders of magnitude lower than previous similarly sized mesa-geometry Ge-on-Si SPADs. This technology can potentially offer a lowcost, Si foundry compatible SPAD operating at short-wave infrared wavelengths, with potential applications in quantum technologies and autonomous vehicle LIDAR.

3D single-photon LiDAR imaging plays an important role in numerous applications. However, long acquisition times and significant data volumes present a challenge to LiDAR imaging. This paper proposes a task-optimized adaptive sampling framework that enables fast acquisition and processing of high-dimensional single-photon LiDAR data. Given a task of interest, the iterative sampling strategy targets the most informative regions of a scene which are defined as those minimizing parameter uncertainties. The task is performed by considering a Bayesian model that is carefully built to allow fast per-pixel computations while delivering parameter estimates with quantified uncertainties. The framework is demonstrated on multispectral 3D single-photon LiDAR imaging when considering object classification and/or target detection as tasks. It is also analysed for both sequential and parallel scanning modes for different detector array sizes. Results on simulated and real data show the benefit of the proposed optimized sampling strategy when compared to fixed sampling strategies.

We present a pseudo-planar geometry 26µm diameter Ge-on-Si single-photon avalanche diode (SPAD) detector with temperature insensitive single photon detection efficiency of 29.4% at 1310nm wavelength for applications including free-space LIDAR. A record low dark count rate of 104 counts/s at 125K at an excess bias of 6.6% is demonstrated, with temporal jitter reaching 134ps. The noise-equivalent power is measured to be 7.7x10-17WHz-12 which is a 2 orders of magnitude reduction when compared to comparable 25µm mesa devices. This device represents the state-of-the-art for Ge-on-Si SPADs, and highlights that these Si foundry compatible devices have enormous potential for SWIR single-photon applications.

Single-photon detection when used in picosecond resolution time-correlated mode is now a candidate technology for a number of depth imaging applications in the visible, near-infrared and short-wave infrared regions. This approach has been used in a range of challenging sensing scenarios including imaging though highly scattering underwater conditions, free-space imaging through obscurants such as smoke or fog, and depth imaging of complex multiple surface scenes. I will review recent developments in this field and describe recent experiments and computational imaging approaches that highlight the long-term potential of this work.

Single-Photon Avalanche Diode (SPAD) detectors are of significant interest for a range of applications [1] , in particular for quantum technologies (e.g. quantum-key distribution, quantum information processing), and light detection and ranging (LIDAR) for defence, terrain mapping, and autonomous vehicles. These applications either require, or benefit from, operation at wavelengths in the short-wave infrared (SWIR). Previous SWIR single-photon LIDAR has typically used InGaAs/InP SPAD detector technology, which has relatively low efficiency and suffers from afterpulsing. Previously, a pseudo-planar design for a Ge-on-Si SPAD was demonstrated [2] , yielding a huge improvement in performance for Ge-on-Si SPADs at 1310 nm and demonstrating the potential for Si foundry compatible SWIR SPADs. Furthermore, reduced afterpulsing was demonstrated compared to a commercial InGaAs/InP device when measured in nominally identical conditions. Here we present a further step change in performance, with reduced dark count rate (DCR), record low noise-equivalent-power (NEP) and low jitter by scaling the technology and developing 26 µm diameter pixels [3] .

Inspired by plant grafting, grafted vortex beams can be formed through grafting two or more helical phase profiles of optical vortex beams. Recently, grafted perfect vortex beams (GPVBs) have attracted much attention due to their unique optical properties and potential applications. However, the current method to generate and manipulate GPVBs requires a complex and bulky optical system, hindering further investigation and limiting its practical applications. Here, a compact metasurface approach for generating and manipulating GPVBs in multiple channels is proposed and demonstrated, which eliminates the need for such a complex optical setup. A single metasurface is utilized to realize various superpositions of GPVBs with different combinations of topological charges in four channels, leading to asymmetric singularity distributions. The positions of singularities in the superimposed beam can be further modulated by introducing an initial phase difference in the metasurface design. The work demonstrates a compact metasurface platform that performs a sophisticated optical task that is very challenging with conventional optics, opening opportunities for the investigation and applications of GPVBs in a wide range of emerging application areas, such as singular optics and quantum science.

Recently, time-of-flight LiDAR using the single-photon detection approach has emerged as a potential solution for three-dimensional imaging in challenging measurement scenarios, such as over distances of many kilometres. The high sensitivity and picosecond timing resolution afforded by single-photon detection offers high-resolution depth profiling of remote, complex scenes while maintaining low power optical illumination. These properties are ideal for imaging in highly scattering environments such as through atmospheric obscurants, for example fog and smoke. In this paper we present the reconstruction of depth profiles of moving objects through high levels of obscurant equivalent to five attenuation lengths between transceiver and target at stand-off distances up to 150 m. We used a robust statistically based processing algorithm designed for the real time reconstruction of single-photon data obtained in the presence of atmospheric obscurant, including providing uncertainty estimates in the depth reconstruction. This demonstration of real-time 3D reconstruction of moving scenes points a way forward for high-resolution imaging from mobile platforms in degraded visual environments.

We present a method of improving the spatial resolution of a single-photon counting light detection and ranging system using a sub-pixel micro-scanning approach. The time-correlated single-photon counting technique was used to measure photon time-of-flight from remote objects. The high-sensitivity and picosecond timing resolution of this approach allows for high-resolution depth and intensity information to be obtained from targets with very low average optical power levels. The system comprised a picosecond pulsed laser source operated at a wavelength of 1550 nm and a 32 × 32 InGaAs/InP single-photon avalanche diode detector array. The detector array was translated along two orthogonal axes in the image plane of the receive channel objective lens using two computer-controlled motorized translation stages. This allowed for sub-pixel scanning, resulting in a composite image of the scene with improved spatial resolution. This paper presents preliminary measurements of depth and intensity profiles taken at stand-off distances of approximately 2.5 meters in laboratory conditions using average optical power levels in the micro-watt regime. A standard test chart was used to evaluate the resolving power of the system for both standard and micro-scanned images to assess performance improvements in spatial resolution. Depth profiles of targets were also obtained to investigate improvements in resolving small details and the quality of target edges.

Experimental quantum key distribution through free-space channels requires accurate pointing-and-tracking to co-align telescopes for efficient transmission. The hardware requirements for the sender and receiver could be drastically reduced by combining the detection of quantum bits and spatial tracking signal using two-dimensional single-photon detector arrays. Here, we apply a two-dimensional CMOS single-photon avalanche diode detector array to measure and monitor the single-photon level interference of a free-space time-bin receiver interferometer while simultaneously tracking the spatial position of the single-photon level signal. We verify an angular field-of-view of 1.28° and demonstrate a post-processing technique to reduce background noise. The experimental results show a promising future for two-dimensional single-photon detectors in low-light level free-space communications, such as quantum communications.

The reliable detection of optical radiation is an essential element of laser technology. Most standard optoelectronics textbooks cover the subject of photodetection to some degree. The sensitivity of a photodetector can be defined in terms of the incident optical power required to achieve a given signal-to-noise ratio (SNR). A figure of merit commonly used is the noise-equivalent power (NEP), defined as the mean optical power required to be incident on the detector to generate an SNR of unity. The responsivity of a detector indicates the change in the output of the detector per unit change in the incident optical power and is, therefore, unrelated to noise considerations. When photons of sufficient energy are incident on a solid, electrons are emitted from the surface. Electrons emitted by the photocathode are accelerated towards the first dynode by a large potential difference. Dynode materials are chosen for their low work functions to enable efficient secondary emission.