This PDF file contains the front matter associated with SPIE Proceedings Volume 10978, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Imaging through scattering media in the visible part of the electromagnetic spectrum holds many applications in various industries. For example, seeing through fog would enable autonomous vehicles to navigate in degraded weather conditions, augment human drivers and allow airplanes to operate in dense fog conditions. Another domain is medical imaging, where the ability to see into the body in the visible spectrum would reduce ionizing radiation exposure and provide more clinical meaningful data. Recent advances in single photon avalanche diode (SPAD) counters, and specifically time-resolved single photon counters enabled various challenging imaging applications. The main advantages provided by SPAD devices for imaging are improved noise models and sensitivity, both are essential in low signal-to-noise (SNR) imaging modalities. Another interesting property of SPAD detectors is the ability to measure single photon events which exposes the statistical nature of light. Moreover, the ability to manufacture SPAD arrays naturally lead to a faster and simpler acquisition process as they alleviate the need for scanning. Here, we leverage a time-resolved SPAD camera and demonstrate its advantages for imaging through scattering media. Specifically we experimentally demonstrate seeing through fog and imaging through scattering layers. These examples directly leverage the single photon sensitivity in modeling and rejecting scattered light.

Gathering information of objects hidden from the field of view is an extremely relevant problem in many areas of science and technology. Some state-of-the-art techniques are able to detect and image an object behind an obstacle at the cost of high computational and processing times. Alternatively, other methods can track the object in real-time without giving information on the objects shape. Here we make use of a non-scanning ultrashort pulsed light source, a Single-Photon Avalanche Diode (SPAD), and artificial neural networks (ANNs) to demonstrate a system that can detect, identify, and track objects hidden from view. SPAD technology, characterised by a temporal resolution of 100 ps, provides us with the time traces of the light back-scattered by the environment (including the hidden object). By using different known objects placed at different known positions, we generate a library of time traces that are used to train the ANN algorithm. The application of the trained ANN algorithm in an experimental scenario allow us to identify unknown objects hidden from view in real time with cm resolution. These results open new routes for exciting novel machine learning applications with high impact in the fields of machine vision, self-driving cars, and defence.

Time-resolved imaging is a valuable tool for biomedical applications such as Diffused Optical Tomography (DOT) and Fluorescence Lifetime Imaging (FLIM). The first one characterizes and localizes absorption/scattering heterogeneities, which can be representative of tumors, and is routinely used for brain functional imaging. FLIM provides relevant information (e.g. pH, ion concentration and FRET) in cell biology and find application in molecular imaging for preclinical studies in small animals. Beyond biomedical applications, time-resolved imaging is exploited for environmental monitoring, LIDAR and characterization of combustion processes. Structured light illumination and compressive-sensing detection have been recently proposed as new strategies to preserve information content while significantly reducing the number of measurements. One possible implementation of this approach is the Single Pixel Camera (SPC), where the inner product between the image of the subject and appropriate patterns is measured by using a spatial modulator (e.g. DMD, SLM) and focusing the light on a single pixel detector. In this work, we present a time-resolved imaging system for DOT applications based on structured light illumination and SPC detection, implementing an adaptive scheme based on Singular-Value Decomposition for optimal generation of input/output patterns. Moreover, a novel scheme of time-resolved camera, with ps temporal resolution, is proposed and experimentally validated. The device consists of a high-density matrix of single photon detection elements which can be selectively enabled/disabled. Spatial modulator and detector are combined into a single chip improving cost and compactness. In conclusion, the proposed time-resolved imaging approach can have significant impact on biomedical, environmental and LIDAR applications as an alternative to gated cameras or scanning systems.

A 3D imaging pulsed LADAR with Geiger mode APD array was assembled, flight-tested and deployed in response to a FEMA request for data collection and debris estimation analysis to support the Hurricane Harvey relief effort in TX. Here we report on the rapid response and application of this Geiger Mode APD system to collect high area coverage rate data for geo-mapping and debris volume estimation. MIT Lincoln Laboratory's Airborne Optical Systems Testbed (AOSTB) hosted on a DeHavilland Twin Otter aircraft was flown to collect LADAR imagery of Houston TX area inundated with over 50 inches of rainfall in 4 days and the Port Arthur coastal vicinity that weathered Harvey's initial landfall. This testbed, which serves to advance the Laboratory's effort to develop EO Sensor architectures, along with the actions of a large dedicated team demonstrated the usefulness of this sensor modality for Humanitarian Aid and Disaster Relief response.

Single-photon counting lidar using Geiger-mode avalanche photodiode (GmAPD) arrays can provide high resolution 3D images at kilometer stand-off distances through coincidence processing. 3D data is useful for detection and identification of targets, especially those so occluded by vegetation that only small patches, smaller than the instantaneous field-of-view of a sensor pixel, have free line-of-sight. To cover an area of interest, e.g. the edge of a forest, with spatial resolution high enough to identify targets, a multimegapixel 3D image is needed. Current GmAPD arrays are limited to tens of kilopixels. Even if the technical challenges of larger arrays could be solved, the necessary pulse energy per pixel will still limit the effective number of pixels at longer ranges, especially if nominal ocular hazard distance (NOHD) is a concern or if short pulse fiber lasers should be used. Thus scanning of the sensor field-of-view will probably always be necessary. In this paper we describe activities at FOI to explore the potential of single-photon counting panoramic 3D imaging using a GmAPD array detector. Results from outdoor experiments at up to 1.2 km stand-off distances, in day and night conditions, are shown. The impact of background light, and how this is handled by changing the aperture stop size, is considered. Signal processing techniques to go from scattered photon detections via 3D point clouds to voxel-based scene analysis are described. The results support the position that single-photon counting with GmAPD arrays is suitable for 3D imaging in military applications with kilometer stand-off distances.

The Extremely Low-Resource Optical Identifier (ELROI) beacon is a concept for a milliwatt optical license plate" that can provide unique ID numbers for everything that goes into space. ELROI is designed to help address the problem of space object identification in the crowded space around the Earth, where over 16,000 objects - from active satellites to rocket bodies and debris - are currently tracked and monitored. Using photon counting to enable extreme background rejection in real time, the ID number can be uniquely identified from the ground in a few minutes, even if the ground station detects only a few photons per second. The ELROI concept has been validated in long-range ground tests. A first orbital prototype, integrated into the student CubeSat NMTSat, was launched in December 2018. We discuss our most recent attempts to observe this prototype, including our ground station and an outline of data analysis techniques, as well as the most recent optical signal characteristics for those interested in making observations with their own ground stations.

The Photon Counting Camera (PCC) is a single-photon sensitive laser communication camera that will launch on board the NASA PSYCHE spacecraft, part of the Deep-Space Optical Communication (DSOC) technology demonstration mission. The PCC comprises a single-photon sensitive Geiger-mode Avalanche Photo Diode (GmAPD) array connected to an electronics board designed to power, configure, and read out the array. The logic on the electronics board prevents accidental damage to the array, provides health and status information about the array and provides a simple interface to the downstream data processing modules. The array and electronics board are mounted into the chassis, which provides precise alignment between the optics bench and the detector as well as a path to radiate waste heat. We discuss the current design of the camera, including the electronic, thermal, and structural design. We also discuss some of the design challenges and our roadmap to building the flight unit.

The SAPHIRA is currently the only astronomical device capable of counting photons in the NIR while showing other performance easily comparable to the ubiquitous HAWAII arrays. Photon counting was previously only available in astronomy with high dark currents, prohibiting observation of many astronomical targets. Initiated by the European Southern Observatory for work on the VLT’s GRAVITY instrument, it was further developed by the University of Hawai’i and greatly improved, including a reduction of dark current by 3+ orders of magnitude. Development continues, with further improvements in dark current relative to avalanche gain and larger array sizes to be shown. Since initial deployments, it has now become a vital device in several astronomical instruments, and remains the only array capable of counting NIR photons for low-background astronomical targets.

Photon-counting receivers are deployed on the NASA Ice, Cloud and land Elevation Satellite-2 (ICESat2) Advance Topographic Laser Altimeter System (ATLAS). The ATLAS laser altimeter design has total six ground tracks with three strong and three weak tracks. The strong track has nominally 4 times more laser power than the weak track. The receiver is operated in photon counting mode. There are 16 photon-counting channels for each strong track and 4 photon-counting channels for each weak track. Hamamatsu photomultiplier with a 4x4-array anode was used as photon counting detector. This receiver design has high counting efficiency (>15%) at 532 nm, low dark count rate (<400 counts per second), low jitter (less than 285ps), short dead time (<3 ns), long lifetime under large solar background radiation, radiation harden for space operation, and ruggedized for survives the harsh vibration during the launch. In this paper, we will present the initial on-orbit performance of this photon-counting receiver.

High-speed periodic gating of InGaAs/InP single-photon avalanche diodes (SPADs) has allowed these detectors to operate at count rates above 10⁸ per second with low afterpulsing. However, a drawback of high-speed periodic gating is that bias gates are applied continuously, regardless of whether an avalanche has occurred or not. This is disadvantageous because gates immediately following an avalanche have elevated afterpulse probabilities, and the additional charge from these secondary events contributes to the overall afterpulse probability. We investigate this phenomenon in a proof-of-principle experiment in which the series of bias gates is briefly interrupted after an avalanche, and we measure the resulting impact on the afterpulse probability. We observe a significant reduction in afterpulsing when such a bias-gate hold-o_ is applied to an InGaAs/InP SPAD gated at 1.25 GHz; when one bias gate is omitted after an avalanche the per-gate afterpulse probability is reduced by more than 40 %. These results indicate that afterpulsing noise at high count rates can be further reduced in high-speed-gated SPADs.

Single-photon detection capabilities of Single Photon Avalanche Diodes (SPADs), in laser ranging, enable the measurement of significantly long ranges, the minimization of the optical power of the laser source and the implementation of high frame rate 3D imaging systems, thanks to the possibility of using an array. However, some disadvantages intrinsically affect the Geiger mode operation: above all, each time a photon is detected, the SPAD cannot record another photon during the so-called dead time. The minimization of this dead time is of paramount importance in many cases. For example, in airborne LIDAR altimeters that scan the terrain topography through a semiporous obscuration (e.g. tree canopies, clouds, ground fog, etc.), the first photons that are reflected can mask the photons actually scattered by the terrain: in this scenario, a dead time in the nanosecond range allows the record of photons reflected by surfaces having a distance of few meters. Moreover, a fast recovery of the detector is crucial in presence of a strong background when the LIDAR receiver can fall into paralyzation due to the high rate of photon detections. Here, we present a new Active Quenching Circuit (AQC) able to operate external high-performance custom technology SPAD detectors at extremely high rates. In particular, the circuit can drive a thin custom-technology SPAD with a dead time as low as 6.2ns, corresponding to a maximum photon count rate of more than 160 Mcps, and a RED-Enhanced SPAD up to 100Mcps.

Low noise silicon reach-through avalanche photodiodes are designed and implemented through 0.35 μm high voltage CMOS process. Seperated absorption charge multiplication (SACM) structure with vertical n++/p+/pi/p four layers is adopted. The light sensitive area of two different size element detectors is 200 μm and 500 μm in diameter, respectively. Regardless of the size of the light sensitive area, the typical reach-through voltage and the breakdown voltage for both element detectors is tested to be 60 V, and 172 V, respectively. The distribution of the breakdown voltage across the wafer is like a superposition of two Gaussian distributions of one peak at 168 V corresponding to the fewer detectors close to the rim and the other at 172 V corresponding to the most detectors in the center. The temperature coefficient of the breakdown voltage is tested down to be 0.32 V/K. The dark current at the gain M=100 is tested to be 50 pA and 500 pA for each detector. The responsive wavelength is 400-1100 nm. The peak responsivity of the 500 μm diameter detector is tested to be 57 A/W at 900 nm wavelength and with gain 100 to show the successful near infrared enhanced reponsivity. The excess noise factor is tested to be 3 - 4, much lower than those in the reported high voltage CMOS avalanche photodiodes. The yield is 100%. The devices are applied to a multiple-line Lidar to show the feasibility. Two 16×1 linear arrays of different pixel pitches are also designed and fabricated in the same way.

Photodetection plays a key role in basic science and technology, with exquisite performance having been achieved down to the single photon level. Further improvements would open new possibilities across a broad range of scientific disciplines, and enable new types of applications. However, it is still unclear what is possible in terms of ultimate performance, and what properties are needed to achieve such performance. Here, we present a general modeling framework for photodetectors whereby the photon field, the absorption process, and the amplification process are all treated as one coupled quantum system. The formalism naturally handles field states with single or multiple photons as well as a variety of detector configurations, and includes a mathematical definition of ideal photodetector performance. The framework reveals how specific photodetector architectures introduce limitations and tradeoffs for various performance metrics, providing guidance for optimization and design.

The growing demand for 3D time of flight (ToF) imaging LiDAR (Light Detection and Ranging) systems, based on CMOS Silicon Photomultiplier (SiPM), poses an engineering and scientific challenge. SiPM, which is composed of a mosaic array of passive quenched SPADs (single photon avalanche diode in Geiger Mode) combined in parallel, is the building block of 3D-optical radars. It is the leading solid- state sensor in systems requiring simultaneously photon counting as well as photon timing. An open essential design parameter is the required number of sub-pixels for adequate detection of a packet of m photons, considering that each sub-pixel, composed of a single SPAD, can detect only the first photon. This study evaluates this design parameter based on a stochastic approach, where the random number of incident photons as well as the detection probability of each SPAD is taken into consideration.

Single Photon Avalanche Diodes (SPADs) have been proven to be extremely powerful sensors for single photon detection. Thanks to the advantages of solid state devices (ruggedness, small size, low supply voltage, high reliability) combined to photon detection efficiency inherently higher than PMTs, especially in the red and near-infrared regions of the spectrum, SPADs have become the detectors of choice in a steadily increasing number of applications, such as Förster Resonance Energy Transfer (FRET), Laser Imaging Detection and Ranging (LIDAR) and Quantum Key Distribution (QKD). The development of specific fabrication processes, usually referred to as custom technologies, has given the designers the degrees of freedom necessary to pursue the best device performance. Nevertheless, custom processes do not easily allow the integration of complex front-end and processing electronics on the same chip of the detector. Therefore, external high-performance electronics is required to extract the best performance from these sensors. We report the latest results we achieved with a fully-planar custom technology process, that allows the fabrication of SPAD arrays, and specifically designed external front-end and timing electronics with particular focus on solutions to achieve high speed in counting and timing applications.

Single photon avalanche diode arrays have achieved extraordinary performance and are beginning to replace vacuum tube photomultipliers in almost every application. While silicon based single photon avalanche diode arrays are a rapidly maturing technology, similar arrays in compound semiconductors have met with only limited success. This is partly due to the intrinsic high defect densities in compound semiconductors and partly due to the immaturity of the fabrication techniques available. Newly developed planar processing technologies hold the potential to substantially improve the performance of compound semiconductor SPAD arrays, including decreasing dark count rates, increasing single photon detection efficiencies, and increasing dynamic range. These new techniques have been applied to GaInP SPAD arrays, enabling the SPAD array pitch to be decreased to five microns and 40,000 SPADs/mm. The performance characteristics of these GaInP SPAD arrays will be described.

Detection of few-photon signals requires photodetectors with high gain, typically achieved with bulk devices such as avalanche photodiodes, superconducting systems, and photomultiplier tubes. Recently, there has been interest in exploiting the properties of nanoscale materials and devices to realize new types of high gain photodetectors. In this presentation, a new photodetector is reported based on C60-sensitized aligned carbon nanotube (CNT) transistors with extremely high responsivity of 108 A W-1 (gain > 108) in the UV and visible, and 720 A W-1 (gain = 940) in the IR. In contrast to most sensitized phototransistors that operate on the photogating effect, the new photodetector operates on modulation of the electron scattering in the CNTs, leading to negative photoconductivity. Comparison with similar photodetectors using random CNT networks show the benefit of using aligned CNTs. At room temperature the aligned CNT photodetectors are demonstrated to detect a few tens of photons per CNT, and recent results with single CNT devices show detection of 200 photon pulses. Quantum transport modeling reveals the photodetection mechanism and establishes a path towards single photon detection.

A fast timing MCP-PMT detector has been developed and characterized for LIDAR applications. A field of application for which it is needed to demonstrate the capability to measure photon fluxes in single photon counting mode at low and high rates on one hand, and on the other hand to detect high intensity peaks, containing up to thousands photons within a ns-time window, as it is the case in the response signal of highly reflective surfaces. Based on high quantum efficiency (above 30% in peak) and low dark rates (about 30 Hz/cm²) s-20 photocathodes from our Hi-QE series optimized for 200-550 nm spectral range, our fast MCP-PMT detectors are demonstrating efficient single photon counting from very low rates and up to few GHz for burst pulses. Thanks to the implementation of high linearity MCPs the linear detection range is extended up to several 100 MHz of averaged photon fluxes. The detection of high intensity peak is a challenge for devices optimized for single photon counting mode; here we propose a two-channel detection scheme that allows performing accurate measurements in photon counting mode and in parallel that is enabling the detection and counting intense multi-photon pulses with sub-ns time resolution.

Superconducting nanowire single-photon detectors (SNSPDs) are excellent single-photon detectors from the ultraviolet to the near-infrared. System detection efficiencies of ~ 90% are typical, with jitters on the order of 100 ps and maximum count rates of a few MHz. Recently we have begun exploring the use of SNSPDs for the detection of single mid-infrared photons in the 2 - 11 μm wavelength range for applications in astronomy and chemical sensing. In particular, we are developing arrays of SNSPDs which could potentially be used for exoplanet spectroscopy in order to identify elements in the atmospheres of exoplanets outside our solar system. Improved sensitivity for these low-energy photons has been made possible by the recent development of amorphous WSi which is now used in the fabrication of superconducting nanowire detectors. I will discuss the optimization of these detectors to enhance their detection efficiency in the midinfrared, with the ultimate goal of building a single-photon focal plane array of SNSPDs in the 2 - 11 μm band.

We present innovative planar geometry Ge-on-Si single-photon avalanche diode (SPAD) detectors. These devices provide picosecond timing resolution for applications operating in the short-wave infrared wavelength region such as quantum communication technologies and three-dimensional imaging. This new planar design successfully reduces the undesirable contribution of surface defects to the dark current. This has allowed for the use of large excess biases, resulting in a single-photon detection efficiency of 38% when operated at 125 K using 1310 nm wavelength illumination. A record low noise equivalent power of 2 × 10-16 WHz-1/2 was achieved, more than a fifty-fold improvement compared to the previous best Ge-on-Si mesa geometry SPADs when operated under similar conditions. These Ge-on-Si SPAD detectors have operated in the range of 77 K to 175 K, and we will discuss ways in which the operating temperature can be raised to that consistent with Peltier cooling. We will present analysis of Ge-on-Si SPADs, which has revealed much reduced afterpulsing compared with SPAD detectors in other material systems. Laboratory trials have demonstrated these Ge-on-Si SPAD devices in short-range LIDAR and depth profiling measurements. Estimations of the performance of these detectors in longer range measurements will be presented. We will discuss the potential for the development of high efficiency arrays of Ge-on-Si SPADs for the use in eye-safe automotive LIDAR and quantum technology applications.

Initial results of electrical and optical characterization of Voxtel’s first generation 256 x 256 dual-mode silicon singlephoton avalanche diode (SPAD) image sensor are presented. The SPAD image sensor is a dual-mode device capable of sequential passive single-photon-counting (2D) and active single-photon lidar (3D) range imaging at greater than 250 frames per second, full-frame. The sensor was developed in 180-nm complementary metal-oxide semiconductor imagesensor technology with a pixel pitch of 30 μm and fill factor of 9%; and it achieves room temperature per-pixel dark count rate of less than 55 Hz (0.63 Hz/μm2), peak photon detection probability of 29% (at 480 nm) and timing jitter of 268 ps full width at half maximum at the optimal operating point. Preliminary imaging results in 2D and 3D mode are presented.