This PDF file contains the front matter associated with SPIE Proceedings Volume 9492 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Intensity interferometry, which was first used for obtaining ultra—high-resolution image information in astronomy in the 1960's and 1970's, is now being revived using modern detectors and electronics. This paper explores the possibility of wireless optical interferometry made possible by technological advancements in timing correlation, signal processing, and detector technology. If this can be achieved, then baselines of one to several kilometers may be possible in optical interferometry in the coming years. This would improve the resolution over the current generation of amplitude-based optical interferometers by a factor of at least ten.

The Lunar Laser Communication Demonstration (LLCD) successfully demonstrated for the first time duplex laser communications between a lunar-orbiting satellite and ground stations on Earth with error-free downlink data rates up to 622 Mb/s utilizing an optical receiver based on photon-counting superconducting nanowires and operating near 1550 nm.

We present a software based control system for Geiger-mode avalanche photodiodes (GM-APDs) that enables constant photon detection efficiency irrespective of the diode's junction temperature. Furthermore, we demonstrate that this control system enables passively quenched GM-APDs to double the rate of photon detection events before saturation compared to the standard control method that fixes the junction temperature and applied bias voltage. We present data demonstrating the robustness of the GM-APD control system when tested in near-space conditions using a correlated photon pair source carried by a weather balloon to an altitude of 35.5 km.

Superconducting circuits comprising SNSPDs placed in parallel—superconducting nanowire avalanche photodetectors, or SNAPs—have previously been demonstrated to improve the output signal-to-noise ratio (SNR) by increasing the critical current. In this work, we employ a 2-SNAP superconducting circuit with narrow (40 nm) niobium nitride (NbN) nanowires to improve the system detection efficiency to near-IR photons while maintaining high SNR. Additionally, while previous 2-SNAP demonstrations have added external choke inductance to stabilize the avalanching photocurrent, we show that the external inductance can be entirely folded into the active area by cascading 2-SNAP devices in series to produce a greatly increased active area. We fabricated series-2-SNAP (s2-SNAP) circuits with a nanowire length of 20 μm with cascades of 2-SNAPs providing the choke inductance necessary for SNAP operation. We observed that (1) the detection efficiency saturated at high bias currents, and (2) the 40 nm 2-SNAP circuit critical current was approximately twice that for a 40 nm non-SNAP configuration.

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.

The application of non-line-of-sight vision has been demonstrated in the recent past on laboratory level with round trip path lengths on the scale of 1 m as well as 10 m. This method uses a computational imaging approach to analyze the scattered information of objects which are hidden from the direct sensor field of view. In the present work, the authors evaluate the application of recent single photon counting devices for non-line-of-sight sensing and give predictions on range and resolution. Further, the realization of a concept is studied enabling the indirect view on a hidden scene. Different approaches based on ICCD and GM-APD or SPAD sensor technologies are reviewed. Recent laser gated viewing sensors have a minimal temporal resolution of around 2 ns due to sensor gate widths. Single photon counting devices have higher sensitivity and higher temporal resolution.

Hybrid receivers that enable switching between direct and coherent detection provide many imaging functions beneficial to scientific and defense applications. A hybrid receiver system is presented wherein a single detector is switched between the Geiger-mode and linear amplification modes of operation. This system benefits from enhanced functionality and lower size, weight, power, cost, and complexity compared with dual receiver implementations. The hybrid receiver sensing modality is reconfigurable on-the-fly between single photon direct detection and amplitude/phase coherent detection. The reconfiguration is achieved by adjusting detector bias (electrically) and by simultaneously enabling or disabling the local oscillator (optically). This work describes these two sensing scenarios, discusses the operation of the receiver system and shows laboratory-scale imaging results for each mode of hybrid receiver operation.

Geiger-mode avalanche photodiodes (GMAPDs) are capable of detecting single photons. They can be operated to directly trigger all-digital circuits, so that detection events are digitally counted or time stamped in each pixel. An imager based on an array of GMAPDs therefore has zero readout noise, enabling quantum-limited sensitivity for photon-starved imaging applications. In this review, we discuss devices developed for 3D imaging, wavefront sensing, and passive imaging.

In this study, a CMOS Single Photon Avalanche Diode (SPAD) 2D array is used to record and sample muzzle flash events in the visible spectrum, from representative weapons. SPADs detect the emission peaks of alkali salts, potassium or sodium, with spectral emission lines around 769nm and 589nm, respectively. The alkali salts are included in the gunpowder to suppress secondary flashes ignited during the muzzle flash event. The SPADs possess two crucial properties for muzzle flash imaging: (i) very high photon detection sensitivity, (ii) a unique ability to convert the optical signal to a digital signal at the source pixel, thus practically eliminating readout noise. The sole noise sources are the ones prior to the readout circuitry (optical signal distribution, avalanche initiation distribution and nonphotonic generation). This enables high sampling frequencies in the kilohertz range without significant SNR degradation, in contrast to regular CMOS image sensors. This research will demonstrate the SPAD’s ability to accurately sample and reconstruct the temporal behavior of the muzzle flash in the visible wavelength, in the presence of sunlight. The reconstructed signal is clearly distinguishable from background clutter, through exploitation of flash temporal characteristics and signal processing, which will be reported. The frame rate of ~16 KHz was chosen as an optimum between SNR degradation and temporal profile recognition accuracy. In contrast to a single SPAD, the 2D array allows for multiple events to be processed simultaneously. Moreover, a significant field of view is covered, enabling comprehensive surveillance and imaging.

Optical crosstalk is a major factor limiting the performance of Geiger-mode avalanche photodiode (GmAPD) focal plane arrays (FPAs). This is especially true for arrays with increased pixel density and broader spectral operation. We have performed extensive experimental and theoretical investigations on the crosstalk effects in InP-based GmAPD FPAs for both 1.06-μm and 1.55-μm applications. Mechanisms responsible for intrinsic dark counts are Poisson processes, and their inter-arrival time distribution is an exponential function. In FPAs, intrinsic dark counts and cross talk events coexist, and the inter-arrival time distribution deviates from purely exponential behavior. From both experimental data and computer simulations, we show the dependence of this deviation on the crosstalk probability. The spatial characteristics of crosstalk are also demonstrated. From the temporal and spatial distribution of crosstalk, an efficient algorithm to identify and quantify crosstalk is introduced.

A model for the turn-on characteristics of separate-absorber-multiplier InP-based Geiger-mode Avalanche Photodiodes (APDs) has been developed. Verilog-A was used to implement the model in a manner that can be incorporated into circuit simulations. Rather than using SPICE elements to mimic the voltage and current characteristics of the APD, Verilog-A can represent the first order nonlinear differential equations that govern the avalanche current of the APD. This continuous time representation is fundamentally different than the piecewise linear characteristics of other models. The model is based on a driving term for the differential current, which is given by the voltage overbias minus the voltage drop across the device’s space-charge resistance RSC. This drop is primarily due to electrons transiting the separate absorber. RSC starts off high and decreases with time as the initial breakdown filament spreads laterally to fill the APD. With constant bias voltage, the initial current grows exponentially until space charge effects reduce the driving function. With increasing current the driving term eventually goes to zero and the APD current saturates. On the other hand, if the APD is biased with a capacitor, the driving term becomes negative as the capacitor discharges, reducing the current and driving the voltage below breakdown. The model parameters depend on device design and are obtained from fitting the model to Monte-Carlo turn-on simulations that include lateral spreading of the carriers of the relevant structure. The Monte-Carlo simulations also provide information on the probability of avalanche, and jitter due to where the photon is absorbed in the APD.

In recent years, many applications have been proposed that require detection of light signals in the near-infrared range with single-photon sensitivity and time resolution down to few hundreds of picoseconds. InGaAs/InP singlephoton avalanche diodes (SPADs) are a viable choice for these tasks thanks to their compactness and ease-of-use. Unfortunately, their performance is traditionally limited by high dark count rates (DCRs) and afterpulsing effects. However, a recent demonstration of negative feedback avalanche diodes (NFADs), operating in the free-running regime, achieved a DCR down to 1 cps at 10 % photon detection efficiency (PDE) at telecom wavelengths. Here we present our recent results on the characterization of NFAD detectors for temperatures down to approximately 150 K. A FPGA controlled test-bench facilitates the acquisition of all the parameters of interest like PDE, DCR, afterpulsing probability etc. We also demonstrate the performance of the detector in different applications: In particular, with low-temperature NFADs, we achieved high secret key rates with quantum key distribution over fiber links between 100-300 km. But low noise InGaAs/InP SPADs will certainly find applications in yet unexplored fields like photodynamic therapy, near infrared diffuse optical spectroscopy and many more. For example with a large area detector, we made time-resolved measurements of singlet-oxygen luminescence from a standard Rose Bengal dye in aqueous solution.

Results of characterization data on linear mode photon counting (LMPC) HgCdTe electron-initiated avalanche photodiode (e-APD)focal plane arrays (FPA) are presented that reveal an improved understanding and the growing maturity of the technology. The first successful 2x8 LMPC FPA was fabricated in 2010 [1]. Since then a process validation lot of 2x8 arrays was fabricated. Five arrays from this lot were characterized that replicated the previous 2x8 LMPC array performance. In addition, it was unambiguously verified that readout integrated circuit (ROIC) glow was responsible for most of the false event rate (FER) of the 2010 array. The application of a single layer metal blocking layer between the ROIC and the detector array and optimization of the ROIC biases reduced the FER by an order of magnitude. Photon detection efficiencies (PDEs) of greater than 50% were routinely demonstrated across 5 arrays, with one array reaching a PDE of 70%. High resolution pixel-surface spot scans were performed and the junction diameters of the diodes were measured. The junction diameter was decreased from 31 μm to 25 μm resulting in a 2x increase in E-APD gain from 470 on the 2010 array to 1100 on one of the 2013 FPAs. Mean single photon signal to noise ratios of >12 were demonstrated at excess noise factors of 1.2-1.3. NASA Goddard Space Flight Center (GSFC) performed measurements on the delivered FPA that verified the PDE and FER data.