This PDF file contains the front matter associated with SPIE Proceedings Volume 7320, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

This paper describes a rapid data acquisition photon-counting time-of-flight ranging technique that is designed for the avoidance of range ambiguity, an issue commonly found in high repetition frequency timeof- flight systems. The technique transmits a non-periodic pulse train based on the random bin filling of a high frequency time clock. A received pattern is formed from the arrival times of the returning single photons and the correlation between the transmitted and received patterns was used to identify the unique target timeof- flight. The paper describes experiments in free space at over several hundred meters range at clock frequencies of 1GHz. Unambiguous photon-counting range-finding is demonstrated with centimeter accuracy.

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.

Space-based lidar instruments must be able to detect extremely weak laser return signals from orbital distance. The signals have a wide dynamic range caused by the variability in atmospheric transmission and surface reflectance under a fast moving spacecraft. Ideally, lidar detectors should be able to detect laser signal return pulses at the single photon level and produce linear output for multiple photon events. They should have high quantum efficiency in the nearinfrared wavelength region where high-pulse-energy space-qualified lasers are available. Silicon avalanche photodiode (APD) detectors have been used in most space lidar receivers to date. Their sensitivity is typically hundreds of photons per pulse at 1064 nm, and is limited by the quantum efficiency, APD gain noise, dark current, and preamplifier noise. NASA is investigating photon-sensitive near-infrared detectors with linear response for possible use on the next generation direct-detection space lidars. We have studied several types of linear mode avalanche photodiode detectors that are sensitive from 950 nm to 1600 nm and potentially viable for near term space lidar missions. We present our measurement results and a comparison of their performance.

Early applications driving the development of single photon sensitive detectors, such as fluorescence and photoluminescence spectroscopy, simply required low noise performance with kiloHertz and lower count rate requirements and minimal or no timing resolution. Newer applications, such as high data rate photon starved free space optical communications require photon counting at flux rates into megaphoton or gigaphoton per second regimes coupled with sub-nanosecond timing accuracy. With deep space optical communications as our application driver, we have developed and implemented systems to both characterize gigaHertz bandwidth single photon detectors as well as process photon count signals at rates beyond 100 megaphotons per second to implement communications links at data rates exceeding 100 megabits per second with efficiencies greater than two bits per detected photon. With these systems, we have implemented high bandwidth real-time systems using intensified photodiodes, visible light photon counter detectors, superconducting nanowire detectors, Geiger-mode semiconductor avalanche photodiodes, and negative avalanche feedback photon counters.

We present an experimental scheme for the reconstruction of the Wigner function of optical states. The method is based on direct intensity measurements by non-ideal photodetectors operated in the linear regime. We mix, at a beam-splitter, the signal state with a set of coherent probes of known complex amplitudes, and measure the probability distribution of the detected photons for each probe. The Wigner function is given by a suitable sum of those probability distributions. For comparison, the same data are analyzed to obtain the number distributions and the Wigner functions for photons.

A snake photon detection based imaging technique developed by our group is explained in details and its performances compared with those obtained by other experimentalists. The technique is based on simultaneous application of time and spatial-mode selection. We also show that in these very particular working conditions commonly used plastic tissue phantoms display non tissue-like scattering properties.

The recent advances in superconducting nanowire single-photon detector (SNSPD or SSPD) technology has enabled long distance quantum key distribution (QKD) over an optical fiber. We point out that the performance of SNSPDs play a crucial role in achieving a secure transmission distance of 100 km or longer. We analyze such an impact from a simplified model and use it to interpret results from our differential-phase-shift (DPS) QKD experiment. This allows us to discuss the optimization of the detection time window and the clock frequency given the detector characteristics such as dark count rate, detection efficiency, and timing jitter.

Over the past few years there has been a growing interest in monolithic arrays of single photon avalanche diodes (SPAD) for spatially resolved detection of faint ultrafast optical signals. SPADs implemented in CMOS-compatible planar technologies offer the typical advantages of microelectronic devices (small size, ruggedness, low voltage, low power, etc.). Furthermore, they have inherently higher photon detection efficiency than PMTs and are able to provide, beside sensitivities down to single-photons, very high acquisition speeds (i.e. either high frame-rates or very short integration time-slots). SPADs offer several advantages over other commercially available imagers. For example, CCDs and similar imagers lack in speed because their readout process is based on a slow charge-transfer mechanisms. CMOS APS, on the other hand, are unable to detect very faint optical signals, due to poor sensitivity and noisy electronics. In order to make SPAD array more and more competitive it is necessary to face several issues: dark counts, quantum efficiency, crosstalk, timing performance. These issues will be discussed in the context of two possible approaches to such a challenge: employing a standard industrial CMOS technology or developing a dedicated technology. Advances recently attained will be outlined with reference to both photon counting and Time correlated single photon counting detector arrays.

Detection of low-level ultraviolet (UV) light has been the focus of numerous research and development efforts in recent years. To date, the most promising solid-state solution is SiC avalanche photodiodes. We report 4H-SiC avalanche photodiodes with low dark current and high gain. Geiger mode operation with high single photon detection efficiency and low dark count probability has been achieved. The dark current behavior of a 4x4 array of SiC APDs is also presented.

Photon counting detectors are used in many diverse applications and are well-suited to situations in which a weak signal is present in a relatively benign background. Examples of successful system applications of photon-counting detectors include ladar, bio-aerosol detection, communication, and low-light imaging. A variety of practical photon-counting detectors have been developed employing materials and technologies that cover the waveband from deep ultraviolet (UV) to the near-infrared. However, until recently, photoemissive detectors (photomultiplier tubes (PMTs) and their variants) were the only viable technology for photon-counting in the deep UV region of the spectrum. While PMTs exhibit extremely low dark count rates and large active area, they have other characteristics which make them unsuitable for certain applications. The characteristics and performance limitations of PMTs that prevent their use in some applications include bandwidth limitations, high bias voltages, sensitivity to magnetic fields, low quantum efficiency, large volume and high cost. Recently, DARPA has initiated a program called Deep UV Avalanche Photodiode (DUVAP) to develop semiconductor alternatives to PMTs for use in the deep UV. The higher quantum efficiency of Geiger-mode avalanche photodiode (GM-APD) detectors and the ability to fabricate arrays of individually-addressable detectors will open up new applications in the deep UV. In this paper, we discuss the system design trades that must be considered in order to successfully replace low-dark count, large-area PMTs with high-dark count, small-area GM-APD detectors. We also discuss applications that will be enabled by the successful development of deep UV GM-APD arrays, and we present preliminary performance data for recently fabricated silicon carbide GM-APD arrays.

Improving SPAD performances, such as dark count rate and quantum efficiency, without degrading the photontiming jitter is a challenging task that requires a clear understanding of the physical mechanisms involved. In this paper we investigate the contribution of the avalanche buildup statistics and the lateral avalanche propagation to the photon-timing jitter in silicon SPAD devices. Recent works on the buildup statistics focused on the uniform electric field case, however these results can not be applied to Si SPAD devices in which field profile is far from constant. We developed a 1-D Monte Carlo (MC) simulator using the real non-uniform field profiles derived from Secondary Ion Mass Spectroscopy (SIMS) measurements. Local and non-local models for impact ionization phenomena were considered. The obtained results, in particular the mean multiplication rate and jitter of the buildup filament, allowed us to simulate the statistical spread of the avalanche current on the device active area. We included space charge effects and a detailed lumped model for the external electronics and parasitics. We found that, in agreement with some experimental evidences, the avalanche buildup contribution to the total timing jitter is non-negligible in our devices. Moreover the lateral propagation gives an additional contribution that can explain the increasing trend of the photon-timing jitter with the comparator threshold.

We report new results on the design, fabrication and characterization of a novel midinfrared sensor called quantum dot avalanche photodiode (QDAP). The QDAP consists of a quantum dots-in-a-well (DWELL) detector coupled with an avalanche photodiode (APD) through a tunnel barrier. In the QDAP, the photons are absorbed in the DWELL active region while the APD section provides photocurrent gain. Spectral response and photocurrent measurements at 77 K were taken to characterize the response of the device. The increase of the spectral response and the nonlinear increase in the photocurrent as the APD voltage increases support theoretical predictions about the QDAP capability to work in Geiger mode. The QDAP photocurrent is similar to the IV characteristic of the APD section, indicating gain in the device.

Arrays of InP-based avalanche photodiodes operating at 1.06-μm wavelength in the Geiger mode have been fabricated in the 128x32 format. The arrays have been hermetically packaged with precision-aligned lenslet arrays, bump-bonded read-out integrated circuits, and thermoelectric coolers. With the array cooled to -20C and voltage biased so that optical cross-talk is small, the median photon detection efficiency is 23-25% and the median dark count rate is 2 kHz. With slightly higher voltage overbias, optical cross-talk increases but the photon detection efficiency increases to almost 30%. These values of photon detection efficiency include the optical coupling losses of the microlens array and package window.

We report on the development of 32 x 32 focal plane arrays (FPAs) based on InGaAsP/InP Geiger-mode avalanche photodiodes (GmAPDs) designed for use in three-dimensional (3-D) laser radar imaging systems at 1064 nm. To our knowledge, this is the first realization of FPAs for 3-D imaging that employ a planar-passivated buried-junction InP-based GmAPD device platform. This development also included the design and fabrication of custom readout integrate circuits (ROICs) to perform avalanche detection and time-of-flight measurements on a per-pixel basis. We demonstrate photodiode arrays (PDAs) with a very narrow breakdown voltage distribution width of 0.34 V, corresponding to a breakdown voltage total variation of less than +/- 0.2%. At an excess bias voltage of 3.3 V, which provides 40% pixel-level single photon detection efficiency, we achieve average dark count rates of 2 kHz at an operating temperature of 248 K. We present the characterization of optical crosstalk induced by hot carrier luminescence during avalanche events, where we show that the worst-case crosstalk probability per pixel, which occurs for nearest neighbors, has a value of less than 1.6% and exhibits anisotropy due to isolation trench etch geometry. To demonstrate the FPA response to optical density variations, we show a simple image of a broadened optical beam.

LAser Detection And Ranging (LADAR) is a promising tool for precise 3D-imaging, which enables field surveillance and target identification under low-light-level conditions in many military applications. For the time resolution and sensitivity requirements of LADAR applications, InGaAsP/InP Geiger-mode (GM) avalanche photodiodes (APDs) excel in the spectrum band between 1.0~1.6 μm. Previously MIT Lincoln Laboratory has demonstrated 3D LADAR imaging in the visible and near infrared (1.06 μm) wavelengths with InP/InGaAsP GM-APD arrays. In order to relieve the design tradeoffs among dark count rate (DCR), photo detection efficiency (PDE), afterpulsing, and operating temperature, it is essential to reduce the DCR while maintaining a high PDE. In this paper we will report the progress of GM-APD detectors and arrays with low DCR and high PDE at 1.06 μm. In order to improve both DCR and PDE, we optimized the multiplication layer thickness, substrate, and epitaxial growth quality. With an optimized InP multiplier thickness, a DCR as low as 100 kHz has been demonstrated at 4V overbias at 300 °C. and at 240 K, less than 1 kHz DCR is measured. A nearly 40% PDE can be achieved at a DCR of 10 kHz at the reduced temperature.

Arrays of photon-counting Geiger-mode avalanche photodiodes (APDs) sensitive to 1.06 and 1.55 μm wavelengths and as large as 256 x 64 elements on 50 μm pitch have been fabricated for defense applications. As array size, and element density increase, optical crosstalk becomes an increasingly limiting source of spurious counts. We characterize the crosstalk by measurement of emitted light, and by extracting the spatial and temporal focal plane array (FPA) response to the light from FPA dark count statistics. We discuss the physical and geometrical causes of FPA crosstalk, suggest metrics useful to system designers, then present measured crosstalk metrics for large FPAs as a function of their operating parameters. We then present FPA designs that suppress crosstalk effects and show more than 40 times reduction in crosstalk.

Arrays as large as 256 x 64 of single-photon counting avalanche photodiodes have been developed for defense applications in free-space communication and laser radar. Focal plane arrays (FPAs) sensitive to both 1.06 and 1.55 μm wavelength have been fabricated for these applications. At 240 K and 4 V overbias, the dark count rate (DCR) of 15 μm diameter devices is typically 250 Hz for 1.06 μm sensitive APDs and 1 kHz for 1.55 μm APDs. Photon detection efficiencies (PDE) at 4 V overbias are about 45% for both types of APDs. Accounting for microlens losses, the full FPA has a PDE of 30%. The reset time needed for a pixel to avoid afterpulsing at 240 K is about 3-4 μsec. These devices have been used by system groups at Lincoln Laboratory and other defense contractors for building operational systems. For these fielded systems the device reliability is a strong concern. Individual APDs as well as full arrays have been run for over 1000 hrs of accelerated testing to verify their stability. The reliability of these GM-APDs is shown to be under 10 FITs at operating temperatures of 250 K, which also corresponds to an MTTF of 17,100 yrs.

Avalanche Photodiode (APD) photon counting arrays are finding an increasing role in defense applications in laser radar and optical communications. As these system concepts mature, the need for reliable screening, test, assembly and packaging of these novel devices has become increasingly critical. MIT Lincoln Laboratory has put significant effort into the screening, reliability testing, and packaging of these components. To provide rapid test and measurement of the APD devices under development, several custom parallel measurement and Geiger-mode (Gm) aging systems have been developed. Another challenge is the accurate attachment of the microlens arrays with the APD arrays to maximize the photon detection efficiency. We have developed an active alignment process with single μm precision in all six degrees of freespace alignment. This is suitable for the alignment of arrays with active areas as small as 5 μm. Finally, we will discuss a focal plane array (FPA) packaging qualification effort, to verify that single photon counting FPAs can survive in future airborne systems.

We report the first entanglement swapping experiment using entangled photon-pair sources based on spontaneous four-wave mixing (SFWM). The 1.5-μm band entangled photon pairs generated by SFWM in two independent 500-m dispersion shifted fibers exhibited quantum interference, thanks to the negligible walk-off between the pump and photon pairs. The use of 500-MHz gated-mode InGaAs/InP avalanche photodiodes based on the sine-wave gating technique increased the fourfold coincidence rate significantly. As a result, the formation of an entanglement between photons from independent sources was successfully observed.

There are many applications where the ability to detect optical signals in the 1.65 - 3 μm wavelength range would be of considerable interest. In this paper we discuss two technologies that offer considerable promise for high speed, high sensitivity detection in this region utilising avalanche gain. InGaAs/GaAsSb Type II superlattices as the absorption region and InAlAs as the multiplication region can be combined to form a separate absorption and multiplication (SAM) avalanche photodiode (APD), all grown lattice matched on InP substrates. Detection at room temperature up to 2.4 μm can be readily achieved as can gains in excess of 40. InAs homojunction p-i-n diodes are capable of detecting light with wavelengths > 3 μm, even at 77 K. Although controlling the surface leakage current is a major challenge in mesa devices of InAs, gains in excess of 40 have also been obtained in these devices at room temperature. InAs is also the only III-V semiconductor material that appears to show excess noise-free avalanche gain when electrons are used to initiate the avalanche multiplication. We will discuss recent developments in these two material systems to date and the current state of the technology.

We report an improved passive-quenching-with-active-reset (PQAR) circuit that can operate in a free-running mode with reduced afterpulsing. A dynamic range of approximately 80 dB has been achieved. A model that reveals the factors that determine the dynamic range is described. The PQAR circuit approach can also be utilized in gated mode, which we refer to as gated-PQAR circuit. Compared to conventional gated quenching, the gated-PQAR circuit can significantly reduce the current flow during avalanche. This will reduce afterpulsing and provide the capability of utilizing wider ac bias pulses, which will ease restrictions on synchronization with the arrival of incident photons.

A single photon receiver module combining an InGaAsP/InP avalanche photodiode with peak responsivity at 1064nm and a CMOS integrated electronic circuit for operation in both gated and free running modes is presented. The standalone module exhibits a single photon detection probability as high as 30% at 1064nm that is by far higher than silicon devices. The dark count rate mean value over eight devices cooled down to -40°C is about 100Hz at 7.5% detection probability and 1.2kHz at 30%. Dark count rate versus temperature measurements show that trap-assisted tunneling in the InP multiplication layer progressively dominates the total dark count rate when the device is cooled down. At medium cooling, the thermal generation in the absorber is the dominant mechanism. Afterpulsing rate is relatively high when compared to silicon devices. However, the integration of a dedicated pulser in close-proximity with the APD makes possible free-running operation. The timing resolution was measured at 430ps FWHM at 30% detection probability. Though comparing favorably with silicon reach-through avalanche photodiodes, we believe that a large uncertainty stands on this measurement. A timing resolution of less than 300ps is expected with the developed receiver module.

A new family of photodetectors with a Discrete Amplification (DA) mechanism allows the realization of very high gain and low excess noise factor in the visible and near infrared spectral regions and offers an alternative to conventional photomultiplier tubes and Geiger mode avalanche photodetectors. These photodetectors can operate in linear detection mode with gain-bandwidth product in excess of 4X10¹⁴ and in photon counting mode with count rates up to 10⁸ counts/sec. Potential benefits of this technology over conventional avalanche photodetectors include ultra low excess noise factor, very high gain, and lower reset time (<< 1 μs). In the photon counting mode, the devices can be operated in the non-gated mode under a constant dc bias. Because of its unique characteristics of self-quenching and self-recovery, no external quenching circuit is needed. We present the discrete amplification design approach used for the development of a solid state photomultiplier in the near infrared wavelength region. The demonstrated device performance far exceeds any available solid state photodetectors in the near infrared wavelength range. The measured devices have the following performance characteristics: gain > 2X10⁵, excess noise factor < 1.05, rise time < 350ps, fall time < 500ps, dark current < 2X10⁶ cps, operating voltage < 60V. These devices are ideal for researchers in the field of deep space optical communication, spectroscopy, industrial and scientific instrumentation, Ladar/Lidar, quantum cryptography, night vision and other military, defence and aerospace applications.

The requirement for external quenching circuits adds substantially to the complexity and processing difficulty for InGaAs single-photon detectors, particularly in array configurations. Using bandgap engineering, we have developed InGaAs SPADs with self-quenching and self-recovering capabilities. The quenching process occurs in less than 100 ps, determined by the gain buildup time and the magnitude of device overbias. On the other hand, the recovery time is determined by the carrier escape time over an energy barrier that is typically tens of meVs. The recovery time can range from 1 ns to > 100 ns from the design of device and material structures. The optimal recovery time is a function of dark count rate and afterpulsing rate. Our data show that a recovery time of around 10 ns is near the optimum in most operation conditions. The self-quenched SPADs also show great suppression in excess noise, yielding a very uniform intensity distribution of output response to single photons. This unique property favors resolving photon number in an array device. As in conventional InGaAs SPADs, the single-photon detection efficiency increases with the amount of overbias (bias above breakdown voltage) and so does the dark count rate. A detection efficiency of 13-16% is obtained while still keeping the dark count and afterpulsing rates low. To our knowledge, the self-quenched InGaAs SPAD is the only device in its class to be able to operate under DC bias without gating or external circuits. As a result, the device is particularly suitable for array structures often used in communications, sensing, and imaging.

In recent years significant progress has been made in near-infrared single photon detection using Geiger-mode InP-based single photon avalanche diodes (SPADs). A more detailed understanding of these detectors with regard to device design, material growth and device fabrication has led to continual performance improvements. A variety of circuits for enabling SPAD Geiger-mode operation have been proposed and demonstrated as well. However, due to the inherent positive feedback nature of the avalanche process, Geiger-mode SPADs are constrained by certain performance limitations, particularly with regard to counting rate and the absence of photon number resolution. These limitations hinder the use of SPADs in certain applications. By incorporating a negative feedback mechanism into InP-based SPADs, these SPAD performance limitations can be overcome. In this paper, we present a negative feedback avalanche diode (NFAD), which is formed by monolithically integrating a passive negative feedback element with a high-performance InP-based SPAD. We describe the design and operation of the NFAD device, along with basic characteristics such as pulse response and quenching dynamics, as well as the dependence of these characteristics on excess bias voltage and input photon number. We will also review the results of near-infrared single photon counting performance for fundamental performance parameters such as photon detection efficiency, dark count rate, and afterpulsing.

HgCdTe and InGaAs linear-mode avalanche photodiodes (APDs) were fabricated and tested for properties suitable for high-speed photon counting when integrated with commercially available 2-GHz resistive transimpedance amplifiers (RTIAs). The 2.71-μm, 100-μm-diameter HgCdTe APDs were fabricated in using an n⁺/p vertical carrier transport architecture designed to reduce carrier drift time and facilitate high-speed operation. At 215 K, a gain of 100 was measured with an excess noise of 2.5. The InGaAs/InAlAs APDs were fabricated using two absorber alloy compositions, one optimized for 950-1300 nm operation and the other for 950-1550 nm operation. Both were fabricated using multiple, cascaded gain regions that allowed for high gain and low avalanche-induced shot noise. Gain exceeding 6000 was observed, and the excess noise factor was measured to be below 20 at a gain of M = 1200 (effective k ~ 0.03). The InGaAs/InAlAs APDs were integrated into receivers consisting of a multi-gain-stage APD coupled to a commercial 2-GHz RTIA and were operated as thresholded photon counters. At a linear gain of M = 1800, a single photon detection efficiency greater than 85% was measured at a maximum count rate of 70 MHz; at a linear gain of M = 1200, single photon detection efficiencies greater than 20% were measured at maximum count rates of 80 MHz. At the temperature tested, 185 K, the receiver's dark count rate (DCR) is dominated by electronic amplifier noise from the TIA for low threshold settings, and by dark counts from the APD at high threshold settings.

This paper reports the demonstration of single photon counting receivers with pulse detection efficiency as high as 68% for 2 photons and single photon counting probabilities as high as 44% at 1550-nm, 1 MHz rate and room temperature and with linear-mode (below the breakdown voltage), high speed response in the 450-1700 nm spectral band. The developed single photon counting receiver is based on Epitaxial Technologies' ultra high gain (>300000), low excess noise, linear-mode APDs, which have been fabricated in dimensions ranging from 25 to 200-μm and array formats up to 32 x 32.

By tens-of-picosecond resolved fluorescence detection (TCSPC, time-correlated single-photon counting) we study Förster resonance energy transfer between a donor and a black-hole-quencher acceptor bound at the 5'- and 3'-positions of a synthetic DNA oligonucleotide. This dual labelled oligonucleotide is annealed with either the complementary sequence or with sequences that mimic single-nucleotide polymorphic gene sequences: they differ in one nucleotide at positions near either the ends or the center of the oligonucleotide. We find donor fluorescence decay times whose values are definitely distinct and discuss the feasibility of single nucleotide polymorphism genotyping by this method.