This PDF file contains the front matter associated with SPIE Proceedings Volume 7681, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

In this review we present the instrumental and theoretical developments for functional diffuse reflectance spectroscopy at small source-detector distances. We proposed the possibility to perform photon migration measurements at null or small inter-fiber distances demonstrating the improvement of this novel approach in terms of achievable contrast, spatial resolution and number of detected photons. We developed a novel system to perform time-resolved diffuse reflectance measurement at small source detector separation based on a single photon avalanche photodiode (SPAD) operated in fast time gated mode and a broadband fiber laser. By means of time gating it is possible to detect longer lived photons neglecting initial ones. We show results both on homogeneous and inhomogeneous tissue phantoms demonstrating a dynamic range of 7 orders of magnitude and a temporal range of 6 nanoseconds. Furthermore, this approach proved valuable to detect brain activity.

The full characterisation of photon counting detection systems is important because it allows the identification and subsequent adoption of the system with the optimum performance. It also allows the uncertainty contributions introduced by a particular detection system to be calculated and used in the estimation of the combined uncertainty of the measurement in which that detection system is being used. The Optical Metrology Group at the National Physical Laboratory (NPL) has assembled dedicated facilities, which are able to characterise the critical operating parameters of photon counting systems anywhere in the 250 nm to 1600 nm wavelength region. These include the absolute and relative spectral responsivity over the wavelength range of interest, the spatial uniformity of response at the wavelengths of interest, the deviation from a true linear response as a function of incident radiant power/irradiance and the stability of response as a function of time or ageing. Using these facilities, the performance of a number of photon counting systems has been evaluated in an effort to identify the most appropriate detector technologies for the various radiometric applications NPL is currently addressing. This document describes the dedicated facilities which exist at NPL and highlights how they are being used to provide traceable measurements of the key performance parameters of photon counting systems. Examples of characterisations of photon counting systems are presented.

Time correlated single photon counting (TCSPC) has made tremendous progress during the past ten years enabling improved performance in precision time-of-flight (TOF) rangefinding and lidar. In this review the development and performance of several ranging systems is presented that use TCSPC for accurate ranging and range profiling over distances up to 17km. A range resolution of a few millimetres is routinely achieved over distances of several kilometres. These systems include single wavelength devices operating in the visible; multi-wavelength systems covering the visible and near infra-red; the use of electronic gating to reduce in-band solar background and, most recently, operation at high repetition rates without range aliasing- typically 10MHz over several kilometres. These systems operate at very low optical power (<100μW). The technique therefore has potential for eye-safe lidar monitoring of the environment and obvious military, security and surveillance sensing applications. The review will highlight the theoretical principles of photon counting and progress made in developing absolute ranging techniques that enable high repetition rate data acquisition that avoids range aliasing. Technology trends in TCSPC rangefinding are merging with those of quantum cryptography and its future application to revolutionary quantum imaging provides diverse and exciting research into secure covert sensing, ultra-low power active imaging and quantum rangefinding.

Single-photon detection technologies in conjunction with low laser illumination powers allow for the eye-safe acquisition of time-of-flight range information on non-cooperative target surfaces. We previously presented a photon-counting depth imaging system designed for the rapid acquisition of three-dimensional target models by steering a single scanning pixel across the field angle of interest. To minimise the per-pixel dwelling times required to obtain sufficient photon statistics for accurate distance resolution, periodic illumination at multi- MHz repetition rates was applied. Modern time-correlated single-photon counting (TCSPC) hardware allowed for depth measurements with sub-mm precision. Resolving the absolute target range with a fast periodic signal is only possible at sufficiently short distances: if the round-trip time towards an object is extended beyond the timespan between two trigger pulses, the return signal cannot be assigned to an unambiguous range value. Whereas constructing a precise depth image based on relative results may still be possible, problems emerge for large or unknown pixel-by-pixel separations or in applications with a wide range of possible scene distances. We introduce a technique to avoid range ambiguity effects in time-of-flight depth imaging systems at high average pulse rates. A long pseudo-random bitstream is used to trigger the illuminating laser. A cyclic, fast-Fourier supported analysis algorithm is used to search for the pattern within return photon events. We demonstrate this approach at base clock rates of up to 2 GHz with varying pattern lengths, allowing for unambiguous distances of several kilometers. Scans at long stand-off distances and of scenes with large pixel-to-pixel range differences are presented. Numerical simulations are performed to investigate the relative merits of the technique.

We have implemented cross strip readout microchannel plate detectors in 18 mm active area format including open face (UV/particle) and sealed tube (optical) configurations. These have been tested with a field programmable gate array based parallel channel electronics for event encoding which can process high input event rates (> 5 MHz) with high spatial resolution. Using small pore MCPs (6 μm) operated in a pair, we achieve gains of >5 x 10⁵ which is sufficient to provide spatial resolution of <35 μm FHWM, with self triggered event timing accuracy of ~2 ns for sealed tube optical sensors. A peak quantum efficiency of ~19% at 500 nm has been achieved with SuperGenII photocathodes that have response over the 400 nm to 900 nm range. Local area counting rates of up to >200 events/mcp pore sec^-1 have been attained, along with image linearity and stability to better than 50 μm.

The review of photon counting detectors based missions for the laser time transfer ground to space in Chinese and European programs will be presented. The new self-calibration scheme of detector package will be introduce together with experimental results showing several days ground campaign. The produced data set allows applied post-processing algorithms to enhance final long-term stability of a range signal down to 2 ps over several days. As a near-future outlook, proposed instruments for new missions will be presented. The first one designed for the Galileo program - optical detector for the laser time transfer ground to space. The second one is designed to provide one-way ranging of unprecedented performance: sub-centimeter precision and accuracy may be achieved over distances of several AU. Data products from this instrument will provide an optical reference for distance measurements based on traditional ranging systems, being free from dispersive plasma effects normally encountered in standard RF-based ranging systems. And it also provide a complement radio science observations, by up-link signal time-of-arrival measurements vs. the on-board ultra stable oscillator. It will allow the use of standard TM/TC signals and down-link data transmissions for ranging and radio science purposes, while increasing significantly the observation time and the amount of data available for ranging and radio science. And last but not least it will enable also time transfer capability ground to space.

Superconducting photon detectors have emerged as a powerful new option for detecting single photons. System detection efficiency that incorporates the quantum efficiency of the device and system losses is one of the most important single-photon detector performance metrics for quantum information applications. Superconducting transition-edge sensors (TESs) are microcalorimeters that have the ability of distinguishing single photons with negligible dark counts. In addition, TESs are capable of directly measuring the photon number in a pulse of light. We have achieved near-unity system detection efficiency with TESs at particular wavelengths in the near-infrared by using multilayer structures that enhance the absorption of light into the active device material. We describe the design of the multilayer structure enabling high detection efficiency TESs including issues and requirements for obtaining detection efficiency values higher than 99 %. We describe the device fabrication and finally, show recent results of devices optimized for high detection efficiency using the multilayer structure.

Single-photon sources and detectors are key enabling technologies for photonics in quantum information science and technology (QIST). QIST applications place high-level demands on the performance of sources and detectors; it is therefore essential that their properties can be characterized accurately. Superconducting nanowire single-photon detectors (SNSPDs) have spectral sensitivity from visible to beyond 2 μm in wavelength, picosecond timing resolution (Jitter <100 ps FWHM) and the capacity to operate ungated with low dark counts (<1 kHz). This facilitates data acquisition at high rates with an excellent signal-to-noise ratio. We report on the construction and characterization of a two-channel SNSPD system. The detectors are mounted in a closed-cycle refrigerator, which eliminates reliance on liquid cryogens. Our specification was to deliver a system with 1% efficiency in both channels at a wavelength of 1310 nm with 1 kHz dark count rate. A full width at half maximum timing jitter of less than 90 ps is achieved in both channels. The system will be used to detect individual photons generated by quantum-optical sources at telecom wavelengths. Examples include single-photon sources based on quantum dots (emitting at 1310 nm). The SNSPD system's spectral sensitivity and timing resolution make it suited to characterization of such sources, and to wider QIST applications.

Single photon sources are key devices for optical quantum information processing, miniaturized optical elements, as well as light standards. Several systems have been exploited so far such as semiconductor quantum dots, defect centers in diamond, alkali atoms, and parametric down-conversion sources. In this contribution we will review some of these sources and highlight their unique properties with respect to applications in quantum information processing. A focus lies on two different room temperature sources based on cavity-enhanced parametric downconversion and on nitrogen-vacancy centers in diamond.

Important avalanche breakdown statistics for Single Photon Avalanche Diodes (SPADs), such as avalanche breakdown probability, dark count rate, and the distribution of time taken to reach breakdown (providing mean time to breakdown and jitter), were simulated. These simulations enable unambiguous studies on effects of avalanche region width, ionization coefficient ratio and carrier dead space on the avalanche statistics, which are the fundamental limits of the SPADs. The effects of quenching resistor/circuit have been ignored. Due to competing effects between dead spaces, which are significant in modern SPADs with narrow avalanche regions, and converging ionization coefficients, the breakdown probability versus overbias characteristics from different avalanche region widths are fairly close to each other. Concerning avalanche breakdown timing at given value of breakdown probability, using avalanche material with similar ionization coefficients yields fast avalanche breakdowns with small timing jitter (albeit higher operating field), compared to material with dissimilar ionization coefficients. This is the opposite requirement for abrupt breakdown probability versus overbias characteristics. In addition, by taking band-to-band tunneling current (dark carriers) into account, minimum avalanche region width for practical SPADs was found to be 0.3 and 0.2 μm, for InP and InAlAs, respectively.

In recent years a growing number of applications demands always better timing resolution for Single Photon Avalanche Diodes. The challenge is pursuing the improved timing resolution without impairing the other device characteristics such as quantum efficiency and dark counts. This task requires a clear understanding of the physical mechanisms necessary to drive the device engineering process. Past studies state that in Si-SPADs the avalanche injection position statistics is the main contribution to the photon-timing jitter. However, in recent re-engineered devices, this assumption is questioned. For the purpose of assessing for good this contribution we developed an experimental setup in order to characterize the photontiming jitter as a function of the injection position by means of TCSPC measurements with a laser focused on the device active area. Results confirmed not only that the injection position is not the main contribution to the photon-timing jitter but also evidenced a radial dependence never observed before. Furthermore we found a relation between the photon-timing jitter and the specific resistance of the devices. To characterize the resistances we studied the avalanche current density distribution in the device active area by imaging the photo-luminescence due to hot-carrier emission.

We will report on our advances on the development of a new planar silicon SPAD with high photon detection efficiency (PDE) and good photon timing resolution. We will show that a 10μm thick epitaxial layer allows for the absorption of a significant fraction of the incident photons even at the longer wavelengths, while a suitable electric field profile limits the breakdown voltage value and the timing jitter. Simulations show that the new devices can attain a PDE higher than 30% at a wavelength of 800nm.

Silicon based avalanche photodiodes (APDs) have exhibited impressive performance over the visible spectrum for more than a decade. Photon counting with these devices has progressed to the level where room-temperature operation and low dark count rates (< 100 Hz) are commonplace. Several commercial enterprises have been established to capitalise on these devices and many niche markets are now serviced by incorporating these devices into suitable systems. This paper describes one approach that allows the performance of silicon based Geigermode avalanche photodiodes (GM-APDs) to be extended into the near-infra-red. The process development is described whereby Ge absorbers are incorporated into adapted silicon APD designs to provide separate absorption and multiplication devices. Simulation results are presented outlining the performance of these devices at wavelengths between 1 μm and 1.6 μm. The performance results from silicon APD designs are presented for visible wavelengths. A silicon-germanium bonding process is described and the challenges presented in developing the hybrid absorber/multiplier structure are detailed. Finally, a summary of appropriate custom application integrated circuits for various applications is discussed.

We present a unique hybridization process that permits high-performance back-illuminated silicon Geiger-mode avalanche photodiodes (GM-APDs) to be bonded to custom CMOS readout integrated circuits (ROICs) - a hybridization approach that enables independent optimization of the GM-APD arrays and the ROICs. The process includes oxide bonding of silicon GM-APD arrays to a transparent support substrate followed by indium bump bonding of this layer to a signal-processing ROIC. This hybrid detector approach can be used to fabricate imagers with high-fill-factor pixels and enhanced quantum efficiency in the near infrared as well as large-pixel-count, small-pixel-pitch arrays with pixel-level signal processing. In addition, the oxide bonding is compatible with high-temperature processing steps that can be used to lower dark current and improve optical response in the ultraviolet.

At MIT Lincoln Laboratory, avalanche photodiodes (APDs) have been developed for both 2-μm and 3.4-μm detection using the antimonide material system. These bulk, lattice-matched detectors operate in Geiger mode at temperatures up to 160 K. The 2-μm APDs use a separate-absorber-multiplier design with an InGaAsSb absorber and electron-initiated avalanching in the multiplier. These APDs have exhibited normalized avalanche probability (product of avalanche probability and photo-carrier-injection probability) of 0.4 and dark count rates of ~150 kHz at 77 K for a 30-μm-diameter device. A 1000- element imaging array of the 2-μm detectors has been demonstrated, which operate in a 5 kg dewar with an integrated Stirling-cycle cooler. The APD array is interfaced with a CMOS readout circuit, which provides photon time-of-arrival information for each pixel, allowing the focal plane array to be used in a photon-counting laser radar system. The 3.4-μm APDs use an InAsSb absorber and hole-initiated avalanching and have shown dark count rates of ~500 kHz at 77 K but normalized avalanche probability of < 1%. Research is ongoing to determine the cause of the low avalanche probability and improve the device performance.

InGaAs/InP Single-Photon Avalanche Diodes (SPADs) have recently shown good performances in terms of dark count rate and detection efficiency, making them suitable for many NIR single-photon counting applications. However, it is mandatory to operate InGaAs/InP SPADs in optimized working conditions and in association with proper dedicated electronics. A complete characterization of primary dark count rate, afterpulsing, detection efficiency and timing jitter is required in order to be able to tailor the working conditions to the specific request. Moreover, very fast quenching circuits can efficiently minimize afterpulsing, while low-jitter front-end circuits detect the avalanche pulse with high timing precision.

We report reduced afterpulsing for a high-performance InGaAs/InP single photon avalanche photodiode (SPAD) using a gated-mode passive quenching with active reset (gated-PQAR) circuit. Photon detection efficiency (PDE) and dark count probability (DCP) were measured at a gate repetition rate of 1 MHz. With a double-pulse measurement technique, the afterpulsing probability was measured for various hold-off times. At 230K, 0.3% afterpulsing probability for a 10 ns hold-off time was achieved with 13% PDE, 2×10^-6 DCP and 0.4 ns effective gate width. For the same hold off time, 30% PDE and 1×10^-5 DCP was achieved with 6% afterpulsing probability for an effective gate width of 0.7 ns.

Output pulse jitter from single photon detection events in single photon sensitive detectors sets an upper limit to the useful bandwidth of a photon counting signal processing system. Unlike counting losses, single photon jitter is not improved by splitting the signal across a detector array, but rather degrades due to the introduction of additional variable propagation delays in additional wiring. We have observed that both the mean delay from photon arrival to output pulse and the delay variance (jitter) can be a strong function of detector bias conditions, as well as incident illumination conditions. We have characterized samples of both Geiger mode and negative-avalanche feedback (NAF) InGaAs(P) single photon detectors for single photon timing jitter at both 1.06 and 1.5 microns at temperatures ranging from 298K to below 200K. Using pulse-picked mode-locked laser sources, we attenuate the beam greatly to ensure that we are measuring true single photon mean delay and jitter, not a multi-photon response.

We report on progress in improving fundamental properties of InP-based single photon avalanche diodes (SPADs) and recent trends for overcoming dominant performance limitations. Through experimental and modeling work focused on the trade-off between dark count rate (DCR) and photon detection efficiency (PDE), we identify the key mechanisms responsible for DCR over a range of operating temperatures and excess bias voltages. This work provides a detailed description of temperature- and bias-dependent DCR thermal activation energy E_a(T,V), including the crossover from low E_a for trap-assisted tunneling at temperatures below ~230 K to larger E_a for thermal generation at temperatures approaching room temperature. By applying these findings to new device design and fabrication, the fundamental tradeoff between PDE and DCR for InP/InGaAs SPADs designed for 1.55 μm photon detection has been managed so that for PDE ~ 20%, devices routinely exhibit DCR values of a few kHz, while "hero" devices demonstrate that it is possible to achieve sub-kHz DCR performance at temperatures readily accessible using thermoelectric coolers. However, important limitations remain, particularly with respect to maximum count rates. Strategies adopted recently to circumvent some of these present limitations include new circuit-based solutions involving high-speed very short-duration gating as well as new monolithic chip-level concepts for obtaining improved performance through avalanche self-quenching. We discuss these two approaches, and we describe recent results from devices with monolithically integrated quench resistors that achieve rapid self-quenching, accompanied by evidence for a partial discharge of the detector capacitance leading to charge flows as low as ~3 ×10⁵ carriers associated with each avalanche event.

Recently, considerable attention has been placed upon exploiting the negative-feedback effect in accelerating the quenching time of the avalanche current in passively quenched single-photon avalanche-diode (SPAD) circuits. Reducing the quenching time results in a reduction in the total charge generated in the SPAD, thereby reducing the number of trapped carries; this, in turn, can lead to improved after-pulsing characteristics. A passively quenched SPAD circuit consists of a DC source connected to the SPAD, to provide the reverse bias, and a series load resistor. Upon a photon-generated electron-hole pair triggering an avalanche breakdown, current through the diode and the load resistor rises quickly reaching a steady state value, after which it can collapse (quench) at a stochastic time. In this paper we review recent analytical and Monte-Carlo based models for the quenching time. In addition, results on the statistics of the quenching time and the avalanche pulse duration of SPADs with arbitrary time-variant field across the multiplication region are presented. The calculations of the statistics of the avalanche pulse duration use the dead-space multiplication theory (DSMT) to determine the probability of the avalanche pulse to quench by time t after the instant s at which the electron-hole pair that triggers the avalanche was created. In the analytical and Monte-Carlo based models for the quenching time, the dynamic negative feedback, which is due to the dynamic voltage drop across the load resistor, is taken into account. In addition, in the Monte-Carlo simulations the stochastic nature of the avalanche current is also considered.

We have designed and developed a new family of photodetectors and arrays with Internal Discrete Amplification (IDA) mechanism for the realization of very high gain and low excess noise factor in the visible and near infrared spectral regions. These devices surpass many limitations of the Single Photon Avalanche Photodetectors such as ultra low excess noise factor, very high gain, lower reset time (< 200 ns). These devices are very simple to operate in the non-gated mode under a constant dc bias voltage. Because of its unique characteristics of self-quenching and self-recovery, no external quenching circuit is needed. This unique feature of self quenching and self-recovery makes it simple to less complex readout integrated circuit to realize large format detector arrays. In this paper, we present the discrete amplification design approach used for the development of self reset, high gain photodetector arrays in the near infrared wavelength region. The demonstrated device performance far exceeds any available solid state Photodetectors in the near infrared wavelength range. These devices are ideal for researchers in the field of spectroscopy, industrial and scientific instrumentation, Ladar, quantum cryptography, night vision and other military, defense and aerospace applications.

Silicon avalanche photodiode (APD) detectors have been used in most space lidar receivers to date with a sensitivity that 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. We have purchased and tested InGaAs avalanche photodiode based receivers from several US vendors as possible alternatives. We present our measurement results and a comparison of their performance to our baseline silicon APD. Using a multichannel scalar instrument, we observed undesired dark counts in some devices, even though the APDs were biased below the breakdown voltage. These effects are typically associated with over-biased Geiger-mode photoncounting, but we demonstrate that the probability distribution indicates their necessity at the high gains typically associated with operation slightly below the breakdown voltage. We measured the following parameters for our 0.8 mm diameter baseline silicon APD receiver: excess noise factor 2.5, bandwidth 210 MHz, minimum detectable pulse (10 ns) in incident photons 110 photons, noise equivalent power 30 fW/rt-Hz. We present our test procedures and results for the InGaAs based APD receivers.

We implement an InGaAs/InP single-photon avalanche diode (SPAD) for single-photon detection with the fastest gating frequency reported so far, of 2.23GHz, which approaches the limit given by the bandwidth of the SPAD - 2.5 GHz. We propose a useful way to characterize the afterpulsing distribution for rapid gating that allows for easy comparison with conventional gating regimes. We compare the performance of this rapid gating scheme with free-running detector and superconducting single-photon detector (SSPD) for the coherent one-way quantum key distribution (QKD) protocol. The rapid gating system is well suited for both high-rate and long-distance QKD applications, in which Mbps key rates can be achieved for distances less than 40km with 50 ns deadtime and the maximum distance is limited to ~190km with 5 μs deadtime. These results illustrate that the afterpulsing is no longer a limiting factor for QKD.

Iqueye is a single photon counting very high speed photometer built for the ESO 3.5m New Technology Telescope (NTT) in La Silla (Chile) as prototype of a 'quantum' photometer for the 42m European Extremely Large Telescope (E-ELT). The optics of Iqueye splits the telescope pupil into four portions, each feeding a Single Photon Avalanche Diode (SPAD) operated in Geiger mode. The SPADs sensitive area has a diameter of 100 μm, with a quantum efficiency better than 55% at 500 nm, and a dark count less than 50 Hz. The quenching circuit and temperature control are integrated in each module. A time-to-digital converter (TDC) board, controlled by a rubidium oscillator plus a GPS receiver, time tags the pulses from the 4 channels. The individual times are stored in a 2 TeraByte memory. Iqueye can run continuously for hours, handling count rates up to 8 MHz, with a final absolute accuracy of each time tag better that 0.5 ns. A first very successful run was performed in Jan 2009; both very faint and very bright stars were observed, demonstrating the high photometric quality of the instrument. The first run allowed also to identify some opto-mechanical improvements, which have been implemented for a second run performed in Dec 2009. The present paper will describe the first version, the improvements implemented in the second one, and some of the obtained astronomical results.