This PDF file contains the front matter associated with SPIE Proceedings Volume 11721, including the Title Page, Copyright information and Table of Contents

LIDAR (Light Detection and Ranging) is emerging as a necessity for fully automated self-driving automotive applications. In order to sample the far field with sufficient resolution for this application the system must incorporate many optical elements, leading to challenges for manufacturability and size. Due to the density of optical components required, LIDAR is well suited for photonic integration in order to achieve miniaturization and scalable manufacturability. This talk will give an overview of LIDAR, the components required for a chip-scale solution, and silicon photonics progress with respect to this goal.

CMOS SPADs have appeared in 2003 but it is only the availability of deep-submicron SPAD technology that has made megapixel SPAD cameras possible. Large-format SPAD image sensors have enabled more applications and the introduction of proximity sensing is accelerating the process, so much so that today SPADs are in almost every smartphone and the promise is that they will be in every car by 2022. The digital nature of SPADs and the increased density of computation over multiple silicon layers will soon enable deep-learning processors on chip, thus enabling complex processing while reducing power consumption. Another recent trend is the use of SPADs in qubit readout and control, thus making them amenable to interface with quantum processors, due to the capability of operating normally at cryogenic temperatures. The talk will conclude with a technical and economic perspective for SPAD imagers and a vision statement for photon counting in CMOS and other hybrid technologies.

3D sensing devices are becoming increasingly prevalent in robotics, self-driving cars, human-computer interfaces, as well as consumer electronics. Recent years have seen single-photon avalanche diodes (SPADs) emerging as one of the key technologies underlying 3D time-of-flight sensors, with the capability to capture accurate 3D depth maps in a range of environmental conditions, and with low computational overhead. In particular, direct ToF SPADs (dToF), which measure the return time of back-scattered laser pulses, form the backbone of many automotive LIDAR systems. We here consider an advanced direct ToF SPAD imager with a 3D-stacked structure, integrating significant photon processing. The device generates photon timing histograms in-pixel, resulting in a maximum throughput of 100's of giga photons per second. This advance enables 3D frames to be captured at rates in excess of 1000 frames per second, even under high ambient light levels. By exploiting the re-configurable nature of the sensor, higher resolution intensity (photon counting) data may be obtained in alternate frames, and depth upscaled accordingly. We present a compact SPAD camera based on the sensor, enabling high-speed object detection and classification in both indoor and outdoor environments. The results suggest a significant potential in applications requiring fast situational awareness.

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

Imaging through strongly diffuse media can be extremely challenging yet has many applications ranging from seeing though thick fog to imaging inside the human body. A number of approaches have been demonstrated for imaging over a relatively small number of transport mean free paths, TMFPs, defined as the distance over which scattering leads to complete loss information of the original photon propagation direction. However, imaging deep inside the body, for example all the way through the head, requires transmission through more than 100 TMFPs. Whereas late photons degrade image retrieval with weak diffusion, we show how in the presence of deep diffusion including the late photons in the inverse retrieval enhances the image reconstruction. An analysis using information theory sheds light on how information moves from the early photons towards later arrival times and provides insight into how to improve inverse image retrieval.

Development of a novel Time Correlated Single Photon Counting system with 256 independent channels is presented. The system integrates a UV/VIS sensitive Microchannel Plate based Multi Anode Photomultiplier Tube having a 16 × 16 array of 1.656 mm pitch pixels with the TOFPET2 ASIC. Each pixel acts as an independent detector with < 70 ps RMS single photon timing and per pixel maximum photon rate of 480 KHz. Characterisations presented include the pixel response uniformity, pixel-to-pixel crosstalk and LiDAR demonstration of single photon depth resolution of < 3 cm.

Time-resolved imaging by means of Time Correlated Single Photon Counting (TCSPC) is the enabling technology for several powerful analytical techniques, such as Fluorescence Lifetime Imaging Microscopy (FLIM), F¨orster Resonance Energy Transfer (FRET) and Fluorescence Lifetime Correlation Spectroscopy (FLCS). Even more, TCSPC could further extend its application also to other research fields, for example in-vivo analysis, if only its relatively long acquisition time could be reduced. To achieve such a challenging result, multi-channel integrated systems have been developed to increase the measurement speed, but posing at the same time tight constraints on power dissipation, area occupation and interconnection complexity inside the chip. Moreover, the high rate of events potentially produced by a huge number of detectors can easily saturate the transfer bandwidth. To address this problem, we designed a dynamic router optimized to use the transfer bandwidth of the system at its best. The router is based on a fully-integrated, 150nm-technology, distributed routing logic that permits to interconnect different parts of the system, each one optimized by exploiting a different technology node selected on purpose. In this work, we present a 32-channel prototype of the router, but we’ll show that it has been designed to be easily up scalable and to be the building block of a densely integrated system.

Time-Correlated Single Photon Counting (TCSPC) is generally recognized as a powerful tool for Fluorescence Lifetime Imaging (FLIM), thanks to its inherently high sensitivity and timing precision. Nevertheless, one of the major drawbacks of the technique is represented by the so-called pile-up distortion, that typically limits the acquisition rate to few percent of the laser stimulation rate. In recent years, an innovative methodology has been proposed to overcome this restriction: by matching the detector dead time to the laser period an average acquisition rate of 40 Mcps is achieved, along with negligible distortion. In this work, we present the first single-channel system that implements the new measurement technique. To this aim, two modules have been specifically developed to accommodate a custom-technology Single-Photon Avalanche Diode (SPAD) and its dedicated acquisition chain. On one hand, a compact Detection Module hosts both a fully-integrated Active Quenching Circuit (AQC) to provide a finely-tunable dead time and a differential Pick-Up Circuit (PUC) to extract a picosecond-precision timing signal. On the other hand, a Time Conversion module is intended to acquire the fast timing signal thanks to a mixed-architecture Fast Time to Amplitude Converter (F-TAC). The experimental characterization proved that the modules feature excellent performance both in terms of timing precision (55 ps FWHM) and Differential Nonlinearity (4 % LSB peak to peak) and we’re now ready to compare the new technique with the classic pile-up limited approach in a real application on field.

The continuous time-tagging of photon arrival times for high count rate sources is necessary for applications such as optical communications, quantum key encryption, and astronomical measurements. Detection of Hanbury- Brown Twiss photon correlations from thermal sources such as stars requires a combination of high dynamic range, long integration times and low systematics in the photon detection and time tagging system. The continuous nature of the measurements and the need for highly accurate timing resolution requires a customized time-to-digital converter (TDC). We used a custom built, two-channel, FPGA-based TDC to continuously time tag single photons with sub clock cycle timing resolution for correlation measurements. We used autocorrelation and cross-correlation measurement tools to constrain spurious systematic effects in the pulse count data as a function of system variables. These variables included, but were not limited to, incident photon count rate, incoming signal attenuation, and measurements of fixed signals. We present an overview of the results of these tests, the types of systematic effects that the results imply, and how those effects may be accounted for and corrected to levels below those required for photon correlation measurements.

In this work, a dynamic metallic filamentary resistive switch (MFRS) is used to quench the avalanche in a single photon avalanche photodiode (SPAD). The experimental results and simulations are consistent with an interpretation that, the MFRS is in a high resistance state when the avalanche occurs. This enables the quenching of the avalanche sufficiently within a short time. This increases the voltage drop across the MFRS, which switches the MFRS to its low resistance on-state and the recharging process is greatly accelerated because of the lowered R-C time constant. This leads to a sharp avalanche pulse shape and a fast detection speed.

Superconducting nanowire single photon detectors stand today as the best technology, due to their near-unity detection efficiency, low dark count rates, and low timing jitter. In this work, we demonstrate the improvement of the superconducting properties of NbN thin films on 8” silicon-on-insulator wafers by using an ultra-thin (10-15 nm) sputtered AlN buffer layer. The higher crystalline quality of NbN, leads to an increase of the superconducting critical temperature up to 10 K for 9-nm-thick NbN films. The material was validated for single photon detection using a fiber-coupled vertical SNSPD with half-cavity architecture. This results opens the way for the development of CMOS compatible waveguide-integrated detectors. The implementation of such guided devices is a keystone for the development of a fully integrated quantum photonics platform able to generate, manipulate and detect a large number of photonic qubits for secure communications and quantum computing applications.

Publisher’s Note: This conference presentation, originally published on 12 April 2021, was withdrawn on 7 May 2021 per author request.

The quest to enlarge the capabilities of quantum information experiments based on non-classical states of light has directed the research toward an integrated approach. Thanks to integrated photonics circuits, photons can be efficiently generated, manipulated and detected within the same integrated chip. SNSPDs are the unique detectors that showed an integration compatibility with standard PICs. Nowadays the increasing complexity of integrated quantum optics experiments requires the manipulation of many modes and the simultaneous readout of an increasing number (N) of integrated detectors, thus posing new challenges regarding the simultaneous readout of different channels. In this presentation, we will discuss the advantages and drawbacks of our approach based on the amplitude multiplexing of N active pixels, each one consisting of a SNSPD with in parallel an on-chip AuPd resistor (Ri), to read (measure) simultaneously the output of several optical modes using only one coax cable.

Superconducting Nanowire Single Photon Detectors (SNSPDs) excel at a wide variety of performance criteria for single photon counting. They combine unprecedented high detection efficiency, high timing resolution, high count rates, low intrinsic dark count rates, and are sensitive to ultraviolet through mid-infrared single-photons. At JPL, we are working on several projects to push the performance limits of SNSPDs to achieve higher maximum count rates, larger active areas, higher timing resolution, and a wider spectral range. Our recent advances enable new applications for dark matter detection, imaging, and space-to-ground communication and provide insight into the fundamental physics of single-photon detection in superconducting nanowires.

Quantum-enhanced optical technologies operating within the 2- to 2.5-μm spectral region have the potential to revolutionize emerging applications in communications, sensing, and metrology. Currently, sources of entangled photons are available at visible, near-infrared and telecom wavelengths but are strongly underdeveloped at longer wavelengths. Here, using custom-designed lithium niobate crystals for spontaneous parametric down-conversion and tailored superconducting nanowire single-photon detectors, we demonstrate two-photon interference and polarization-entangled photon pairs at 2090 nm. These results open the 2- to 2.5-μm mid-infrared window for the development of optical quantum technologies such as quantum key distribution in next-generation mid-infrared fiber communication systems and future Earth-to-satellite communications.

We present modeling and analysis of carrier dynamics in a single-photon avalanche diode (SPAD) operated with a capacitive quenching (CQ) method. The CQ method is regarded as a conventional resistive quenching (RQ) with its quenching resistance infinite or an open circuit. The SPAD is modelled as a lumped circuit consisting of a voltage dependent charge generator representing an avalanching depletion region and a capacitance of the depletion region and parasitic components. The carrier dynamics inside the device is described by time-dependent bipolar continuity equations (BCE) derived from the carrier continuity equations. We solve the BCE numerically with a 0.1 ps time resolution and investigate numbers of carriers in each circuit element as functions of time and of excess bias voltage (|𝑉_ex|). We find two important characteristics of the CQ method; (1) a single-photon triggered Geiger-mode pulse is guaranteed to be quenched in a stable state (2) a voltage drop of the internal bias of SPAD due to the charges stored on the capacitance is proportional to |𝑉_ex| with the proportionality factor of two. The results, in turn, enables one to design a SPAD free from after-pulse and from overflow. Such a SPAD pixel is shown to be compatible with a conventional complementary metal-oxide semiconductor (CMOS) image sensor (CIS) with a four transistors configuration pixel circuit. Finally, effectiveness of the present methodology is demonstrated by the subrange synthesis (SRS) time-of-flight (ToF) ranging experiments using a 6 μm size 400 × 400 pixels SPAD-based CIS.

The NASA Psyche mission is set to explore an asteroid located between Mars and Jupiter with a launch date in 2022. Onboard the Psyche spacecraft is experimental demonstrator technology that will allow scientists to explore the capabilities of optical communications – a program called Deep Space Optical Communication (DSOC) led by Jet Propulsion Laboratory (JPL). DSOC seeks to improve communications performance by developing a space-based Flight Laser Transceiver (FLT) and a ground-based transceiver to enable photon-efficient communications with equipment in deep space. An integral part to this FLT system is a high-efficiency photon-counting camera (PCC) that is able to detect both the 1064nm uplink/beacon laser photons and 1550nm downlink laser photons with low background noise, and is capable of withstanding the rigors of space-travel. The paper details the characterization of several asynchronous Geiger-Mode Avalanche Photodiode (GmAPD) arrays developed by MIT Lincoln Laboratory for use in the PCC- specifically evaluating the temperature dependence of background noise, photon detection efficiency at 1064nm and 1550nm wavelengths, pixel lifetime testing, and angle of acceptance measurements. The results of this characterization are used to determine the nominal conditions for the device to operate in while in flight to maintain an efficient link with the ground-based transceiver.

Time-correlated single-photon counting (TCSPC) has been established as the preferred detection approach for high performance lidar and depth profiling due to its excellent surface to surface resolution and high optical sensitivity. This presentation will show the results of laboratory-based experiments obtained using different optical transceiver configurations. Particular attention will be given to underwater depth imaging using silicon single photon avalanche diode (Si-SPAD) detector arrays fabricated in complementary metal-oxide semiconductor (CMOS) and Si SPAD detector arrays built in custom fabrication technology.

A system-level performance evaluation of Geiger-mode avalanche photodiode (GmAPD) arrays requires accurate measurement and prediction of the background rate of the device due to dark counts and other spurious detection events. Since a GmAPD detector reports only a binary value and timestamp associated with an avalanche event, dark count rates are typically measured by averaging thousands of frames to support a statistically significant measurement. For both synchronous and asynchronous detector, the Poisson distributed background rates are referenced to the time each pixel is armed. Unlike for synchronous GmAPD imagers where all the pixels are armed to an array-wide arm signal, an asynchronous pixel operates independently from its neighboring pixels; requiring the background rates to be calculated using an interarrival histogram. For both types of imagers, the background rate is typically evaluated by fitting an exponential distribution to a fixed window within a measured histogram of time intervals between detection events However, if the statistics of the background rate are insufficient – whether that is due to low population sizes, saturation, or a large dynamic range of population size across the array, the pixel, or array-wide, performance metrics may report results with varying accuracy. This paper reports on an implementation of an algorithm that evaluates GmAPD background rates based on statistical metrics rather than fixed windows. The algorithm functions by determining the appropriate integration window within the interarrival time histogram based on a per-pixel count rate set by a predetermined tolerable measurement error. The implementation of the algorithm allows us to characterize GmAPD arrays with orders of magnitude spread in background rates across the detector using common statistical parameters.

A 128x192 SPAD array (QuantiCam) with an on-chip time-to-digital converter in each pixel is used as a camera in a single-photon time-resolved fluorescence microscope. The SPAD array introduces systematic nonlinearities and timing offset to the measured photon arrival times. This limits the fidelity of the experimental results. A Monte-Carlo algorithm was developed to transform the raw photon time-stamp stream coming from the SPAD array into a corrected virtual “photon” time-stamp stream devoid of the systematic measurement errors. This data is compatible with existing downstream data processing pipelines used in time-correlated single-photon counting. We discuss the calibration measurement, the algorithm, their performance and application to live fluorescence lifetime imaging of photosynthetic organisms.