This PDF file contains the front matter associated with SPIE Proceedings Volume 10229 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

This paper reviews a few novel designs of superconducting nanowire single-photon detectors (SNSPDs). For polarization-insensitive SNSPDs, we present a fractal-inspired design and a waveguide-integrated design; for lowtiming-jitter SNSPDs, we present SNSPDs integrated with current reservoirs.

Superconducting nanowire single-photon detectors (SNSPDs) have shown to be promising in applications such as quantum communication and computation, quantum optics, imaging, metrology and sensing. They offer the advantages of a low dark count rate, high efficiency, a broadband response, a short time jitter, a high repetition rate, and no need for gated-mode operation. Several SNSPD designs have been proposed in literature. Here, we discuss the so-called parallel nanowires configurations. They were introduced with the aim of improving some SNSPD property like detection efficiency, speed, signal-to-noise ratio, or photon number resolution. Although apparently similar, the various parallel designs are not the same. There is no one design that can improve the mentioned properties all together. In fact, each design presents its own characteristics with specific advantages and drawbacks. In this work, we will discuss the various designs outlining peculiarities and possible improvements.

High-resolution position-sensitive detectors have been proposed to correct response non-uniformities in Cadmium Zinc Telluride (CZT) crystals by virtually subdividing the detectors area into small voxels and equalizing responses from each voxel. 3D pixelated detectors coupled with multichannel readout electronics are the most advanced type of CZT devices offering many options in signal processing and enhancing detector performance. One recent innovation proposed for pixelated detectors is to use the induced (transient) signals from neighboring pixels to achieve high sub-pixel position resolution while keeping large pixel sizes. The main hurdle in achieving this goal is the relatively low signal induced on the neighboring pixels because of the electrostatic shielding effect caused by the collecting pixel. In addition, to achieve high position sensitivity one should rely on time-correlated transient signals, which means that digitized output signals must be used. We present the results of our studies to measure the amplitude of the pixel signals so that these can be used to measure positions of the interaction points. This is done with the processing of digitized correlated time signals measured from several adjacent pixels taking into account rise-time and charge-sharing effects. In these measurements we used a focused pulsed laser to generate a 10-micron beam at one milliwatt (650-nm wavelength) over the detector surface while the collecting pixel was moved in cardinal directions. The results include measurements that present the benefits of combining conventional pixel geometry with digital pulse processing for the best approach in achieving sub-pixel position resolution with the pixel dimensions of approximately 2 mm. We also present the sub-pixel resolution measurements at comparable energies from various gamma emitting isotopes.

The analysis of optical signals by means of Single Photon Avalanche Diodes (SPADs) has been subject to a widespread interest in recent years. The development of multichannel high-performance Time Correlated Single Photon Counting (TCSPC) acquisition systems has undergone a fast trend. Concerning the detector performance, best in class results have been obtained resorting to custom technologies leading also to a strong dependence of the detector timing jitter from the threshold used to determine the onset of the photogenerated current flow. In this scenario, the avalanche current pick-up circuit plays a key role in determining the timing performance of the TCSPC acquisition system, especially with a large array of SPAD detectors because of electrical crosstalk issues. We developed a new current pick-up circuit based on a transimpedance amplifier structure able to extract the timing information from a 50-μm-diameter custom technology SPAD with a state-of-art timing jitter as low as 32ps and suitable to be exploited with SPAD arrays. In this paper we discuss the key features of this structure and we present a new version of the pick-up circuit that also provides quenching capabilities in order to minimize the number of interconnections required, an aspect that becomes more and more crucial in densely integrated systems.

In recent years, the Time-Correlated Single Photon Counting (TCSPC) technique has gained a prominent role in many fields, where the analysis of extremely fast and faint luminous signals is required. In the life science, for instance, the estimation of fluorescence time-constants with picosecond accuracy has been leading to a deeper insight into many biological processes. Although the many advantages provided by TCSPC-based techniques, their intrinsically repetitive nature leads to a relatively long acquisition time, especially when time-resolved images are obtained by means of a single detector, along with a scanning point system. In the last decade, TCSPC acquisition systems have been subjected to a fast trend towards the parallelization of many independent channels, in order to speed up the measure. On one hand, some high-performance multi-module systems have been already made commercially available, but high area and power consumption of each module have limited the number of channels to only some units. On the other hand, many compact systems based on Single Photon Avalanche Diodes (SPAD) have been proposed in literature, featuring thousands of independent acquisition chains on a single chip. The integration of both detectors and conversion electronic in the same pixel area, though, has imposed tight constraints on power dissipation and area occupation of the electronics, resulting in a tradeoff with performance, both in terms of differential nonlinearity and timing jitter. Furthermore, in the ideal case of simultaneous readout of a huge number of channels, the overall data rate can be as high as 100 Gbit/s, which is nowadays too high to be easily processed in real time by a PC. Typical adopted solutions involve an arbitrary dwell time, followed by a sequential readout of the converters, thus limiting the maximum operating frequency of each channel and impairing the measurement speed, which still lies well below the limit imposed by the saturation of the transfer rate towards the elaboration unit. We developed a novel readout architecture, starting from a completely different perspective: considering the maximum data rate we can manage with a PC, a limited set of conversion data is selected and transferred to the elaboration unit during each excitation period, in order to take full advantage of the bus bandwidth toward the PC. In particular, we introduce a smart routing logic, able to dynamically connect a large number of SPAD detectors to a limited set of high-performance external acquisition chains, paving the way for a more efficient use of resources and allowing us to effectively break the tradeoff between integration and performance, which affects the solutions proposed so far. The routing electronic features a pixelated architecture, while 3D-stacking techniques are exploited to connect each SPAD to its dedicated electronic, leading to a minimization of the overall number of interconnections crossing the integrated system, which is one of the main issues in high-density arrays.

The laser time transfer ground to space is an attractive technique to compare the ultra-stable clocks on ground and in space. The photon counting approach enables to reduce significantly the systematic errors of the measurement chain. For the space mission nominated for the next decade the precision and long term detection delay stability requirements are on sub-picosecond level. We have developed a new SPAD detector package for laser time transfer ground to space with extremely high timing precision and stability. It is based on 100 μm or 200 μm diameter K14 series SPAD chips. The device presented is primarily dedicated for a ground segment of the laser time transfer instrument chain. Applying such a detector the limiting precision of laser time transfer characterized by time deviation TDEV is well below 100 fs. The long term timing stability is better than 1 ps over days of operation. The detector package is constructed on a basis of electronics components for which the space qualified equivalents are available. The device construction, tests and results will be presented in detail.

Time-Correlated Single Photon Counting (TCSPC) is one of the most effective techniques for measuring weak and fast optical signals. It outperforms traditional “analog” techniques due to its high sensitivity along with high temporal resolution. Despite those significant advantages, a main drawback still exists, which is related to the long acquisition time needed to perform a measurement. In past years many TCSPC systems have been developed with higher and higher number of channels, aimed to dealing with that limitation. Nevertheless, modern systems suffer from a strong trade-off between parallelism level and performance: the higher the number of channels the poorer the performance. In this work we present the design of a 32x32 TCSPC system meant for overtaking the existing trade-off. To this aim different technologies has been employed, to get the best performance both from detectors and sensing circuits. The exploitation of different technologies will be enabled by Through Silicon Vias (TSVs) which will be investigated as a possible solution for connecting the detectors to the sensing circuits. When dealing with a high number of channels, the count rate is inevitably set by the affordable throughput to the external PC. We targeted a throughput of 10Gb/s, which is beyond the state of the art, and designed the number of TCSPC channels accordingly. A dynamic-routing logic will connect the detectors to the lower number of acquisition chains.

In recent years the development of Single-Photon Avalanche Diodes (SPADs) had a big impact on single-photon counting applications requiring high-performance detectors in terms of Dark Count Rate (DCR), Photon Detection Efficiency (PDE), afterpulsing probability, etc. Among these, it is possible to find applications in singlemolecule fluorescence spectroscopy that suffer from long-time measurements. In these cases SPAD arrays can be a solution in order to shorten the measurement time, thanks to the high grade of parallelism they can provide. Moreover, applications in other fields (e.g. astronomy) demand for large-area single-photon detectors, able also to handle very high count rates. For these reasons we developed a new single-photon detection module, featuring an 8×8 SPAD array. Thanks to a dedicated silicon technology, the performance of the detector have been finely optimized, reaching a 49% detection efficiency at 550 nm, as well as low dark counts (2 kcount/s maximum all over the array). This module can be used in two different modes: the first is a multi-spot configuration, allowing the acquisition of 64 optical signals at the same time and considerably reducing the time needed for a measurement. The second operation mode instead exploits all the pixels in a combined mode, allowing the detection of a 64-times higher maximum photon rate (up to 2 Gcount/s). In addition, this configuration provides also an extended dynamic range and allows to attain photon number resolving capabilities. Dark counts, detection efficiency, linearity, afterpulsing and crosstalk probability have been characterized at different operating conditions.

Achievement of the ultimate sensitivity along with a high spectral resolution is one of the frequently addressed problems, as the complication of the applied and fundamental scientific tasks being explored is growing up gradually. In our work, we have investigated performance of a superconducting nanowire photon-counting detector operating in the coherent mode for detection of weak signals at the telecommunication wavelength. Quantum-noise limited sensitivity of the detector was ensured by the nature of the photon-counting detection and restricted by the quantum efficiency of the detector only. Spectral resolution given by the heterodyne technique and was defined by the linewidth and stability of the Local Oscillator (LO). Response bandwidth was found to coincide with the detector’s pulse width, which, in turn, could be controlled by the nanowire length. In addition, the system noise bandwidth was shown to be governed by the electronics/lab equipment, and the detector noise bandwidth is predicted to depend on its jitter. As have been demonstrated, a very small amount of the LO power (of the order of a few picowatts down to hundreds of femtowatts) was required for sufficient detection of the test signal, and eventual optimization could lead to further reduction of the LO power required, which would perfectly suit for the foreseen development of receiver matrices and the need for detection of ultra-low signals at a level of less-than-one-photon per second.

Thermodynamic fluctuations of the superconducting order parameter in NbN/NiCu and NbTiN/NiCu superconductor/ferromagnet (S/F) thin bilayers patterned to microbridges are investigated. Plain NbN and NbTiN films served as reference materials for the analyses. The samples were grown using dc-magnetron sputtering on chemically cleaned sapphire single-crystal substrates. After rapid thermal annealing at high temperatures, the superconducting films were coated with NiCu overlays, using co-sputtering. The positive magnetoresistance of the superconducting single layers is very small in the normal state but with a sharp upturn close to the superconducting transition, a familiar signature of superconducting fluctuations. The fluctuation-enhanced conductivity (paraconductivity) of the NbN and NbTiN single layer films is slightly larger than the prediction of the parameter-free Aslamazov-Larkin theory for order-parameter fluctuations in two-dimensional superconductors. The addition of a ferromagnetic top layer, however, changes the magnetotransport properties significantly. The S/F bilayers show a negative magnetoresistance up to almost room temperature, while the signature of fluctuations is similar to that in the plain films, demonstrating the relevance of both ferromagnetic and superconducting effects in the S/F bilayers. The paraconductivity is reduced below theoretical predictions, in particular in the NbTiN/NiCu bilayers. Such suppression of the fluctuation amplitude in S/F bilayers could be favorable to reduce dark counts in superconducting photon detectors and lead the way to enhance their performance.

The phenomenon of dark counts in nanostripes of different superconductor systems such as high-temperature superconducting YBa₂Cu₃O_7-x and superconductor/ferromagnet hybrids consisting of either NbN/NiCu or YBa₂Cu₃O_{7- x}/L_0.7Sr_0.3MnO₃ bilayers have been investigated. For NbN/NiCu the rate of dark-count transients have been reduced with respect to pure NbN nanostripes and the events were dominated by a single vortex entry from the edge of the stripe. In the case of nanostripes based on YBa₂Cu₃O_7-x, we have found that thermal activation of vortices was also, apparently, responsible for triggering dark-count signals.

Satellite Laser Ranging Systems typically operate on the second harmonic wavelength of a pulsed Nd:YAG laser at a wavelength of 532 nm. The absence of sufficiently sensitive photo-detectors with a reasonably large active area made it beneficial to trade the conversion loss of frequency doubling against the higher quantum efficiency of the detectors. Solid state silicon detectors in the near infra-red regime at λ = 1.064 µm also suffered from high thermal noise and slow signal rise times, which increased the scatter of the measurements by more than a factor of 3 over the operation at λ = 532 nm. With the availability of InGaAs/InP compound - Single Photon Avalanche Diodes the situation has changed considerably. Their quantum efficiency has reached 70% and the compound material of these diodes provides a response bandwidth, which is commensurate with high high speed detectors in the regime of 532 nm. We have investigated the properties of such a diode type Princeton Lightwave PGA-200-1064 for its suitability for SLR at the Nd:YAG fundamental wavelength with respect to the quantum efficiency and their timing properties. The results are presented in this paper. Furthermore, we provide remarks to on the performance of the diode compared to state of the art detectors, that operate at the Nd:YAG second harmonic wavelength. Finally, we give an estimate of the photoelectron statistics in satellite laser ranging for different operational parameters of the Wettzell Laser Ranging System.

We report on the implementation of a new infrared detection at the Grasse lunar laser ranging station and describe how infrared telemetry improves the situation. We present our first results on the lunar reflectors and show that infrared detection permits us to densify the observations and allows measurements during the new and the full moon periods. We also present the benefit obtained on the ranging of Global Navigation Satellite System (GNSS) satellites and on RadioAstron which have a very elliptic orbit.

Silicon Photomultipliers (SiPMs) are a well-recognized new generation of photon number resolving avalanche photodetectors. Many advantages – a high gain with an ultra-low excess noise of multiplication, multi-pixel architecture, relatively low operating voltage – make SiPMs very competitive in a growing number of applications. Challenging demands of LIDAR applications for a receiver having high sensitivity starting from single photons, superior time-offlight resolution, robustness including surviving at bright light flashes, solid-state compactness and more, are expected to be feasible for the SiPMs. Despite some known drawbacks, namely crosstalk, afterpulsing, dark noise, limited dynamic range, SiPMs are already considered as promising substitutes for conventional APDs and PMTs in LIDAR applications. However, these initial considerations are based on a rather simplified representation of the SiPM as a generic LIDAR receiver described by generic expressions. This study is focused on a comprehensive evaluation of a SiPM potential considering essential features of this new technology, which could affect applicability and performance of SiPMs as LIDAR receivers. Namely, an excess noise due to correlated processes of crosstalk and afterpulsing, are included into account utilizing the well-established framework of analytical probabilistic models. The analysis of SiPM performance in terms of a photon number and time resolution clarifies their competitiveness over conventional APD and PMT and anticipates the development of next SiPM generations.

The asymmetric nano-channel diode (ANCD) is the 2-dimensional electron gas (2DEG) semiconductor nanodevice that, unlike a conventional diode, relies on the device nanostructure and field-controlled transport in a ballistic nanometerwidth channel instead of barriers to develop its asymmetric, diode-like current-voltage (I-V) characteristics. We focus on ANCD optoelectronic properties, and demonstrate that the devices can act as very sensitive, single-photon-level, visiblelight photodetectors. Our test structures consist of 2-μm-long and ~230-nm-wide channels and were fabricated using electron-beam lithography on a GaAs/AlGaAs heterostructure with a 2DEG layer, followed by reactive ion etching. The I-V curves were collected by measuring the transport current under the voltage-source biasing condition, both in the dark and under light illumination. The experiments were conducted inside a cryostat, in a temperature range from 300 K to 78 K. As an optical excitation, we used a 800-nm-wavelength, generated by a commercial Ti:sapphire laser operated either at a quasi-continuous–wave mode or as a source of 100-fs-wide pulses. The impact of the light illumination was very clear, and at low temperatures we observed a significant photocurrent I_ph ~ 0.25 μA at temperature 78 K for the incident optical power as low as 1 nW, with a limited dark-current background. The magnitude of the device optical responsivity increased linearly with the decrease of the optical power, reaching for 1-nW optical excitation the value as high as ~400 A/W at room temperature and >800 A/W at 78K. The physics of the photoresponse gain mechanism in the ANCD arises from a vast disparity between the sub-picosecond transit time of photo-excited electrons travelling in the 2DEG nanochannel and the up to microsecond lifetime of photo-excited holes pushed towards the device substrate.

The results of computer simulation of heat distribution processes taking place after the absorption of single photons of 1-1000 eV energy in the three-layer detection pixel of the thermoelectric detector are presented. We considered different geometries of the detection pixel in which (La,Ce)B6 hexaboride is used as a thermoelectric layer and superconducting absorber and heat sink layers are of Nb, Pb and YBCO. The calculations are based on the heat conduction equation and are carried out by the matrix method for differential equations. It is shown, that by changing materials and sizes of the detection pixel’s layers it is possible to obtain transducers for the registration of photons within the given spectral range with the required energy resolution and count rate. It is demonstrated that such detector has a number of advantages, as well as improved characteristics, which give ground to consider the thermoelectric detector as a real alternative to superconducting nanowire single-photon detectors.

The paper presents studies results of a complex algorithm for the detection of highly orbital space objects. Before the implementation of the algorithm, a series of frames with weak tracks of space objects, which can be discrete, is recorded. The algorithm includes pre-processing, classical for astronomy, consistent filtering of each frame and its threshold processing, shear transformation, median filtering of the transformed series of frames, repeated threshold processing and detection decision making. Modeling of space objects weak tracks on of the night starry sky real frames obtained in the regime of a stationary telescope was carried out. It is shown that the permeability of an optoelectronic device has increased by almost 2^m.

The compact single photon lidar demonstrator dedicated for asteroid rendezvous missions has been designed and realized in our laboratory two years ago. The instrument provides crucial data on altitude and terrain profile for altitudes exceeding 5 km with a precision of less than 10 cm fulfilling the Rayleigh criterion. One of the calibration procedure of demonstrator is the positioning of receiver and transmitter optics related to detector and laser and the aligning of transmitter and receiver optical common paths. To improve this particular indoor calibration procedure the new simulator of single photon energy budget during short range operation has been created. The comparison of simulated and experimental data will be presented and discussed.

We are reporting on timing parameters of commonly used standard Small Form Factor Pluggable (SFP) laser modules using single photon counting method. Photon counting is a promising approach for laser time transfer via optical fiber communication hardware. The sub-picosecond precision and stability may be achieved. We have performed several experiments with the aim to measure main parameters of the modules, such as time delay precision, time stability and temperature stability, all being critical for optical time transfer applications. Two standard 16 and 10 Gbit/s at 850 nm SFP modules were examined. The ultimate precision of possible time transfer of 800 fs for averaging times of hours was achieved. The modules together with their driving circuits exhibited very good temperature stability. The temperature drift as low as 300±200 fs/K was measured. The achieved timing parameters will enable to use the standard SFP modules for a new method of two way time transfer where the time differences between two distant time scales are measured in parallel to data transfer on existing optical data links without any communication interference.