KEYWORDS: Sensors, Single photon detectors, Superconductors, Photons, Temperature metrology, Optical testing, Very large scale integration, Signal detection, Integrated circuits, Nanowires
Today Superconducting Nanowire Single-Photon Detectors (SNSPDs) are commonly used in different photon-starved applications, including testing and diagnostics of VLSI circuits. Detecting very faint signals in the near-infrared wavelength range requires not only good detection efficiency, but also very low Dark Count Rate (DCR) and jitter. For example, low noise is crucial to enable ultra-low voltage optical testing of integrated circuits. The effect of detector temperature and background thermal radiation on the noise of superconducting single-photon detectors made of NbN meanders is studied in this paper. It is shown that two different regimes can be identified in the DCR vs. bias current characteristics. At high bias, the dark count rate is dominated by the intrinsic noise of the detector, while at low bias current it is dominated by the detection of stray photons that get onto the SNSPD. Changing the detector temperature changes its switching current and only affects the high bias branch of the characteristics: a reduction of the DCR can be achieved by lowering the SNSPD base temperature. On the other hand, changing the temperature of the single-photon light source (e.g. the VLSI circuit under test) only affects the low bias regime: a lower target temperature leads to a smaller DCR.
KEYWORDS: Sensors, Signal to noise ratio, Switching, Signal attenuation, Electronics, Superconductors, Picosecond phenomena, Single photon detectors, Photons, Nanowires
Time-Resolved Emission (TRE) is a truly non-invasive technique based on the detection of intrinsic light emitted by
integrated circuits that is used for the detection of timing related faults from the backside of flip-chip VLSI circuits.
Single-photon detectors with extended sensitivity in the Near Infrared (NIR) are used to perform time-correlated
single-photon counting measurements and retrieve the temporal distribution of the emitted photons, thus identifying
gates switching events. The noise, efficiency and jitter performance of the detector are crucial to enable ultra-low voltage
waveform sensitivity. For this reason, cryogenically cooled Superconducting Nanowire Single-Photon Detectors
(SNSPDs) offer superior performance compared to state-of-the-art Single-Photon Avalanche Diodes (SPADs). In this
paper we will discuss how detector front-end electronics parameters, such as bias current, RF attenuation and comparator
threshold, can be tailored to optimize the measurement Signal-to-Noise Ratio (SNR), defined as the ratio between the
switching emission peak amplitude and the standard deviation of the noise in the time interval in which there are no
photons emitted from the circuit. For example, reducing the attenuation and the threshold of the comparator used to
detect switching events may lead to an improvement of the jitter, due to the better discrimination of the detector firing,
but also a higher sensitivity to external electric noise disturbances. Similarly, by increasing the bias current, both the
detection efficiency and the jitter improve, but the noise increases as well. For these reasons an optimization of the SNR
is necessary. For this work, TRE waveforms were acquired from a 32 nm Silicon On Insulator (SOI) chip operating
down to 0.4 V using different generations of SNSPD systems.
Presently time-resolved optical spectroscopy is applied with increasing success for non-invasive medical diagnostics mainly up to 1100 nm. We extended the investigation range beyond this limit, employing a supercontinuum fiber laser source and a Single-Photon Avalanche Diode in InGaAs/InP operated in gated mode. First in-vivo measurements were performed on the forearm and the breast of two healthy volunteers, reaching up to 1360 nm.
We present a photon-counting module based on InGaAs/InP SPAD (Single-Photon Avalanche Diode) for detecting
single photons up to 1.7 μm. The module exploits a novel architecture for generating and calibrating the gate width,
along with other functions (such as module supervision, counting and processing of detected photons, etc.). The gate
width, i.e. the time interval when the SPAD is ON, is user-programmable in the range from 500 ps to 1.5 μs, by means of
two different delay generation methods implemented with an FPGA (Field-Programmable Gate Array). In order to
compensate chip-to-chip delay variation, an auto-calibration circuit picks out a combination of delays in order to match
at best the selected gate width. The InGaAs/InP module accepts asynchronous and aperiodic signals and introduces very
low timing jitter. Moreover the photon counting module provides other new features like a microprocessor for system
supervision, a touch-screen for local user interface, and an Ethernet link for smart remote control. Thanks to the fullyprogrammable
and configurable architecture, the overall instrument provides high system flexibility and can easily
match all requirements set by many different applications requiring single photon-level sensitivity in the near infrared
with very low photon timing jitter.
Combined 2D imaging and 3D ranging sensors provide useful information for both long (some kms) and short (few tens of m) distance, in security applications. To this aim, we designed two different monolithic imagers in a 0.35 μm costeffective CMOS technology, based on Single Photon Avalanche Diodes (SPADs), for long-range time-of-flight (TOF)
and short-range phase-resolved depth ranging. The single pixel consists of a SPAD (30 μm diameter), a quenching
circuit, and a Time-to-Digital Converter (TDC) for TOF measurements or three up/down synched counters for phaseresolved depth assessments. Such smart pixels operate in two different modalities: single photon-counting for 2D “intensity” images; while either photon-timing or phase-resolved photon-counting for 3D “depth” images. In 2D
imaging, each pixel has a counter that accumulates the number of photons detected by the SPAD in the pixel, thus
providing single-photon level sensitivity and high (100 kframe/s) frame-rate. In the TOF 3D imager, each pixel measures
the photon arrival time with a 312 ps resolution, thanks to a two-stage TDC (with 6 bit coarse counter plus a 4 bit fine
interpolator), with a 320 ns full-scale range. The resulting spatial resolution is 9 cm within a 50 m range, centered at any user-selectable distance (e.g. 100 m – 5 km), with linearity of DNLrms=4.9% LSB and INLrms=11.7% LSB, and 175 ps
precision. In the phase-resolved 3D imager, the in-pixel electronics measures the phase difference between the
modulated light emitted by a laser and the back-reflected light, with both continuous-wave and pulsed-light modulation techniques.
The growing interest for fast, compact and cost-effective 3D ranging imagers for automotive applications has prompted to explore many different techniques for 3D imaging and to develop new system for this propose. CMOS imagers that exploit phase-resolved techniques provide accurate 3D ranging with no complex optics and are rugged and costeffective. Phase-resolved techniques indirectly measure the round-trip return of the light emitted by a laser and backscattered from a distant target, computing the phase delay between the modulated light and the detected signal. Singlephoton detectors, with their high sensitivity, allow to actively illuminate the scene with a low power excitation (less than 10W with diffused daylight illumination). We report on a 4x4 array of CMOS SPAD (Single Photon Avalanche Diodes) designed in a high-voltage 0.35 μm CMOS technology, for pulsed modulation, in which each pixel computes the phase difference between the laser and the reflected pulse. Each pixel comprises a high-performance 30 μm diameter SPAD, an analog quenching circuit, two 9 bit up-down counters and memories to store data during the readout. The first counter counts the photons detected by the SPAD in a time window synchronous with the laser pulse and integrates the whole echoed signal. The second counter accumulates the number of photon detected in a window shifted with respect to the laser pulse, and acquires only a portion of the reflected signal. The array is readout with a global shutter architecture, using a 100 MHz clock; the maximal frame rate is 3 Mframe/s.
We present new circuital solutions for operating InGaAs/InP SPADs at high speed with very fast avalanche quenching
time. A compact wide-band pulse generator (mounted close to the detector) is able to gate the SPAD at a repetition
frequency from 200 Hz up to 133 MHz. An adjustable amplitude gate-driver allows to trade-off between photon
detection efficiency and dark count rate, while a variable gate-width precisely selects the time interval during which the
detector is ON. A fast avalanche-quenching scheme, working on both SPAD's anode and cathode, is able to minimize
quenching action to less than 1 ns, thus effectively reducing afterpulsing through a decreased total charge flowing
through the junction. We integrated all such circuits into a compact detection module, together with a previouslyreported
differential read-out electronics for low time-jitter response. The performance of the overall module is good in
many different setting points, thus being able to satisfy a wide variety of applications.
Time-domain diffuse optical spectroscopy has become a powerful tool to study highly scattering media, mainly in the
fields of non-invasive medical diagnostics and quality assessment of food and pharmaceutical products. Up to now this
technique has been exploited mostly up to 1100 nm: we extend the spectral range by means of a continuously tunable
pulsed laser source at a high repetition rate and a custom InGaAs/InP Single-Photon Avalanche Diode operated in time-gated
mode, working up to 1700 nm. The characterization of the system is presented. As a first example of application,
we measured the absorption spectrum of collagen powder in the range 1100 - 1700 nm, which could prove useful for
breast density assessment.
Time domain diffuse optical spectroscopy provides the absorption and scattering properties of biological tissues
and diffusive materials. Few measurements are available at discrete wavelengths beyond 1100 nm, and just one
time-domain system continuously tuneable up to 1400 nm. We developed a time-domain system, based on a
continuously tuneable supercontinuum pulsed source, and a custom InGaAs/InP Single-Photon Avalanche Diode.
Operation was demonstrated in the 1100-1700 nm range with a spectral resolution of 15 nm, a temporal resolution
of 150 ps and a background of 6000 counts/s. A first example of application on the optical characterization of
collagen powder is given.
In order to acquire low-level optical signals with picosecond resolution, Single-Photon Avalanche Diodes (SPADs) are
exploited thanks to their extreme performance. For many demanding applications, there is a growing need to operate
such detectors with advanced instrumentation, specifically designed for efficiently exploiting the best performance in
terms of sensitivity, timing resolution, fast-gating capabilities, etc. To this purpose we designed, tested and employed an
ultra-fast pulse generator, a fast gated-counter and a wide-band delayer. The pulse generator is designed for gating
SPADs with fast transition times (less than 100 ps), when it is needed to avoid unwanted photons that either precede or
follow the useful signal. The gated counter acquires photons in well-defined time windows, programmable from 100 ps
up to 10 ns. Finally, a wide-band delayer provides programmable delays, ranging from 25 ps up to 6.4 ns in steps of 25
ps. Such a delayer can be used to synchronize signals in many different experimental setups.
InGaAs/InP Single-Photon Avalanche Diodes (SPADs) have good enough performance to be successfully employed in
many applications that demand to detect single photons in the 1 - 1.7 μm wavelength range. However, in order to fully
exploit such InGaAs/InP SPADs, it is mandatory to operate them in optimized working conditions by means of dedicated
electronics. We present the design and experimental characterization of a high-performance compact detection module
able to operate at best InGaAs/InP SPADs. The module contains a pulse generator for gating the detector, a front-end
circuit for avalanche sensing, a fast circuitry for detector quenching and resetting, a counting electronics, and some subcircuits
for signal conditioning. Experimental measurements prove the state-of-the-art performance and its great
flexibility to adapt it to the different applications.
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