Quantum photonic integrated circuits (PICs) exploit the virtually null photon-photon interaction to realize systems that are robust to external disturbance. While this resilience is particularly interesting for a development towards room-temperature systems, many experiments rely on superconducting nanowires that need cryogenic temperatures to operate. For photons in the near infrared spectral region, single photon avalanche diodes (SPADs) could be used as a room temperature alternative.
We show a novel method of PIC-detector coupling that allows for the monolithic fabrication of substrate-integrated photodiodes and a silicon nitride PIC on the same chip. With the use of an engineered wet-etching process, we shape the bottom cladding of the photonic layer into a basin with shallow wedge borders on top of the region of the detectors. In this way, the waveguides are gently laid on top of the detectors, allowing for a strong waveguide-detector optical coupling. We show experimental results of the first PIC-diode coupling with a total efficiency exceeding 40%, and the first promising results concerning the coupling with SPADs paving the way for on-chip, room-temperature, single photon detection.
Time domain diffuse optics (TD-DO) relies on the injection of ps laser pulses and on the collection of the arrival times of scattered photons. To reach the ultimate limits of the technique (allowing to investigate even structures at depth <5 cm), a large area detector is needed. To this extent, we realized and present a new silicon photomultiplier featuring a 1 cm2 area. To the best of our knowledge, it represents the largest detector ever proposed for TD-DO and shows a light harvesting capability which is more than 1 decade larger than the state-of-the-art technology system. To assess its suitability for TDDO measurements, we tested the detector with several procedures from shared protocols (BIP, nEUROPt and MEDPHOT). However, the light harvesting capability of a detector with large area can be proficiently exploited only if coupled to timing electronics working in sustained count-rate CR (i.e., well above the single photon statistics). For this reason, we study the possibility to work in a regime where (even more than) one photon per laser pulse is detected (i.e., more than 100% laser repetition rate) exploiting in-silico technology. The results show that the possibility to use sustained count-rate represents a dramatic improvement in the number of photons detected with respect to current approaches (where count-rate of 1-5% of the laser repetition rate are used) without significant losses in the measurement accuracy. This represents a new horizon for TD-DO measurements, opening the way to new applications (e.g., optical investigation of the lung or monitoring of fast dynamics never studied before).
KEYWORDS: Sensors, Optical properties, In vivo imaging, Diffuse optical imaging, Signal detection, Light harvesting, Absorption, Silicon, Radiography, Radio optics
We present the largest detectors for time-domain diffuse optics, showing superior performances in depth penetration and light-harvesting capability. In-vivo measurements demonstrate their potentialities for futuristic disruptive applications such as optical radiography.
KEYWORDS: Photons, RGB color model, Resistors, Signal detection, Quantum efficiency, Single photon detectors, Silicon photomultipliers, Avalanche photodiodes, Single photon, Photodetectors, Physics, Spectroscopy, High dynamic range imaging, Sensors, Photon counting
Silicon photomultipliers (SiPMs) are arrays of many single-photon avalanche diodes (SPADs), all connected in parallel. Each SPAD is sensitive to single photons and the SiPM gives an output proportional to the number of detected photons. These sensors are becoming more and more popular in different applications, from high-energy physics to spectroscopy, and they have been significantly improved over last years, decreasing the noise, increasing the cell fill-factor (FF) and thus achieving very high photon-detection efficiency (PDE). In FBK (Trento, Italy), we developed new SiPM technologies with high-density (HD) and, more recently, ultra-high-density (UHD) of cells (i.e. density of SPADs). These technologies employ deep-trenches between cells, for electrical and optical isolation. As an extreme case the smallest-cell, SiPM, i.e. with 5μm cell pitch, has about 40000 SPADs per squared millimeter. Such small SPAD dimensions gives a significantly high dynamic range to the SiPM. These small-cells SiPM have a lower correlated noise (including lower afterpulsing probability) and a faster recharge time (in the order of few nanoseconds), and they also preserve a very good detection efficiency (despite the small SPAD dimension).
Recently there has been considerable effort to develop photon-counting detectors for the near-infrared wavelength range
(up to 1700 nm). However, a practical detector, which can be employed in many real applications and has both high
count rates and low noise, is still not available. Here we show a novel technique to operate InGaAs/InP single-photon
avalanche diodes (SPADs) in a free-running equivalent mode at high count rate. The photodetector is enabled with a
915 MHz sinusoidal gate signal: by keeping the SPAD gate signal unlocked from the synchronization reference of the
optical waveform to be reconstructed, a free-running equivalent mode is obtained, that we called gate-free. An extensive
experimental characterization has shown how the SPAD performance improves compared with a classical square-wave
gating scheme, especially for non-periodic high-throughput applications. The resulting single-photon detector achieves a
maximum count rate of 100 Mcount/s, an afterpulsing probability below 0.3 %, a photon detection efficiency of 3 % at
1550 nm, a temporal resolution of 150 ps (Full-Width Half Maximum) and a dark count rate below 2000 count/s (the
SPAD is cooled at 220 K).
We designed, fabricated and tested a new planar InGaAs/InP Single-Photon Avalanche Diode (SPAD). By optimizing
design and fabrication processes, we obtained low afterpulsing and very good timing jitter, with very fast tail. The
detector has a separate absorption, charge and multiplication structure, with double p-type Zn diffusion into n-type InP
for defining the p-n high-field avalanching junction. The SPAD can be operated at temperatures achievable with thermoelectric
coolers mounted in compact packages (like TO-8). When operated in gated mode with 5 V excess bias, the 25
μm active area diameter InGaAs/InP SPAD reaches good performance at 225 K: i) photon detection efficiency of 40% at
1 μm and 25% at 1.55 μm; ii) dark count rate below 100 kcps (counts per second); iii) low afterpulsing allowing to set a
hold-off time as short as 1 μs, corresponding to 1 Mcps; iv) timing jitter less than 90 ps (full width at half maximum) and
time constant of decaying tail of just 30 ps.
Overall this new planar InGaAs/InP SPAD can be exploited in many near-infrared (up to 1.7 μm) applications where low
light, wide dynamic range waveforms have to be acquired, e.g. in Time-Correlated Single-Photon Counting (TCSPC)
measurements or Time-of-Flight LIDAR applications for eye-safe 3D ranging.
InGaAs/InP SPADs are solid-state devices able to detect near-infrared single photons up to 1700 nm. The pn junction is
defined by Zn diffusions in a lightly n-doped InP layer. If a simple Zn diffusion were employed, it would suffer edge
effects; therefore a double diffusion is used in order to smooth the electric field at the periphery of the active area.
However, since most of the main parameters of the SPAD depend on the electric field profile, it is of outmost importance
to properly evaluate the electric field, both in the active area and at its periphery. Currently-available programs for 2D
simulations require heavy and long computations and are not tailored for SPAD performance assessment, thus often 1D
custom simulation is performed for qualitative evaluation of “detection” characteristics.
We present two 1D and 2D device simulators designed for InGaAs/InP SPAD detectors and the models we implemented
therein. We compare the differences in the results between the 1D and 2D approaches in terms of electric field profile,
breakdown voltage, trigger efficiency and dark count rates. The 1D simulator overestimates breakdown voltage by few
percent, while the 2D simulator matches the measured values. We show how trigger efficiency is not constant in the
device and how the high electric field near the edges contributes to increase the dark count rate due to tunneling effects.
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.
We present the characterization results on a recent generation of InGaAs/InP Single-Photon Avalanche Diodes (SPADs)
operating up to 1700 nm. The improved performance makes them very promising for many NIR single-photon counting
applications since they show low dark count rate, good photon detection efficiency and quite low timing jitter.
First we characterized an important drawback of InGaAs/InP SPAD, namely afterpulsing: traps in the InP high-field
region capture carriers during the avalanche current flow and release them with delay, thus triggering another avalanche
and generating additional noise. Using the double pulse method, we measured the afterpulsing probability as a function
of time delay from the avalanche triggering. We carried out measurements at different temperatures and at different
excess bias in order to find the best operating conditions. Moreover, we biased the detector at different voltage levels
during the OFF period, so as to change the electric field during the de-trapping period in order to study how it affects the
carrier release. Then we characterized SPAD timing jitter, that leads to the time spread between photon absorption and
avalanche detection. We measured timing jitter with focused and un-focused light in order to determine the effects of
light absorption position on jitter.
KEYWORDS: Avalanche photodiodes, Performance modeling, Temperature metrology, Signal to noise ratio, Solids, Interference (communication), Single photon detectors, Signal detection, Silicon, Absorption
The afterpulsing noise in Single-Photon Avalanche Diodes (SPADs) is modeled and investigated in order to evaluate its
impact on SPAD performance, in terms of maximum count rate, signal-to-noise ratio, etc. From measurements fitting,
we identified three/four types of defects that we then used to simulate the behavior of the SPAD when operated in
different conditions. We show how the presented modeling is a valuable tool for the estimation of the performance of
different SPADs and the identification of optimal operating conditions, in terms of temperature, voltage bias, gate width,
gate repetition frequency, quenching time, etc.
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.
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.
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