A new integrated SPAD detector is presented. The detector features increased collection efficiencies due to the use of an immersion lens system. Attainable collection improvement factors and application examples are presented.
We present initial evidence of the SOLUS potential for the multimodal non-invasive diagnosis of breast cancer by describing the correlation between optical and standard radiological data and analyzing a case study.
A machine learning classification algorithm is applied to the SOLUS database to discriminate benign and malignant breast lesions, based on absorption and composition properties retrieved through diffuse optical tomography. The Mann-Whitney test indicates oxy-hemoglobin (p-value = 0.0007) and lipids (0.0387) as the most significant constituents for lesion classification, but work is in progress for further analysis. Together with sensitivity (91%), specificity (75%) and the Area Under the ROC Curve (0.83), special metrics for imbalanced datasets (27% of malignant lesions) are applied to the machine learning outcome: balanced accuracy (83%) and Matthews Correlation Coefficient (0.65). The initial results underline the promising informative content of optical data.
InGaAs/InP Single-Photon Avalanche Diodes (SPADs) can achieve high photon detection efficiency (PDE) with a thick absorber, but at the expense of higher dark count rate (DCR). PDE and DCR also depend on the electric field inside the structure, which can be tailored in the design phase and influences the overall performance. We present the design and the experimental characterization of two different 10 μm-diameter InGaAs/InP SPADs. The first one is intended for applications where low noise is the key requirement: at 225 K and 5 V excess bias, it features 1 kcps DCR, 25% PDE at 1550 nm and a timing jitter of 100 ps (FWHM). The second device is an InGaAs/InP SPAD optimized for PDE-enhanced applications, having a PDE up to 50% at 1550 nm, with a DCR of 20 kcps and a timing jitter of 70 ps (FWHM) at 225 K. Alternatively, it features a PDE of 37% at 1550 nm, with a DCR of just 3 kcps and a timing jitter of 100 ps (FWHM). When combined with a custom integrated circuit we developed, both devices show an afterpulsing probability as low as few percent with a gating frequency of 1 MHz and hold-off time of few microseconds at 225 K, allowing to achieve a photon count rate towards 1 Mcps.
New time-correlated single photon counting (TCSPC) applications, like non-line-of-sight imaging, require a new generation of single photon avalanche diodes (SPAD) characterized by an instrument response function (IRF) having not only a narrow peak (< 100 ps FWHM) but also a very fast tail (~75 ps decay time). With such devices it is thus possible to detect two optical pulses as close as 200 ps in time, even if the second one is 2 orders of magnitude weaker than the first one. Such secondary peaks in the TCSPC histogram, can also be caused by reflections from internal optical surfaces of the optoelectronic assembly in which SPADs are mounted and thus are consequently undesirable. Options to mitigate these reflections or reduce the time of flight inside the assembly while not compromising photon detection efficiencies over a wide wavelength range are discussed.
A multimodal instrument for breast imaging was developed, combining ultrasound (morphology), shear wave elastography (stiffness), and time domain multiwavelength diffuse optical tomography (blood, water, lipid, collagen) to improve the non-invasive diagnosis of breast cancer.
To improve non-invasively the specificity in the diagnosis of breast cancer after a positive screening mammography or doubt/suspicious ultrasound examination, the SOLUS project developed a multimodal imaging system that combines: Bmode ultrasound (US) scans (to assess morphology), Color Doppler (to visualize vascularization), shear-wave elastography (to measure stiffness), and time domain multi-wavelength diffuse optical tomography (to estimate tissue composition in terms of oxy- and deoxy-hemoglobin, lipid, water, and collagen concentrations). The multimodal probe arranges 8 innovative photonic modules (optodes) around the US transducer, providing capability for optical tomographic reconstruction. For more accurate estimate of lesion composition, US-assessed morphological priors can be used to guide the optical reconstructions. Each optode comprises: i) 8 picosecond pulsed laser diodes with different wavelengths, covering a wide spectral range (635-1064 nm) for good probing of the different tissue constituents; ii) a large-area (variable, up to 8.6 mm2 ) fast-gated digital Silicon Photomultiplier; iii) the acquisition electronics to record the distribution of time-of-flight of the re-emitted photons. The optode is the basic element of the optical part of the system, but is also a stand-alone, ultra-compact (about 4 cm3 ) device for time domain multi-wavelength diffuse optics, with potential application in various fields.
Optical and mechanical aspects of packaging single photon avalanche diodes for different applications will be discussed. Particular emphasis will be given to fiber coupling at high photon detection efficiencies over a wide wavelength range.
The construction of a new prototype visible-light intensity interferometer for use in stellar astronomy is described. The instrument is located in New Haven, Connecticut, at Southern Connecticut State University, but key components of the system are also portable and have been taken to existing research-class telescopes to maximize sensitivity and baseline. The interferometer is currently a two-station instrument, but it is easily expandable to several stations for simultaneous measurement using multiple baselines. The design features single photon avalanche diode (SPAD) arrays, which increase the throughput and signal-to-noise ratio of the instrument. Predicted system performance and preliminary observations will be discussed.
With the recent progresses in quantum technologies, single photon sources have gained a primary relevance. Here we present a heralded single photon source characterized by an extremely low level of noise photons, realized by exploiting low-jitter electronics and detectors and fast custom-made electronics used to control an optical shutter (a LiNbO3 waveguide optical switch) at the output of the source. This single photon source showed a second-order autocorrelation function g(2)(0) = 0:005(7), and an Output Noise Factor (defined as the ratio of noise photons to total photons at the source output) of 0:25(1)%, among the best ever achieved.
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.
We present the design and performances of a radiation detector based on plastic scintillating fibers with doubleside readout by means of large-area Single Photon Avalanche Diodes (SPAD). This can be the basic step toward the realization of a large-area, cost-effective position sensitive detector to be employed in future space gammaray observatories. SPADs are silicon devices operated above the junction breakdown voltage (with the typical overvoltage of 5V), for which a single photon interacting in the active region is sufficient to trigger a selfsustainable avalanche discharge. SPADs can thus be used for the detection of very low light levels with a fast time response around 50ps FWHM for single photon detection, without spectroscopic capabilities. Large-area SPAD (500 μm in diameter) have been designed and fabricated at the CNR-IMM facility, with an intrinsic noise lower than 10kHz at -15°C, and are optically coupled to both ends of 3-meter long scintillating fibers, with the same diameter. Double-side readout is required to operate the devices in coincidence (10ns coincidence window), in order to reduce the rate of false detections to the level of 1Hz. The detectors have been tested with minimum ionizing particles at CERN PS demonstrating a detection efficiency larger than 90% and a moderate position resolution along the fiber due to the difference in time of arrival between the two photodetectors. Radiation hardness tests on SPADs have also been carried out, showing that large-area SPADs can be safely employed in low-inclination low Earth orbits.
In the last years many progresses have been made in the field of silicon Single Photon Avalanche Diodes (SPAD) thanks to the improvements both in device design and in fabrication technology. Particularly, the Dipartimento di Elettronica e Informazione of Politecnico di Milano and the CNR-IMM of Bologna have been in the forefront of this research activity by designing and fabricating a new device structure enabling the fabrication of SPADs with red enhanced photon detection efficiency. In this paper we present a compact photon counting and timing module that fills the gap between the high temporal resolution and the high detection efficiency systems. The module exploits Red-Enhanced SPAD technology to attain a Photon Detection Efficiency (PDE) as high as 37% at 800 nm (peak of 58% at 600 nm) while maintaining a temporal resolution of about 100 ps FWHM, even with light diffused across the whole active area. A thermo-electric cooling system guarantees a noise as low as few counts per second for a 50 μm diameter SPAD while a low threshold avalanche pick-up circuit assures a limited shift in the temporal response.
KEYWORDS: Sensors, 3D acquisition, Single photon, 3D metrology, Phase measurement, 3D image processing, Cameras, Prototyping, Distance measurement, Modulation
Three dimensions (3D) acquisition systems are driving applications in many research field. Nowadays 3D acquiring
systems are used in a lot of applications, such as cinema industry or in automotive (for active security systems).
Depending on the application, systems present different features, for example color sensitivity, bi-dimensional image
resolution, distance measurement accuracy and acquisition frame rate. The system we developed acquires 3D movie
using indirect Time of Flight (iTOF), starting from phase delay measurement of a sinusoidally modulated light. The
system acquires live movie with a frame rate up to 50frame/s in a range distance between 10 cm up to 7.5 m.
Until very recently Single Photon Avalanche Diodes (SPAD), which yield high detection efficiency in the visible spectrum, provided poor timing performance. This paper will review the current state of the SPAD technology and review new SPAD developments that provide: sub 50ps-timing resolution, are stable with count rate, and yield high detection efficiency. Examples will be provided; comparing timing resolution of PMT's and solid-state photon counting modules, effect of count rate on timing resolution, thus illustrating the stability of these newly developed SPAD's. In addition, the paper will review the basics of photon counting using SPAD's and illustrate how these SPAD's are used in Time-Correlated Single Photon Counting (TCSPC) and the results from these experiments.
In this paper we report the results relative to the design and fabrication of Single Photon Avalanche Detectors (SPAD) operating at low voltage in planar technology. These silicon sensors consist of pn junctions that are able to remain quiescent above the breakdown voltage until a photon is absorbed in the depletion volume. This event is detected through an avalanche current pulse.
Device design and critical issues in the technology are discussed.
Experimental test procedures are then described for dark-counting rate, afterpulsing probability, photon timing resolution, quantum detection efficiency. Through these experimental setups we have measured the electrical and optical performances of different SPAD technology generations. The results from these measurements indicate that in order to obtain low-noise detectors it is necessary to introduce a local gettering process and to realize the diode cathode through in situ doped polysilicon deposition. With such technology low noise detectors with dark counting rates at room temperature down to 10c/s for devices with 10mm diameter, down to 1kc/s for 50mm diameter have been obtained.
Noticeable results have been obtained also as far as time jitter and quantum detection efficiency are concerned.
This technology is suitable for monolithic integration of SPAD detectors and associated circuits. Small arrays have already been designed and fabricated. Preliminary results indicate that good dark count rate uniformity over the different array pixels has already been obtained.
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