|RICCARDO SIMONE||Cycle: XXXIV |
Tutor: GERACI ANGELO
Advisor: TOSI ALBERTO Major Research topic
:Time-gated SPAD systems for visible and near-infrared photon countingAbstract:
The ability to measure very faint and fast light signals has proved to be crucial in large number of applications, from quantum technologies to medical/biological systems, from automotive to security and military fields, and also consumer products. Examples of such applications include: quantum cryptography, quantum computer, fiber-based Optical Time-Domain Reflectometry (OTDR), time-resolved optical spectroscopy, three-dimensional LIght Detection And Ranging (LIDAR), Non-Line Of Sight (NLOS) imaging. All these applications require imagers with single-photon sensitivity in order to either properly retrieve information from the low-light signals or to exploit quantum properties of single photons. Single-Photon Avalanche Diodes (SPADs) provide extremely accurate temporal resolution (picosecond range), high detection efficiency in the visible or Near-InfraRed (NIR, 800 nm to 1800 nm) spectral regions, Photon-Number Resolving capability (PNR), low Dark Count Rate (DCR), as well as very sharp OFF/ON transitions for fast gating operation. The capability of quickly modulating the SPAD bias voltage from below to above breakdown voltage is of utmost importance whenever a signal is expected to be acquired only during well-defined time-windows: keeping the SPAD OFF when no useful photon impinges prevents the device to be blind when a photon of interest is absorbed. This time-filtering technique is known as time-gating. This PhD research focuses on the design of time-gated SPAD systems, customized for different applications. A 16 x 16 SPAD array will be designed and used in order to speed up the acquisition process in state-of-art NLOS imaging setups, which rely on sharp time-filtering of the light backscattered from a relay wall. Given the photon-starved nature of NLOS imaging, only 16 Time-to-Digital-Converters (TDCs) will be integrated on chip, each one shared over multiple detectors in order to optimize area and resources usage. The row conversions (up to 6 Gbps) will be transmitted towards an FPGA, which will reconstruct the histograms of arrival times. Thanks to the high throughput of this system, NLOS scenes will be reconstructed at video rate and, possibly, with lower laser power, thus paving the way towards eye-safe NLOS systems. Another high-throughput fast-gated detection system will be investigated during this PhD studies: a high-count rate detector for photon counting in the NIR range will be based on InGaAs/InP SPADs with the aim of overcoming the detrimental effect of afterpulsing. Thanks to a very fast gating-signal (at a frequency beyond 1 GHz), the detector is turned OFF as soon as an avalanche occurs, thus reducing the number of avalanche carriers and, eventually, strongly suppressing the afterpulsing probability. As a result, high count rates could be achieved in applications like quantum cryptography and LIDAR.