Integrated Multichannel Electronics for High Performance Time Correlated Single Photon Counting
Authors: Acconcia Giulia , Crotti Matteo , Rech Ivan , Ghioni Massimo 
Published: February 2020

The analysis of extremely weak, ultrafast and periodical light signals is increasingly widespread in a large number of fields, from chemistry and biology to telemetry and communication, and in a great variety of applications, from quantum cryptography to fluorescence imaging.
Speaking of fluorescent lifetime analysis, for example, the detection of very faint and fast optical signals is fundamental to investigate the complex interactions between molecules that play a key role in many crucial issues such as the origin and growth mechanisms of tumors.
In this domain the Time-Correlated Single Photon Counting (TCSPC) technique is very effective to reconstruct the light waveform when both extremely high time resolution and linearity are required.
TCSPC is based on the detection of single photons and on the record of their arrival times: if the probability of detecting a photon during one excitation cycle is far less than one, the histogram of photon arrivals per time bin exactly corresponds to the temporal trend of the light signal.
In order to properly reconstruct the light waveform a time measurement block featuring very high performance is necessary: this circuit has to be able to measure time intervals of few hundreds of nanoseconds with picosecond time resolution and a differential non-linearity much lower than few percents of the time bin width.
Moreover a growing number of techniques do now require to collect data from multiple channels at the same time. Förster Resonance Energy Transfer (FRET), Fluorescence Diffuse Optical Tomography (FDOT) and single-molecule spectroscopy, for example, can benefit a lot of a system capable of providing simultaneous analysis in temporal or spectral range. Indeed, concurrency not only reduces the measurement time, revealing dynamic phenomena even on very short timescales but it also opens the way to spectrally resolved measurements that analyze different events occurring simultaneously, like Fluorescence Cross-Correlation Spectroscopy (FCCS).
Nevertheless the development of a truly parallel acquisition system featuring both high performance and a high number of independent channels is still an open challenge: to achieve this goal the design of a time measurement circuit capable of acquiring data from several channels in parallel while providing high performance on each channel is of the utmost importance.
In this work a fully integrated array of 16 time to amplitude converters (TAC) designed in 0.35µm SiGe technology is presented.
The first simulations show very high performance on each channel with a FWHM time resolution lower than 19ps and a differential non linearity lower than 0.4% peak-to-peak of the LSB. In order to reduce the overall area of the chip, pairs of converter stages share all the conversion chain through a 2 input multiplexer. The simulated dead time on the entire full scale range is 191ns, corresponding to a maximum conversion frequency of 5.24MHz per channel.
The obtained results on the designed 16 channel TAC array are extremely good and, if the performance are verified by experimental measurements, this array will be one of the fundamental blocks of a fully parallel high-performance TCSPC acquisition system. Based on high efficiency fully custom single photon detectors (SPAD) and on a front-end electronics specifically designed to properly work with these sensors, the final system will be the upgrade of a few channels high performance acquisition chain already developed in the same technology.