A CMOS SPAD line sensor and timing skew compensation techniques for time-resolved Raman spectroscopy

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

L10, Linnanmaa campus

Topic of the dissertation

A CMOS SPAD line sensor and timing skew compensation techniques for time-resolved Raman spectroscopy

Doctoral candidate

Master of Science Tuomo Talala

Faculty and unit

University of Oulu Graduate School, Faculty of Information Technology and Electrical Engineering, Circuits and systems

Subject of study

Electrical engineering

Opponent

Professor Ari Paasio, Kovilta Ltd.

Custos

Professor Ilkka Nissinen, University of Oulu

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A CMOS SPAD line sensor and timing skew compensation techniques for time-resolved Raman spectroscopy

Time-resolved Raman spectroscopy is a technique in which a pulsed excitation source and a time-resolved sensor are used to measure Raman spectra. In contrast to a conventional continuous-wave Raman measurement, with a time-resolved measurement it is possible to separate Raman and fluorescence signals in time domain, to measure a fluorescence lifetime and to make time-of-flight based depth-resolved measurements.

The objective of this work was to improve the performance of CMOS SPAD based (complementary metal oxide single-photon avalanche diode) time-resolved Raman spectrometer, especially to minimize timing skew of a multichannel sensor, and to add some new functionalities to it. It was shown that spectral distortion caused by timing skew cannot be corrected well with compensation techniques used with conventional Raman spectrometers. As timing skew cannot be completely eliminated in sensor design either, new methods were developed for timing skew characterization and compensation.

In this work, a new 256-channel CMOS SPAD line sensor was designed and tested. A temporal resolution, dynamic range and a full width at half maximum of the temporal instrument response function of the sensor are 26–65 ps, 3.2–8.2 ns and 115 ps, respectively. Owing to parallel-connected time-to-digital converters, the achieved timing skew is lower (median 44 ps) than with any of the previously reported CMOS SPAD line sensors. With characterization and compensation techniques developed in this work, most of the spectral distortion caused by timing skew could be removed. Best reduction in spectral distortion, over 95%, was achieved using fine-tunable time-to-digital converters of the sensor designed in this work. The new sensor was equipped with an operating mode where three consecutive measurements at intervals of tens of nanoseconds were made for each excitation pulse. This operating mode was used for two new functionalities related to Raman measurements. Firstly, with a normal spectrometer setup, it was used for an accurate real-time dark count measurement that does not increase the total measurement time. Secondly, it was combined with a matching fibre-optic setup, and a multipoint time-resolved Raman spectrometer for measuring at two measurement points with a single excitation source and a single spectrometer was demonstrated.
Last updated: 21.11.2023