Time resolving integrated circuit for Raman imaging

Abstract

Time-resolved single photon counting (TRSPC) Raman spectroscopy has shown to be a good alternative to traditional Raman spectroscopy implemented with a continuous-wave laser combined with a CCD-based sensor. By using short laser pulses in comparison to the fluorescence lifetime, TRSPC improves fluorescence rejection through time-resolved measurements. Additionally, the entire sensor could be designed on a single chip as the single photon avalanche diode (SPAD) itself can be implemented using CMOS technology.

TRSPC operates by recording the time of arrival of photons using a Time-to-Digital Converter (TDC) with resolution in tens of picoseconds. Since the entire setup works on time domain functionality, Raman imaging is achieved without sacrificing sensor area by converting image pixels into the time domain through photon delay from different sample pixels.

In this work, a proof-of-concept 1 X 8 TRSPC SPAD array suitable for 10-point imaging was developed to enhance the speed of Raman imaging. The circuit contains following main blocks a 1-to-10 pulse converter for separating photons from different pixels, a 1 x 8 SPAD array, and a TDC. Post-layout simulations showed that the sensor has a resolution of 68 ps and a dynamic range of approximately 2 ns. The current sensor can be scaled to an 8 x 256 SPAD array for 10-point Raman imaging with a rate of 300 kspectra/s.