Single Burst Depth-Resolving Raman Spectrometer Based on a SPAD Array with an On-Chip TDC to Analyse Heterogenous Liquid Samples
A single burst depth-resolving Raman spectrometer based on a CMOS single-photon avalanche diode (SPAD) array with an on-chip 256-channel time-to-digital converter (TDC) has been developed to analyze heterogenous liquid sample without the need for any additional off-chip delay circuits. The depth derivation of Raman sensitive layers on top of and under a water phase is based on measuring the time-of-arrivals of Raman scattered photons generated with short laser pulses (~150 ps) by utilizing the 256 TDCs at every 256 spectral points. Raman spectra of different layers is derived by postprocessing the time domain histograms with the resolution of 200 ps corresponding to 23 mm depth in water. The minimum depth difference between two layers that could be independently identified was 65 mm this being limited by the full-width value of the instrument response function of the whole system (570 ps). The depth derivation error was measured to be less than 19 mm. This technology could enable a compact and fast depth-resolving Raman spectrometer to be used in potential applications within the fields of food, pharmaceuticals and beverage industries, for example.