On the Spectral Quality of Time-Resolved CMOS SPAD-Based Raman Spectroscopy with High Fluorescence Backgrounds
The fluorescence background in Raman spectroscopy can be effectively suppressed by using pulsed lasers and time-gated detectors. A recent solution to reduce the high complexity and bulkiness of the time-gated systems is to implement the detector by utilizing time-resolved single-photon avalanche diodes (SPADs) fabricated in complementary-metal-oxide-semiconductor (CMOS) technology. In this study, we investigate the effects of fluorescence-to-Raman ratio, recording time and excitation intensity on the quality of Raman spectra measured by using one of the furthest developed fluorescence-suppressed Raman spectrometers based on a time-resolved CMOS SPAD line sensor. The objectives were to provide information on the significance of the different causes behind the distortion of the measured Raman spectra with various measurement conditions and to provide general information on the possibilities to exploit the high-intensity non-stationary pulsed laser excitation to gain additional improvement on the spectral quality due to laser-induced fluorescence saturation. It was shown that the distortion of the spectra with samples having short fluorescence lifetimes (~2 ns) and high fluorescence-to-Raman ratios, i.e. with challenging samples, is dominated by the timing skew of the sensor instead of the shot noise caused by the detected events. In addition, the actual reason for the observed improvement in the spectral quality as a function of excitation intensity was discovered not to be the conventionally thought increased number of detected photons but rather the laser-induced fluorescence saturation. At best, 26% improvement to the signal-to-noise ratio was observed due to fluorescence saturation.