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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. [ABSTRACT FROM AUTHOR]

Raman spectroscopy has proved to have potential in deep surface analytical applications. We present here, to the best of our knowledge, the first time depth analysis of semi-transparent media by a depth-resolving Raman spectrometer based on an adjustable time-correlated CMOS SPAD (single-photon avalanche diode) line sensor that can measure the depth of target samples embedded in a centimeter-scale semi-transparent medium simultaneously with a normal Raman depth profiling operation and suppress the fluorescence background by means of adjustable picosecond time gating. The variability of the depth derivation was measured to be ± 0.43 cm at depths ranging from 2 to 9 cm. In addition, the advantages of the adjustable picosecond time gating in terms of depth derivation and fluorescence background suppression performance were shown by comparing gate widths ranging from 100 ps to 13 ns. We believe that the technology concerned could pave the way for a new kind of compact, practical depth-resolving Raman spectrometer for deep subsurface analytical applications. [ABSTRACT FROM AUTHOR]