Improvement to the spectral quality of Rama images of human teeth were achieved with a time-resolved CMOS SPAD-based Raman spectrometer.
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 (singlephoton 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. Index Terms-Depth-resolving Raman spectrometer, depth analysis, time-correlated single photon counting (TCSPC), CMOS single-photon avalanche diode (SPAD).
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 fluorescenceto-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.
Raman analysis of rock samples containing rare earth elements (REEs) is challenging due to the strong fluorescence, which may mask the weaker Raman signal. In this research, time‐gated (TG) Raman has been applied to the construction of the mineral distribution map from REE‐bearing rock. With TG Raman, material is excited with a short subnanosecond laser pulse, and the Raman signal is collected within a picosecond‐scale time window prior to the formation of a strong fluorescent signal by means of single‐photon avalanche diode array. This allows signal readout with a significantly reduced fluorescence background. TG Raman maps are used to reveal the location of valuable minerals and are compared with the elemental distribution given by laser‐induced breakdown spectroscopy. The analysis was carried out from a REE‐bearing rock, nepheline syenite sample from the Norra Kärr deposit, where REEs are mainly hosted in eudialyte and catapleiite. The combination of these two complimentary laser spectroscopic methods offers valuable elemental and mineralogical information about rocks.
Remote Raman spectroscopy is widely used to detect minerals, explosives and air pollution, for example. One of its main problems, however, is background radiation that is caused by ambient light and sample fluorescence. We present here, to the best of our knowledge, the first time a distance-resolving Raman radar device that is based on an adjustable, time-correlated complementary metal-oxide-semiconductor (CMOS) single-photon avalanche diode line sensor which can measure the location of the target sample simultaneously with the normal stand-off spectrometer operation and suppress the background radiation dramatically by means of sub-nanosecond time gating. A distance resolution of 3.75 cm could be verified simultaneously during normal spectrometer operation and Raman spectra of titanium dioxide were distinguished by this system at distances of 250 cm and 100 cm with illumination intensities of the background of 250 lux and 7600 lux, respectively. In addition, the major Raman peaks of olive oil, which has a fluorescence-to-Raman signal ratio of 33 and a fluorescence lifetime of 2.5 ns, were distinguished at a distance of 30 cm with a 250 lux background illumination intensity. We believe that this kind of time-correlated CMOS single-photon avalanche diode sensor could pave the way for new compact distance-resolving Raman radars for application where distance information within a range of several metres is needed at the same time as a Raman spectrum.
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