On the Spectral Quality of Time-Resolved CMOS SPAD-Based Raman Spectroscopy With High Fluorescence Backgrounds

Jere Kekkonen,Jan Nissinen,Ilkka Nissinen,Tuomo Talala

doi:10.1109/jsen.2020.2966119

Abstract

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.

Highlights

R AMAN spectroscopy is a very powerful optical tool that can be used to nondestructively and quantitatively resolve the chemical and molecular composition of a sample
The reason why the filtered signal-to-distortion ratio (SDR) values can beat the signal-to-noise ratio (SNR) values with the lowest fluorescence-to-Raman values is that the filtering does not have a notable effect on the number of detected photons, and on the SNR values, but rather the filtering reduces both the distortion caused by the shot noise and timing skew of the sensor, which decreases the root mean square (RMS) distortion and increases the SDR values
The increase of the distortion caused by the timing skew as a function of overall signal intensity explains why the SDR curves decrease steeper than the SNR curves

Summary

Introduction

R AMAN spectroscopy is a very powerful optical tool that can be used to nondestructively and quantitatively resolve the chemical and molecular composition of a sample. The potential of Raman spectroscopy has been recognized in various fields such as agricultural, food, forensic, security, biomedical and pharmaceutical sciences, just to name a few [1]–[5]. The main advantages of Raman spectroscopy are that it needs minimal sample preparation and it can be Manuscript received December 2, 2019; revised January 9, 2020; accepted January 9, 2020. Date of publication January 13, 2020; date of current version April 3, 2020. The associate editor coordinating the review of this article and approving it for publication was Dr Cheng-Ta Chiang.

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Conclusion