Systematic Investigation and Suppression of Fluorescence in High-Sensitivity Cavity-Enhanced Raman Gas Sensing

TL;DR

This paper presents a cavity-enhanced Raman spectroscopy gas sensor with fluorescence suppression, achieving improved detection limits for trace gases by systematically reducing fluorescence background and optimizing optical setup.

Contribution

The study introduces a fluorescence-minimized cavity-enhanced Raman sensor with a CCD noise model and optical simulations, enabling detection of trace gases at ppm levels.

Findings

Fluorescence background was substantially reduced through systematic elimination of fluorescent optics.

The sensor achieved detection limits of 11 ppm for O2, 5 ppm for N2, and 3 ppm for H2 in 180 seconds.

The setup resolves weak Raman signals in ambient air, including CO2, O2, N2, and CH4.

Abstract

Raman spectroscopy enables broadband, multi-species gas analysis by providing access to an entire vibrational spectrum in a single measurement. However, the sensitivity of gas-phase Raman sensing is often limited by weak signals and fluorescence background from various optical elements that constrain the achievable signal-to-noise ratio (SNR) through signal-dependent noise contributions (e.g. shot noise). Here, we present a cavity-enhanced Raman spectroscopy (CERS) gas sensor employing a 500 mW, 532 nm continuous wave (CW) laser and a simple, non-resonant two-mirror multi-pass cavity (MPC) operated at ambient pressure and near the concentric condition, providing up to 45 internal reflections. To quantitatively capture the impact of fluorescence on performance, a CCD-specific noise model was developed that links fluorescenceinduced baseline levels to measurement noise. Complementary optical simulations were employed to assess the signal collection efficiency in the MPC. Through a systematic analysis of fluorescence sources, the background was reduced substantially by step-wise elimination of fluorescent optics. The fluorescence-minimized setup resolves weak Raman signatures in ambient-air spectra, including CO2 peaks, O2 and N2 overtones, and ambient CH4 (2 ppm). Calibration measurements for O2 (diluted in N2), N2 (in O2) and H2 (in N2) demonstrate detection limits of 11 ppm, 5 ppm and 3 ppm, respectively, with a 180 s measurement time. The results highlight fluorescence mitigation as a key design lever for robust, field-oriented CERS instrumentation for trace gas sensing.