Gas-phase Raman spectroscopy of non-reacting flows: comparison between free-space and cavity-based spontaneous Raman emission

We report on a comparison of free-space and cavity-enhanced Raman spectroscopy for gas-phase measurements of nitrogen and oxygen in ambient air. Real-time analysis capabilities and continuous Raman signals with low power diodes make the technique non-invasive, affordable, compact, and applicable for usage in non-reacting flows. We derive a comprehensive model for estimation of photon emission for both free-space and cavity-based signals and discuss trade-offs in how to organize the cavity geometry for maximum gain relative to free space. Measurements in both free and cavity configurations are compared to the expected signals, demonstrating the usefulness of the model in predicting amplification. The present results can serve as a quick guide on how to use low-power continuous wave lasers in a cavity setup to obtain enhanced laser-induced spontaneous Raman scattering.