Sensitivity comparison of free-space and waveguide Raman for bulk sensing

The sensitivity advantage of waveguide-enhanced Raman spectroscopy (WERS) over free-space Raman, measured by the signal-to-noise ratio, is well established for thin molecular layer sensing, which traditionally relies on confocal Raman setups. However, for bulk liquid or gas samples, WERS must be benchmarked against nonconfocal Raman configurations. We use ray tracing to calculate the power collection efficiency of several model free-space systems, such as microscopes and probes, encompassing both single-objective and dual-lens systems. It is shown that considering only the focal volume of the source beam or the confocal volume of the microscope significantly underestimates the collected power from free-space Raman systems. We show that waveguide-based systems can still outperform high signal collection free-space systems in terms of both the signal collection efficiency and signal-to-noise ratio.