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Optical efficiency in metal-lined capillary waveguide Raman sensors

Researchers have long sought to improve collection efficiencies in scattered-light sensing applications. Herein, we demonstrate efficient collection of Raman scattered light from gaseous samples. This enables the accurate, real-time, simultaneous measurement of otherwise difficult to distinguish molecular gasses or hydrocarbons. Hollow capillary waveguides, lined with a metal and dielectric over-coating, have often been used to deliver IR laser light to a target. We show that these waveguides can be used as both a sample holder for Raman gasses and as a laser-pumped optical cell which can collect Raman scattered light from these gasses. We extend existing low mode-order capillary waveguide analysis to treat higher order modes. This extension allows a robust computer simulation to accurately predict the spontaneous Raman scattering power that can be collected by the waveguide. We verify our new theoretical models with experimental measurements of Raman signals from a nitrogen filled waveguide. We demonstrate a cutback experiment which verifies our new theoretical predictions of the variation of scattering collection efficiency with guide dimensions. The prediction accuracy of our simulations allows us to design spectrometers and detectors to maximize Raman-light throughput in a high-sensitivity gas detection system.