Ambient Hydrocarbon Detection with an Ultra-Low-Loss Cavity Raman Analyzer.

The detection of ambient outdoor trace hydrocarbons was investigated with a multipass Raman analyzer. It relies on a multimode blue laser diode receiving optical feedback from a retroreflecting multipass optical cavity, effectively creating an external cavity diode laser within which spontaneous Raman scattering enhancement occurs. When implemented with ultra-low-loss mirrors, a more than 20-fold increase in signal-to-background ratio was obtained, enabling proximity detection of trace motor vehicle exhaust gases such as H, CO, NO, CH, CH, CH, and CH. In a 10-min-long measurement at double atmospheric pressure, the limits of detection obtained were near or below 100 ppb for most analytes.