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

Abstract

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 2 , CO, NO, CH 4 , C 2 H 2 , C 2 H 4 , and C 2 H 6 . In a 10-minlong measurement at double atmospheric pressure, the limits of detection obtained were near or below 100 ppb for most analytes.

Trace Analysis of C4F7N Insulating Gas Mixtures by Spontaneous Raman Spectroscopy and Gas Chromatography

“…It consists of a multimode blue laser diode (Nichia NUBM44) the emission from which is feedback-coupled via a volume Bragg grating to a retro-reflective multipass cavity. The novel optical characteristics of the setup, in particular a superlinear scaling of the SRS intensity with the number of cavity passes, are explained in prior work . The cavity is located within a sealed chamber into which gases were introduced.…”

“…The novel optical characteristics of the setup, in particular a superlinear scaling of the SRS intensity with the number of cavity passes, are explained in prior work. 65 The cavity is located within a sealed chamber into which gases were introduced. Initially, the chamber was depressurized to 0.03 MPa to minimize any remaining background gas.…”

“…However, numerous recent efforts into the development of enhancement techniques by way of optical capillaries, − fibers, − microcavities, and conventional resonator cavities − that increase the interaction strength and/or the SRS light collection efficiency have dramatically improved the prospects of SRS for trace gas sensing. Among these enhancement strategies, the utilization of a nonresonant multipass cavity offers particular robustness. − This technique has demonstrated trace sensitivity well into the parts-per-billion range, , while requiring minimal power consumption and no active optical resonance stabilization.…”

“…Among these enhancement strategies, the utilization of a nonresonant multipass cavity offers particular robustness. 60 − 66 This technique has demonstrated trace sensitivity well into the parts-per-billion range, 65 , 66 while requiring minimal power consumption and no active optical resonance stabilization.…”

Narrow-Linewidth Pr:YLF Laser for High-Resolution Raman Trace Gas Spectroscopy

“…To circumvent this limitation, numerous approaches to SRS enhancement have been researched and are continually being improved. Approaches based on cavities [9][10][11][12][13][14][15][16][17][18][19] or on waveguides [20][21][22][23][24][25] have demonstrated the ability to routinely detect at parts-per-million (ppm) concentrations. For example, in cavity-enhanced Raman scattering (CERS), high-power green laser light obtained via frequency doubling can be introduced into an external high-finesse optical cavity to create circulating power of 1 kW [26].…”