Gas phase Raman spectroscopy using hollow waveguides

The detection and characterization of molecular gases in a given sample is a relatively difficult challenge. Usually, this task is relegated to expensive and time consuming processes like mass spectrometry and gas chromatography. Furthermore, numerous industrial applications require such gas-phase analysis for pollution and process control. In particular, the operation of large natural gas-fired turbine generators requires real-time analysis of constituent hydrocarbon concentration in order to provide energy content information about the gaseous fuel, and subsequently, inputs for controlling fuel/air ratio and burner temperature. Herein, a novel technique for studying gaseous samples is presented which uses a new collection method for Raman spectroscopy. In our technique, gasses are introduced inside the light-guiding core of a hollow waveguide. Either lengths of hollow-core photonic-bandgap fiber or internally reflective capillary waveguides are used to both contain sample gases and collect Raman photons. The optical confinement characteristics of these types of hollow-waveguides allow a high power-density laser beam to propagate a long distance along with the low-volume gaseous sample. We have shown analytically that the Raman signal strength (power) collected using our gas cells can be hundreds of times larger than that which can be obtained in free-space. Along with this improvement in collected Raman power comes shorter minimum interrogation times and higher sensitivities to trace gasses. In general, the technique paves the way for the construction of a gas Raman spectrometer with low-cost components and high-accuracy