ADVANCED TECHNIQUES FOR GAS-PHASE RAMAN SPECTROSCOPY

The small cross-sections associated with Raman scattering generally hinders the application of Raman spectroscopy to low density gas analysis. The focus of this work is the development of tools and techniques to enhance collection of Raman-scattered light from gaseous samples contained in hollow-core waveguides. The developments are motivated by a need for accurate, real-time composition analysis of natural gas for clean and efficient turbine operation. Methods to increase the Raman signal and to reduce system noise sources are explored. Silver-coated silica waveguides currently used are covered with a highly fluorescent protective plastic coating which is responsible for most of the system noise. One noise reduction method explored is the replacement of these guides by polished, pure or sterling silver tubes. Work directed toward increasing Raman signals began with an analysis of the modal properties of the hollow waveguides used to contain the Raman medium. This work enabled the optimization of Raman power output and the design of the system’s collection optics. Other efforts include development of a new statistical algorithm that applies principal component analysis to extract the gas composition from a collected Raman spectrum. A novel apparatus replaced the spectrometer normally employed in a Raman sensor system by tunable optical filters in a derivative of wavelength modulation spectroscopy