Reduction of Fused-Silica-Fiber Raman Backgrounds in High-Temperature Fiber-Optic Raman Spectroscopy via the Measurement of Anti-Stokes Raman Spectra

Abstract

Measurements of laser Raman spectroscopy using optical fibers have recently become an active area of study. The technique requires minimum alignment of samples with respect to an input laser beam or collection optics, and the sample may be located some distance from the spectrometer in a hostile environment. Most fiber-optic probes employed for such spectroscopic measurements have been constructed by sealing the collecting fibers and one input fiber into a metal or glass protective tube with a special kind of epoxy cement. These materials in a probe impose difficulties in the collection of spectra in some hostile chemical environments because of chemical reactions of the epoxy resin with the surrounding environment. Recently we have reported the fabrication of an all-fused-silica fiber-optic probe that is useful for measuring Raman spectra of molten salt systems at high temperature. A drawback of fiber-optic remote Raman spectroscopy is the large background Raman signals generated from the optical fiber itself. Therefore, the major requirement of remote Raman spectroscopy is the reduction of the unwanted background signals to a level below that of the sample Raman signals. Myrick and Angel have reported the use of optical interference filters to filter out the background fiber signals. In that study, a bandpass filter was used at the tip of the excitation optical fiber to reject any fluorescence or unwanted background signals from the excitation fiber, which, at the same time, allows transmission of the large portion of the laser beam. Similar techniques have also been employed by Sharma and his co-workers in their recent remote fiber-optic studies. Usually, these optical filters cannot survive high-temperature conditions. Here we want to report a simple and yet efficient method to minimize the fiber background signals for high-temperature remote Raman spectroscopy. The performance of the method was characterized by the measurement of Raman spectra of magnesium tetrachloride ion in a high-temperature molten salt medium.