Fiber-optic evanescent-field laser sensor for in-situ gas diagnostics

A compact, rugged and portable fiber-optic evanescent-field laser sensor is developed for the detection of gaseous species in harsh environments such as volcano fumaroles or industrial combustion of glass furnaces. The sensor consists of an optical multi-mode fused silica fiber with jacket and cladding removed and the bare fiber core in direct contact with the surrounding molecules. The beam of a single-mode DFB diode laser with an emission wavelength centered at 1.5705 microm is coupled into the fiber. At the other end of the fiber an infrared detector is used to record the transmitted infrared laser light intensity. Due to the frustrated total reflection (FTR) and the attenuated total reflection (ATR) the laser intensity is attenuated when passing through the fiber. The FTR is related to a change of the index of refraction while the latter one is related to a change of the absorption coefficient. While tuning the DFB laser wavelength across absorption lines of molecules surrounding the fiber a spectral intensity profile is measured. Voigt functions are fitted to the recorded intensity profiles to estimate relative molecule concentrations. In this paper results from first field measurements at the volcano site 'Solfatara' in Italy are reported that use such a sensor device for simultaneous detection of H2S, CO2 and H2O directly in the gas stream of a volcano fumarole.