MIR-evanescent-field fiber sensors

A compact, tunable and single-mode laser for the mid-infrared (MIR) spectral range is developed by difference frequency generation (DFG) in AgGaS2 and two "off-the-shelf" diode lasers. The MIR laser light is coupled into a silverhalide fiber and at the end of the fiber an infrared detector is used to record the transmitted MIR laser light. If the index of refraction of the fiber material is higher than the one of the surrounding medium the light is guided through the fiber due to total reflection. There are two loss mechanisms that will attenuate the laser intensity when passing through the fiber: (1) The frustrated total reflection (FTR) and (2) the attenuated total reflection (ATR). The FTR is related to a change of the index of refraction while the ATR is related to a change of the absorption coefficient. When tuning the MIR laser over an absorption line of a molecule that is outside of the fiber both, FTR and ATR, contribute to the measured spectral line profile that is recorded by the infrared detector. Similar to the direct laser absorption spectroscopy the recorded line profiles in the case of the evanescent-field spectroscopy can be used for estimating concentrations of molecules. A practical application of the evanescent field fiber sensor is shown as H2S is measured online and insitu at the volcano "Solfatara" in Italy.