Optimizing the optics for evanescent wave analysis with laser diodes (EWALD) for monitoring chlorinated hydrocarbons in water

Fiber Evanescent Field Analysis (FEFA) is a novel promising sensing technique for on-line and in situ analysis of hydrocarbons in water. With a conventional IR light source and FTIR spectroscopy it allows multicomponent analysis, while using MIR-tunable diode lasers results in more sensitive and faster single component analysis. Compared to common attenuated total reflection elements, silver halide (AgCl/Br) fibers offer more convenient application for remote sensing and field measurements because the fibers can be used for both, guidance of MIR radiation to and from the sensing part, and the sensing part itself At present these fibers are multimode. The sensor response can be expected to depend strongly on the mode distribution in the fiber. We hence performed a model calculation that allows to compare the FEFA absorption and the intrinsic fiber losses for given mode distributions and dependent on the optical parameters such as the coupling conditions and the fiber design. In order to link theoretical results to experimental data, the theory is based on a internal mode distribution derived from far field fiber emission data. We present far field data and the resulting internal distributions.