Evanescent field fiber optic probe for process analysis

We have demonstrated the use of an optical fiber as a long path internal reflection element in the visible and near-infrared spectral regions. Leaving the polymeric optical cladding intact on the fiber allows it to be coiled into a compact probe geometry and at the same time, may prevent fouling at the surface of the element. The siloxane cladding is penetrated by nonpolar solvents providing a level of selectivity for the probe. The short effective pathlength attainable with the probe makes it ideal for strongly absorbing media. The low-refractive index of the polymer allows operation in solvents with RI's greater than the fiber core which would normally result in the loss of wave guiding. The possibilities become apparent for designing probes for a variety of applications. By choosing a core material of higher RI than fused silica, a range of polymers with desirable properties such as selective permeability or low surface energies could be employed. Layers of non-permeable coatings which are thin relative to the evanescent field penetration and are intimately bound to the core also present numerous possibilities.