The application of a light guiding flexible tubular waveguide in evanescent wave absorption optical sensing

A light guiding flexible fused silica (FFS) capillary has been used in designing evanescent wave (EW) absorption based chemical sensors. The FFS capillary is similar to a conventional silica optical fiber in that it can guide light in the wavelength region from UV to near infrared, but different from a conventional optical fiber in that it is a tubular waveguide. The inner surface of the FFS capillary is fused silica, which was coated with a reagent doped polymer in order to design an optical fiber chemical sensor. The FFS capillary has a cladding layer plus a protection polymer coating on its outside surface. The cladding layer ensured the capability of the FFS capillary for guiding light. The protection coating increases the FFS capillary's mechanical strength and makes it robust in practical applications. This makes the FFS capillary very useful in designing an EW based optical chemical sensor of a long path length. Techniques for activating the inner surface of an FFS capillary, coating the inner surface of the capillary with a polymer, connecting the coated capillary to a light source and a photodetector, and delivering a sample through the capillary have been developed in this work. Three optical sensors for sensing Cu(II), toluene in water samples and ammonia in a gas sample have been fabricated and tested. Preliminary test results obtained in this work illustrate the feasibility of fabricating highly sensitive EW absorption based chemical sensors with the light guiding FFS capillary.