Evanescent wave absorption in an aqueous dye solution has been performed using multimode fused silica fiber which was unclad at the sensing region. Evanescent absorbance values for methylene blue in a concentration range 3×10−8 to 5×10−6 M are reported. In order to produce modes close to cutoff in the sensing region, tunneling modes were launched into the clad fiber. Spatial filtering was used to restrict the light launched to those modes which have substantial power in their evanescent field in the unclad region. The measured evanescent absorbance of the dye solution was found to vary linearly with the exposed core length and to exhibit a square root dependence on concentration. The former effect is predicted from standard theory while the latter is attributed to adsorption on the core surface which obeys a Debye–Huckel-type concentration dependence. In addition, a concentration enhancement of two orders of magnitude was observed due to this surface adsorption. While this effect limits the use of the technique for a reversible sensor it may be exploited in disposable probes.

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Intermodal diffusion in a large-diameter plastic-clad silica step-index fiber is measured with a near-field projection of the modal pattern onto a CCD camera. The diffusion process is found to obey a 1/θ(2) dependence (where θ is the axis-crossing angle of the mode). The diffusion constant is estimated to be 1.3 × 10(-5) rad(2) m-(1). The contribution of mode mixing to evanescent-wave spectroscopy in such fibers is discussed. When all bound meridional modes are launched, the effect of diffusion is a small reduction- some 10% maximum-in the fraction of evanescent-wave power outside the core for the 0.4 numerical aperture fiber used.