Single-mode fiber interferometer for measuring adsorption

We used the analogy between a single-mode optical waveguide and a particle in a quantum mechanical potential well to calculate the effect of adsorption on the outer surface of a fiber on its modal propagation constant by time independent perturbation theory. The results shows that the change in it will be measurable in a straight fiber only if the surface is very close to the core. A very smooth and homogeneous self-developed plasma-etching technique is described to attain this condition. A different approach, which gave more positive results, used bent fibers. In this case, the influence of the adsorbed molecules on the phase of the transmitted wave could be measured, even on the full external diameter of the fiber.