Symmetrical Metal Cladding Waveguide for Absorption Sensing and its Sensitivity Analysis

A sensitivity analysis of absorption sensing based on the symmetrical metal-cladding waveguide is presented. The calculated reflectivity curve shows that any minute variation of the extinction coefficient due to absorption of the sample will easily give rise to a dramatic change in the minimum reflectivity. It is found that thickness of the metal coupling film is a critical factor. With a thin metal coupling film, the minimum reflectivity decreases at first, reaches zero, and then increases gradually when extinction coefficient of the sample increases. However, in another case, the minimum reflectivity always increases as an increasing extinction coefficient of the sample. These characteristics are explained in terms of comparison between the intrinsic and radiative dampings, which are derived in an analytic form by using the first-order perturbation theory. A proof-of-concept experiment, that is, absorption sensing the methylene blue concentration, is also given.