Development of stripped-cladding optical fiber sensors for continuous monitoring: I. Theoretical study of a referencing method for measuring refractive indices of fluids

Intrinsic waveguide optical sensors, IWOS, formed by an exposed core (modified in some cases with a material sensitive to a target parameter), in contact with a measured fluid, have diverse applications. We have identified a novel referencing method for this type of sensor. The referencing method is a self-calibrating procedure that allows for development of portable sensors capable of continuous monitoring. This paper examines the theoretical basis of the proposed referencing method and constitutes a first step in the practical design of such optical sensors. Since the most commonly measured parameter with such sensors is the real term (n) of the complex refractive indices (m = n + ki), real index of refraction for different fluids and glass waveguide materials are used to investigate sensor responses. Case studies, in which ncore > nmeasured fluid and ncore < nmeasured fluid are considered, suggest that an easily configurable optical photo-refractometer can be designed with a variable dynamic range. This means that IWOS physicochemical detection characteristics (extent of reaction, species concentrations, etc.) can be usefully related to refractive index variations through self-calibrating strategies. It is also shown that portable spectrophotometers allow for measurement of spectral signals that are diagnostic of the measured material.