Optimal refractive index design for an optical fibre-based evanescent field sensor

Optical fibre-based evanescent field sensors are proving to be viable and useful in the detection of chemicals, toxins, gases and other species. Such fibre sensors rely on weak interactions between the surrounding environment and the external “evanescent” field associated with bound modes of electromagnetic radiation propagating through an optical fibre. Of fundamental importance in the design of such sensors is the fraction of the total guided field power that propagates as the evanescent field. By increasing this relative evanescent field power, sensor gain can be improved. As the evanescent field is determined by the refractive index profile (RIP) of the sensor and the surrounding environment, design of the sensor RIP is critical. This paper presents a rigorous approach to this RIP design problem, formulated in terms of an optimization problem with the normalized evanescent field power as the payoff, RIP as the input, and a simple electric field model as the constraint. An RIP design procedure utilizing dynamic programming is then presented, along with an example that illustrates the improvement obtainable over existing designs.