Analysis and modeling of an optical fiber loop resonator and an evanescent field absorption sensor for the application for chemical detection

Optical fiber loop resonator (FLR) and fiber loop ring down spectroscopy (FLRDS) are two recent techniques used for chemical and gas sensing application based on evanescent field interaction. In chemical sensing applications, a short region of the fiber core is exposed to the external environment, so that evanescent field can interact with the chemical. The evanescent field access region in single mode fiber has been developed practically by etching a small portion of the cladding and it is used as sensor element in the fiber loop cavity resonator. In this study, a fiber loop cavity resonator has been theoretically modeled and the shift of cavity resonances due to change of the refractive index of the external medium is presented. The theoretical predictions are compared with experimental results. It is observed that when the sample is applied to the evanescent field interaction block (EAB), the cavity resonances remained symmetric while the width of the signal is increased as expected from finesse degradation. In the second step of this paper, modeling on the optimization of the parameters of EAB is carried out for the sensor length (1–1.5mm) and diameter (less than 20μm) of the etched optical fiber. Modeling parameters are similar with practical realization. Finally, EAB as a sensor element for chemical sensing is used with a 419-ppm cobalt nitrate solution and isopropanol.