Experimental Investigation of Microstructured and Capillary Optical Fibers for Refractive Index Measurement from 1.316 to 1.425 RIU

We experimentally investigate the potential of two types of optical fibers for refractive index (RI) measurement from $1.316 < RI < 1.425$, which is a range of interest to detect low to very low concentrations of target gas typically found in the natural environment or in aquatic environments. The first fiber is a microstructured optical fiber (MOF) with a ring of air-holes arranged at the outer periphery of the fiber. The second investigated fiber is the capillary fiber (CF). To simulate the varying RI of the surrounding medium, a glycerolpure water mixture is used. A 5-cm fiber length is utilized as the sensing element to monitor RI changes for the two fiber types and performances are compared with MMF-based refractometer. MOF achieved a sensitivity 95 times greater than that of the MMF, and a resolution of ~ 1.01 × 10⁻⁴ RIU. The CF demonstrates promising results, increasing the sensitivity up to 321 times of that of the MMF and three times that of the MOF over the RI range of 1.316 to 1.425.