Impact of structural parameters on multimode D-shaped optical fiber refractive index sensors for different materials

Abstract

In this work, a D-shaped optical fiber sensor has been experimentally developed to compare how the refractive index (RI) of a water analyte (used as the cladding layer over a small length of the stripped D-polished optical fiber) changes due to the inclusion of an unknown concentration of two distinct materials, one with a low RI and the other with a high RI. No metal film is deposited in the proposed D- shaped optical fiber sensor and hence we demonstrate the sensor's ability to detect the lowest concentration of the added material into the water analyte without employing the surface plasmon modes. A side-polished multi-mode optical fibre with various sensing lengths and polishing depths is used to construct the D-shaped fibre sensor. Experimental results verify that in a tunable refractive index (RI), the sensitivity increases with the sensing length due to the increase in the interaction area between the surrounding material and the sensing area. The sensitivity also increases with polishing depth in the high-RI range for materials with higher concentrations. The D-shaped fibre sensor's maximum sensitivity of 25.076 a.u./RIU is attained in the range of low refractive index values (1.3329) to (1.3345) RIU. Nonetheless, its maximum sensitivity drops to 0. 317 a.u./RIU at the high refractive index region of 1.348–1.4048 RIU. This sensor is predicted to have potential uses in low-RI materials with very low concentrations. Therefore, the best D-shape fibre sensor was used to detect the lowest concentration of the sodium chloride solution in the low refractive index range, and it was found that the lowest detected concentration was 0.012% with a high sensitivity of 327.32 a.u./RIU.