Dual-Core Twisted Photonic Crystal Fiber Salinity Sensor: A Numerical Investigation

The amount of salt dissolved in a body of water is an important factor in determining physical characteristics like the heat capacity of the water. Here, we report a theoretical and numerical investigation on a seawater salinity sensor using a miniaturized fiber-optic probe based on a dual-core photonic crystal fiber (PCF). The sensing mechanism is based on interplay between fiber fundamental mode in the central core and a secondary mode in the water-filled hole of the PCF. Using finite element method analysis with Comsol Multiphysics Software, we show that a spectral sensitivity larger than 5500 nm/RIU can be achieved while varying the salt concentration from 0% to 100%. Furthermore, we demonstrate that the sensitivity for highly saline water bodies can be further improved by applying the permanent twist on the cladding air holes of the fiber. We studied the role of the twist directions and twist rates on the performance of the sensor including sensitivity and the absolute loss value. While the twist does not have any major impact on sensitivity of the sensor for low salinities, our results indicate that twisted PCF has an average higher sensitivity for salt concentrations over 50% and a maximum sensitivity of nearly 7000 nm/RIU is reported around salt concentration of 80%.