Twisted-Bent Refractive Index Sensor With Dual-Wavelength Volume Compensation

This paper presents a twisted-bent fiber optic sensor with volume compensation for precise refractive index (RI) measurement in transformer oil applications. The sensor design combines a twisted structure with an additional controlled bending, enhancing the interaction between the evanescent field and the surrounding medium. A plastic optical fiber (POF) was used, and its performance was systematically characterized through bending diameter variation, RI detection, and oil volume sensitivity analysis. An optimal bending diameter of 4.5 cm was identified, where the sensor achieved RI sensitivities of 2167.4 RIU⁻¹ at 680 nm and 2058.5 RIU⁻¹ at 780 nm, with corresponding linearity values of 0.9172 and 0.9046. The twisted bend configuration also showed strong responsiveness to oil volume variation, with volume sensitivities of 5.9512 ml⁻¹ and 5.5438 ml⁻¹. To address measurement error due to volume effects, a dual-wavelength compensation technique was applied. By utilizing the distinct RI and volume sensitivities at 680 nm and 780 nm, a matrix-based compensation method was applied to separate the RI effects from volume effects and determine the actual RI values. This compensation reduced relative error from 1.7 × 10⁻² % to as low as 1.86 × 10⁻¹³%. Overall, the proposed twisted bend fiber optic sensor demonstrates high sensitivity, reliable compensation for environmental variation, and robust performance. It simple, low-cost design makes it a promising tool for transformer oil monitoring and other chemical or industrial sensing applications requiring high accuracy and stability.