Exploration of assembly-free fabricated single-multi-single mode and multi-single-multi mode fiber structures exhibiting ultra high sensitivities

This study investigates the performance characteristics of two assembly-free optical fiber sensing structures namely the multi-single-multi mode (MSM) and single-multi-single mode (SMS) configurations. To achieve optimal performance, an optimized sensing length is employed for the fiber sections, comprising single-mode (SM) and multimode (MM) fibers in the MSM and SMS structures, respectively. A comprehensive analysis is conducted to evaluate the influence of various external perturbations, including temperature, strain, and refractive index. The experimental results confirm the feasibility of achieving enhanced sensitivity and resolution across a broad dynamic range for these sensing parameters. Key aspects such as fast Fourier transform analysis, fringe visibility, and the free spectral range of the observed transmission spectrum are meticulously examined. Furthermore, theoretical validation is provided to support these findings. Additionally, the self-imaging phenomenon is investigated both experimentally and theoretically. Notably, the MSM fiber structure demonstrates superior sensitivity compared to the SMS configuration. The highest sensitivity is achieved with an SM fiber length of 10 cm, yielding values of 109 pm/°C, -3.04 pm/µε, and -41 nm/RIU for temperature, strain, and refractive index measurements, respectively.