Refractive Index and Curvature Sensing Using SNS and SNSNS Fiber Interferometers in the 2 μm Band

This work comprehensively investigates the refractive-index (RI) and curvature sensing characteristics of the single-mode–no-core–single-mode fiber (SNS) structure and the single-mode–no-core–single-mode–no-core–single-mode fiber (SNSNS) structure in the 2 μm band. The self-imaging effect and mode coupling behavior under varying cladding RIs are numerically simulated using the beam propagation method (BPM), and the RI and curvature sensing mechanisms of the two configurations are systematically compared experimentally. By optimizing the no-core fiber (NCF) length, the SNS structure achieves a maximum RI sensitivity of 294.56 nm/RIU over the range of 1.3245–1.3794 RIU with a 12.5 cm NCF. Meanwhile, the SNSNS structure exhibits a highly linear curvature sensitivity of 38.67 nm/m−1 over the range of 2.500–3.227 m−1 (R2 = 0.9988). Furthermore, rotation responses of both configurations are characterized on a common platform, showing small rotation-induced spectral shifts with sensitivities of −17.72 pm/° and −5.2 pm/°, respectively. This work provides a solid technical foundation for applications in medical monitoring, embodied intelligence, and industrial inspection.