High-Sensitivity Microbend Sensor Based on Light Cones in Coreless Fiber

Eigenmode expansion (EME) is a widely used method for modeling the electromagnetic wave propagation in multimode waveguides, where it breaks down signals into local eigenmodes and calculates them independently. Nevertheless, this methodology may challenge the causality mandated by the theory of special relativity, thus potentially disrupting the cause-and-effect relationship. This study experimentally explored light transmission in the multimode coreless fiber and found discrepancies between the EME method and measurement. To reconcile these inconsistencies, we introduced a light cone model, providing an alternative interpretation guided by the principles of special relativity. Remarkably, this innovative model did not merely resolve the observed discrepancies between the theory and experiments, but also presented a pioneering technique for designing microbend sensors. Through experimentation, we achieved the remarkable sensitivity of 500 dB/m⁻¹ at a bending curvature of 0 m⁻¹. Our research advances the understanding of multimode systems and paves the way for innovative sensing and communications applications in compact devices.