Multimode Interference in a Large-Loop Microfiber Structure and Its Application to Humidity Sensing

A large-loop microfiber structure based on an adiabatic microfiber is proposed. By introducing bending into the microfiber, the fundamental mode can be coupled into high-order modes. The intensities of the high-order modes can be enhanced by increasing bending curvature, leading to a multimode interference (MMI) with tunable intensity. The transmission spectrum characteristics are analyzed, and the relative humidity (RH) sensing application of this structure is demonstrated without any sensitive coating materials. The experimental result shows that the wavelength shifts nonlinearly with RH varying from 30% to 80% RH, and the maximum average sensitivity of 205 pm/%RH is obtained in the range of 70% to 80% RH. The large-loop microfiber structure with high RH sensitivity, low loss, and simple fabrication could provide a cost-effective platform for integrating functional materials or micro/nano-structures for constructing high-performance fiber devices of various applications.