Lyot filter-induced Vernier effect for sensitivity improvement of fiber birefringence sensor

The polarization characteristics of the optical fiber have spurred interest in a plethora of smart fiber optic sensors with extremely compact structures due to the capability of in-line configuration, except that the sensing sensitivity remains too low to meet the practical requirements. In this paper, fiber Lyot filter (FLF) is leveraged to generate Vernier effect for sensitivity enhancement of birefringence-based and versatile fiber optic sensor. The proposed sensing structure is composed of two sections of polarization maintaining fibers (PMF) with the reflective scheme, which can be applied to birefringence sensing scenarios such as strain and temperature measurement. The working principle analysis derived from Jones matrix indicates that the combination of Vernier effect and the reflective structure catalyzes a significant improvement in sensitivity. The length of the PMF used in this sensor, the fusing angle between two PMFs, and the birefringence properties of the PMF are all numerically investigated to generate better Vernier performance. Also, experiments are carried out for validation, and sensitivity as high as −12.96 nm/rad is achieved. The compact and in-line structure of the proposed sensor can enable itself to perform excellent and versatile sensing on various occasions.