Highly Birefringent D-Shaped Microfiber and Its Application in High-Sensitive Optical Sensing

A highly sensitive sensor based on a D-shaped microfiber with high birefringence is theoretically and experimentally demonstrated. The fiber is fabricated by side-polishing a single-mode fiber and then tapering down the polished region into a micrometer size. Through analyzing the electric field distribution of the evanescent wave, the relationship between the structural parameters and the birefringence value of the D-shaped microfiber is established. Simulation results reveal that the maximum birefringence is always achieved at the biggest ratio of the removed depth to the microfiber diameter, i.e., $h/D$ for a certain diameter. By optimizing the diameter and $h/D$ , fantastic birefringence of $4.8\times 10^{-2}$ is achieved, as well as the enhanced refractive index (RI) and temperature sensitivity can be realized when the group birefringence $G$ approaches to zero. By utilizing Sagnac loop interferometer, high RI sensitivity of 9951 nm/RIU and temperature sensitivity up to −0.989 nm/°C are obtained with the microfiber diameter of $2.724~\mu \text{m}$ and the length of only 15 mm in experiment.