Sagnac Interferometer Based on a Highly-Birefringent Two-Core PCF: Theory, Experiment, and Sensing Characteristics

In this article, we present comparative study on the transmission and the sensing characteristics of an all-fiber Sagnac loop interferometer (SLI) made by different kinds of the highly-birefringent (HB) two-core photonic crystal fiber (TCPCF), respectively for strain, torsion, and temperature sensing. The fiber device is analyzed theoretically and experimentally. Transmission spectrum of the SLI using the HB TCPCF shows six spatial frequencies due to superposition of multiple two-beam interferences originating from the intra-core and the inter-core interactions between the fundamental polarization modes propagating in the two birefringent cores. Our results reveal that the transmission characteristics of the fiber device and the sensing features of its multiplexed interferences are strongly dependent on the optical and the structural properties of the two birefringent cores such as their intrinsic birefringences, the effective index contrast between them, and the relative orientation between the (slow) birefringence axes. Finally, the application of the SLI made by the specific HB TCPCF is discussed in the simultaneous measurement of three parameters.