Interference between two orthogonally polarized modes traversing a highly birefringent air-silica microstructure fiber

Highly birefringent fibers can be used as polarization-maintaining fiber in interferometer fiber optic gyros and in pressure sensors. Interference between two orthogonally polarized modes traversing a highly birefringent air-silica microstructure fiber is investigated theoretically and experimentally. The theory includes the effect of the dispersive nature of the modal birefringence of highly birefringent fiber. Measurements are conducted using super luminescent diodes operating at the center wavelengths of 846 and 973 nm with spectral half-widths of 7.4 and 8.1 nm, respectively, and a 9-m highly birefringent fiber. Experiments yield interference fringe visibility V values of 0.4-0.5, even when the effective optical path difference between the two modes is zero. The theory well explains the temporal coherence properties of dispersively propagating waves with regard to both the magnitude of V and the shape of the coherence curve for the highly birefringent fiber. A comparison of the results from a standard single-mode fiber and from an air-silica microstructure fiber with modal birefringence of 3.2×10^-6 at 973 nm is made that addresses temporal coherence. The loss of temporal coherence can be ignored if the fiber has small birefringence. Furthermore, the detection sensitivity of a distributed fiber-optic pressure sensor is measured in order to characterize the temporal coherence response. The results reveal that, in fiber-optic sensors that depend on the interference between the two modes, interference signal sensitivity decreases as the chromatic dispersion difference between the two mode increases.