Birefringent photonic crystal fiber coils and their application to transverse displacement sensing.

We have experimentally investigated the birefringent properties of photonic crystal fiber (PCF) coils in cooperation with a Sagnac loop interferometer. By reducing the bending radius of the PCF coils, very clear interference patterns can be observed for the bending-induced stress effect. Increasing the fiber turns can result in more obvious interference patterns with smaller fringe spacing but has no contribution to the increment of the birefringence value. The fabricated PCF coil is employed in the transverse displacement sensing. Very high sensing sensitivity of 90.4 nm/mm can be achieved due to the large displacement-induced bending radius variations.