PRESSURE-INDUCED BIREFRINGENCE IN A COATED HIGHLY BIREFRINGENT OPTICAL FIBER

Because the stress-applying regions and the surrounding cladding of a highly birefringent optical fiber have different Poisson's ratios, the birefringence in the fiber is expected to be sensitive to the hydrostatic pressure applied to the fiber. An approximate expression is derived to describe this mechanism and highlight the effects of a single fiber coating. It is shown that a sufficiently thick coating with desired elastic properties can significantly increase the pressure-induced birefringence. Several common coating materials are studied and numerical results are compared with existing experimental data. It is found that the change in the differential phase between the two polarized modes of the fiber due to the pressure-induced birefringence is two orders of magnitude larger than that due to the change in the fiber length. A polarimetric sensor using a suitably coated highly birefringent fiber should be useful for the detection of static and low-frequency acoustic pressures.