polarisationmaintaining

Polarisation maintaining refers to optical fibers and components engineered to preserve the polarisation state of light as it propagates. In polarisation-maintaining fibers, two orthogonal linear polarisation modes experience different phase velocities due to built-in birefringence, which suppresses power transfer between the modes and helps keep the output polarisation aligned with the input, within practical limits.

PM fibers achieve this through intentional structural birefringence, created by asymmetries in the core or by surrounding stress-inducing elements. Common designs include Panda fibers, which use three internal stress rods to generate high birefringence, and bow-tie or elliptical-core fibers that support stable orthogonal axes. The strong birefringence reduces coupling between the two principal axes, so a light signal launched along one axis remains predominantly in that axis as it travels, albeit with some intensity loss and phase delay between components.

Performance is described by parameters such as polarization extinction ratio, birefringence value, and polarization-mode dispersion. PM fibers are sensitive to bending, twisting, and temperature changes, which can degrade the maintained polarisation and increase losses or cross-coupling. Accurate splicing and careful handling are required to preserve performance.

Applications include interferometric sensing, fiber-optic gyroscopes, high-stability laser delivery, metrology, and precision communications where a well-defined and stable polarisation state is crucial. While PM fibers improve polarisation control, they are not universally suitable for all wavelengths or configurations and require appropriate design choices and installation practices to minimize environmental effects.