Polarization-maintaining (PM) fiber preserves a stable linear polarization by doing something that may sound counterintuitive: it deliberately introduces strong, well-defined birefringence. Instead of trying to make the fiber perfectly symmetric, PM fiber creates two orthogonal polarization eigenmodes with different effective refractive indices and propagation constants.

This built-in difference is much larger and more controlled than the weak, random birefringence that ordinary fiber can acquire from bending, residual stress, temperature changes, or manufacturing imperfections. When the input polarization is aligned with one of the fiber's principal axes, unwanted coupling into the orthogonal polarization mode is strongly suppressed. For a broader introduction to how light is guided in a fiber, see this overview of fiber-optic principles and structure.