Is polarization-maintaining fiber a single-mode fiber

Polarization-maintaining (PM) fiber is primarily a single-mode optical fiber designed to preserve the polarization state of light.

Polarization-maintaining fibers are specialized single-mode fibers in which linearly polarized light, when properly launched, maintains its polarization throughout propagation with minimal cross-coupling between the two polarization modes (fast and slow axes) due to intentionally induced birefringence . This is achieved by integrating stress elements, such as PANDA or bow-tie structures, into the fiber cladding, which break the circular symmetry and create two well-defined polarization modes with distinct phase velocities . While PM fibers are mostly single-mode, there are rare cases of few-mode PM fibers, but highly multimode PM fibers are not produced because it is difficult to maintain strong and uniform birefringence over a large core area . The single-mode nature ensures that the fiber supports only one spatial mode, which is essential for maintaining a stable polarization state over long distances and under environmental variations like bending, temperature changes, or stress . Applications of PM fibers include fiber-optic gyroscopes, interferometry, quantum key distribution, and modulators in telecommunications, where preserving the polarization state is critical for system performance . The quality of PM fibers is often characterized by the polarization extinction ratio (PER), which measures the ratio of correctly to incorrectly polarized light, typically exceeding 20 dB for high-quality fibers . In summary, PM fibers are single-mode fibers with built-in birefringence to maintain polarization, and their single-mode operation is fundamental to their ability to preserve the polarization state reliably.