TABLE OF CONTENTS In the axially symmetric single-mode fiber, there exist two orthogonally polarized modes, as shown in Section 3.5. They are known as 
and 
modes in accordance with their polarization directions. If the fiber waveguide structure is truly axially symmetric, these orthogonally polarized modes have the same propagation constants and thus they are degenerate. This is why such fiber is called single-mode fiber. In practical fibers, however, an axial nonsymmetry is generated by the core deformation and/or core eccentricity to the outer diameter, and it causes a slight difference in the propagation constants of the two polarization modes. In such fibers, the state of polarization (SOP) of the output light randomly varies, since the mode coupling take place between and modes, which is caused by fluctuations in core diameter along the z-direction, vibration and temperature variations. Therefore, such fibers cannot be used for applications in optical fiber sensors and in coherent optical communications in which SOP and interference effects are utilized.
Birefringent fibers have been proposed and fabricated to solve such polarization fluctuation problems [19]. The difference of the propagation constants between and modes are intentionally made large in birefringent fibers. Birefringent fibers are also called polarization-maintaining fibers.
Mode coupling (or mode conversion) between and modes occurs strongly (refer to Chapter 4) when there exists the same, or nearly equal, frequency components in the longitudinal fluctuations as the difference...
