Unfortunately, the polarization state of light propagating in a length of standard circular fiber varies along the fiber due to the random birefringence induced by the thermal stress, mechanical stress and irregularities of the fiber core.
In many applications this random birefringence has to be avoided, by so called polarization maintaining (PM) fibers.
When a transparent isotropic media is subjected to mechanical stresses it may become optical anisotropic. This effect is known as stress birefringence or the photo-elastic effect.
Several methods can be used in order to induce inner elastic stresses to a fiber during its manufacture. The main idea is to use materials exhibiting different expansion coefficients which induce stress on the core due to their ditterent thermal behavior during the cooling of the fiber. This is reached by doping the stress inducing regions with boron, whereas the core with germanium and the cladding with phosphor. Hence, the asymmetrical stresses lead to birefringence.
Core geometry can be changed in several ways during the manufacturing process. One method is to shape the core elliptically like shown in Figure 4.16 (a).
Another method is to induce asymmetrical stresses by surrounding the circular core ofa single mode fiber by zones having bow-tie shape, fibers hence called bow tie fibers Figure 4.16 (b), or by two circles, fibers called panda fibers like shown in Figure 4.16 (c).
A PM fiber is a highly birefringent fiber with predetermined slow and fast axes. If the polarization of input light is linear and aligned with one of the axes, it remains unchanged after propagation in the fiber. However, if the input polarization is not linear or is linear but not aligned with one of the axes, the polarization varies periodically along the fiber, and the output from the fiber is in general elliptically polarized, with the ellipticity and orientation determined by the fiber length.
Those polarization maintaining fibers (PM) were invented for minimizing the polarization mode dispersion (PMD), which is one of the main obstacles for enhancing the data rate of fiber optic cable networks. PM fibers take more effort at production and have more loss, so that they are mainly used for short fiber length applications like its common in measurement technology.