As discussed in the previous chapter, the so-called single-mode fibers in fact support two modes simultaneously, which are orthogonally polarized. In an ideal circular-core fiber, these two modes will propagate with the same phase velocity; however, practical fibers are not perfectly circularly symmetric. As a result, the two modes propagate with slightly different phase and group velocities. Furthermore, environmental factors such as bend, twist, and anisotropic stress also produce birefringence in the fiber, the direction and magnitude of which keep changing with time due to changes in the ambient conditions such as temperature. These factors also couple energy from one mode to the other mode of the fiber, creating problems in practical applications.
The first issue we discuss is that as the magnitude of the birefringence mentioned earlier keeps changing randomly with time due to fluctuations in the ambient conditions, the output SOP also keeps fluctuating with time. The change in the output SOP is of little consequence in applications where the detected light is not sensitive to the polarization state. However, in many applications such as fiber optic interferometric sensors, coupling between optical fibers and integrated optic devices, coherent communication systems, and so on, the output SOP must remain stable. Secondly, since each mode propagates with a different group velocity, the so-called polarization mode dispersion (PMD) can limit the ultimate bandwidth of a single-mode optical communication system.
We have already discussed the detrimental aspects of the random birefringence present in practical fibers. However, a controlled and deliberate birefringence introduced in the fiber may be used to realize several in-line fiber optical components and devices. In this chapter, we will discuss this aspect, i.e., the applications of the controlled and deliberate birefringence introduced in the fiber. The detrimental aspect - namely, the PMD - will be discussed in the next chapter.
One of the most important applications of the deliberately introduced birefringence in optical fibers is the development of high-birefringence (Hi-Bi) fibers, which are realized by introducing a permanent high birefringence into the fiber during the fabrication stage itself. Such fibers can maintain the SOP of the incident light over large distances and hence are also known as polarization-maintaining fibers (PMFs). In the next section, we will discuss different types of PMFs, their polarization characteristics, and their applications. Devices using controlled birefringence in conventional single-mode fibers are then discussed.