Special Fiber is constructed with a non-cylindrical core or cladding layer, such as Polarization Maintaining Fiber (or PM fiber) and fiber designed to suppress whispering gallery mode propagation.
The Special Points Of PM fiber
PM fiber is a specialty optical fiber with strong built-in birefringence, preserving the properly oriented linear polarization of an input beam.
Polarization maintaining fiber (PM fiber) is a special type of single mode fiber. Normal single mode fibers are capable of carrying randomly polarized light. However, PM fiber is designed to propagate only one polarization of the input light. PM fibers contain a feature not seen in other fiber types. Besides the fiber core, there are stress rods in the fibers. The stress rods are two circles in the Panda PM fiber, an elliptical clad in elliptical-clad PM fiber and two bow-ties in the Bow-Tie type PM fiber.
Polarization-maintaining PANDA fiber (left) and bow-tie fiber (right)
Why PM Fiber Are Needed?
Optical fibers always exhibit some degree of birefringence, even if they have a circularly symmetric design, because in practice there is always some amount of mechanical stress or other effect which breaks the symmetry. As a consequence, the polarization of light propagating in the fiber gradually changes in an uncontrolled (and wavelength-dependent) way, which also depends on any bending of the fiber and on its temperature.
This problem can be fixed by using a polarization-maintaining fiber, which is not a fiber without birefringence, but on the contrary a specialty fiber with a strong built-in birefringence. Provided that the polarization of light launched into the fiber is aligned with one of the birefringent axes, this polarization state will be preserved even if the fiber is bent. The physical principle behind this can be understood in terms of coherent mode coupling. The propagation constants of the two polarization modes are different due to the strong birefringence, so that the relative phase of such copropagating modes rapidly drifts away. Therefore, any disturbance along the fiber can effectively couple both modes only if it has a significant spatial Fourier component with a wavenumber which matches the difference of the propagation constants of the two polarization modes. If this difference is large enough, the usual disturbances in the fiber are too slowly varying to do effective mode coupling.
A disadvantage of using PM fibers is that usually an exact alignment of the polarization direction is required, which makes production more cumbersome. Also, propagation losses are higher than for standard fiber, and not all kinds of fibers are easily obtained in polarization-preserving form.
PM fibers should not be confused with single-polarization fibers, which can guide only light with a certain linear polarization. PM fibers are rarely used for long distance transmission because of their expensive price and higher attenuation than single mode fiber cable, either. Polarization Maintaining Optical Fiber (PMF) have been more and more widespread applied in optical fiber communication and optical fiber sensing system because the strong ability on polarization maintaining to the linearly polarized light and the good compatibility with the ordinary single mode optical fiber. People have done much works and made significant development on the theoretical design and the fabrication technology and the parameter survey of PMF.