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Single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest typically 1300 to 1320nm.
Comparison of single-mode fiber and muti-mode fiber
Constructions are different
source of figure: http://www.rp-photonics.com/multimode_fibers.html
Transmissions are different
source of figure: http://www.arcelect.com/fibercable.htm
What is polarization-maintaining fibers?
Polarization-maintaining fibers is a special purpose single mode fiber, it preserves and transmits the polarization state of light that is launched into it.
Why do we need the polarization-maintaining fibers?
The reason why we need to have polarization-maintaining fibers is that regular single mode fiber cannot maintain the polarization state of input laser beam. Because in a regular single mode fiber, the input polarized laser beam (consist of two perpendicular polarized modes - horizontal and vertical) have the same propagation constant (propagation constant is a measure of the change undergone by the amplitude of the wave as it propagates in a given direction), in other words they both travel at the same velocity (not good). It would cause optical energy from one of these modes (horizontal or vertical mode) to transfer the other or “cross-couple”. That is the reason why single mode fiber cannot maintain the polarization state of input laser beam.
How does polarization-maintaining fibers work?
Then the polarization-maintaining fibers was invented to overcome this problem. The polarization-maintaining fibers are designed in such a way that two perpendicular polarized modes are forced to travel at different propagation constants, which means they travel at different velocity. This difference in velocities makes it very hard for optical energy to transfer between each other. As the result that polarization state of input laser beam is preserved. This special purpose fiber is commonly used in fiber optic sensing, interferometry and slab dielectric waveguides.
Cross Section of Polarization Maintaining Fibers:
Source of the figure: http://www.fiberoptics4sale.com/wordpress/what-are-polarization-maintaining-fibers/
The above three pictures illustrated the three different types of polarization maintaining fibers, which are Panda, Elliptical-clad, Bow-tie. There fibers contain a unique feature; they have stress rods in fibers (a feature not seen in other fiber types). The stress rods are two circles in the Panda PM fiber, a elliptical clad in elliptical-clad PM fiber and two bow-ties in the Bow-Tie type PM fiber. The most important property of stress rods is that it creates stress in the core of the fiber such that the transmission of only one polarization plane of light is favored.
YouTube video:
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A brief introduction of the polarization-maintaining fibers.
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