Polarization-maintaining optical fiber

And since even a short portion of fiber, over which a tiny coupling coefficient may apply, is many thousands of wavelengths long, even that small coupling between the two polarization modes, applied coherently, can lead to a large power transfer to the horizontal mode, completely changing the wave's net state of polarization.

The beat length Lb of such a fiber (for a particular wavelength) is the distance (typically a few millimeters) over which the wave in one mode will experience an additional delay of one wavelength compared to the other polarization mode.

However at point 1/2 Lb along the fiber, the same coupling coefficient between the polarization modes induces an amplitude into mode 2 which is now 180 degrees out of phase with the wave coupled at point zero, leading to cancellation.

The possibility of coherent addition of wave amplitudes through crosstalk over distances much larger than Lb is thus eliminated.

Several different shapes of rod are used, and the resulting fiber is sold under brand names such as "PANDA" and "Bow-tie".

That causes the phase velocity of right and left hand circular polarizations to significantly differ.

Good PM fibers have extinction ratios in excess of 20 dB.

Image of the cross section of a polarization-maintaining optical fiber patch cord, taken with an illuminated microscopic viewer called a fiberscope. The two small, eye-like circles are the stress rods and the tiny circle between them is the core. The larger circle surrounding them is the cladding, usually 125 microns in diameter.
Cross sections of three types of PM fiber.