Polarization-maintaining fibers work by intentionally introducing a systematic linear in the fiber, so that there are two well defined polarization modes which propagate along the fiber with very distinct phase velocities. 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. Thus a length Lb /2 of such fiber is equivalent to a.
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