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Polarization-maintaining optical fiber(2)

Date: 2015-05-12 Source: Read: 0

Principle of operation

Polarization-maintaining fibers work by intentionally introducing a systematic linear birefringence 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 half-wave plate. Now consider that there might be a random coupling between the two polarization states over a significant length of such fiber. At point 0 along the fiber, the wave in polarization mode 1 induces an amplitude into mode 2 at some phase. 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. At point Lb along the fiber the coupling is again in the original phase, but at 3/2 Lb it is again out of phase and so on. The possibility of coherent addition of wave amplitudes through crosstalk over distances much larger than Lb is thus eliminated. Most of the wave's power remains in the original polarization mode, and exits the fiber in that mode's polarization as it is oriented at the fiber end. Optical fiber connectors used for PM fibers are specially keyed so that the two polarization modes are aligned and exit in a specific orientation.

Note that a polarization-maintaining fiber does not polarize light as a polarizer does. Rather, PM fiber maintains the linear polarization of linearly polarized light provided that it is launched into the fiber aligned with one of the fiber's polarization modes. Launching linearly polarized light into the fiber at a different angle will excite both polarization modes, conducting the same wave at a slightly different phase velocities. At most points along the fiber the net polarization will be an elliptically polarized state, with a return to the original polarization state after an integer number of beat lengths. Consequently, if visible laser light is launched into the fiber exciting both polarization modes, scattering of propagating light viewed from the side, is observed with a light and dark pattern periodic over each beat length, since scattering is preferentially perpendicular to the polarization direction.

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