The polarization state of the light
traveling in optical fibers is modified by fiber bending, geometrical
perturbations and material inhomogeneities. The main effect is a changement of
the polarization state. If the rotation angle is different for the reference
and test signals, a reduction of the fringe visibility is observed due to the
partial superposition of orthogonal polarization states. The polarization
controller (POLA) placed in the test arm modifies the polarization state angle
of the test signal in order to optimize the polarization matching with the
reference signal. If q is the polarization angle difference between the reference and the
test lights at the detector input, the measured AC intensity signal is reduced
by a factor cos(q) (see Appendix E for more details). The polarization controller is
manually set to obtain q = 0.
The polarization effect is very
important when an absolute measurement of the OLCR signal is required; in fact,
temperature changes or vibrations can modify q and then reduced the
effective interference amplitude. Finally, we remark that other polarization
coherence effects can be neglected for non-polarized light due to the initial
lack of polarization cross-correlation.
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