Polarization mode couplings in the axial direction are evaluated for polarization-maintaining fibers using optical heterodyne detection. To verify the validity of this approach for fibers with various coupling constants, the method is applied to three fibers with modal birefringence values of 3.0*10/sup -4/, 1.1*10/sup -4/, and 1.5*10/sup -4/, respectively. The coupling constants in the 1.7*10/sup -4/ m/sup -1/ to 6.4*10/sup -7/ m/sup -1/ range are evaluated with a length resolution of 1 m. The extinction ratios are obtained from the coupling constants averaged over the fiber lengths. These values are in good agreement with the values measured directly from power ratios between the orthogonally polarized modes.
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