Polarization mode couplings in the axial direction are evaluated for polarization-maintaining fibers using optical heterodyne detection. This method makes it possible to determine the magnitudes of polarization mode couplings with higher length resolution and wider dynamic range than can be obtained by the POTDR method. To verify the validity of this approach for fibers with various coupling constants, the method is applied to three fibers having different modal birefringence values of 3.0 x 10–4, 1.1 x 10-4, and 1.5 × 10–5. The coupling constants, in the 1.7 x 10–4 m-1 to 6.4 x 10–7 m-1 range, are evaluated with a length resolution of 1 m. Also, 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 measured directly from power ratios between the orthogonally polarized modes. Minimum detectable coupling magnitude, given as the product of coupling constant and fiber length, is 3 x 10–8 in this system.
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