Distributed birefringence sensing at 10^-9 accuracy over ultra-long PMF by optical frequency comb and distributed...

Brillouin dynamic gratings (BDG) can measure the distributed birefringence of polarization-maintaining fibers (PMF), however, its sensing range is limited by both stimulated Brillouin scattering depletion and fiber losses in PMF, which are significantly higher than those in standard single-mode fibers. In this work, we theoretically and experimentally verify that BDG can be sustained over ultra-long distances when assisted by distributed Brillouin amplification, significantly extending the distributed birefringence measurement distance. Using an optical frequency comb pumped by a narrow linewidth laser to both generate and interrogate the amplified BDG, a birefringence measurement accuracy of 7.5 × 10^-9 was achieved over 7 km sensing length, more than double the longest range reported. This opens a new opportunity to investigate small birefringence changes due to nonlinear optics effects and monitoring fiber network security from eavesdropping.