Long-Range and High-Spatial-Resolution Distributed Birefringence Measurement of a Polarization-Maintaining Fiber Based on Brillouin Dynamic Grating

We report a long-range and high-spatial-resolution distributed measurement of the birefringence of a polarization-maintaining fiber based on Brillouin dynamic grating (BDG) through optimizing the pump and probe scheme in the time-domain. The optimal scheme includes a CW pump1, a long pump2 pulse, and a short probe pulse, where the long pump2 pulse can effectively excite a BDG with a relatively low power to avoid other nonlinear effects in a long-length fiber, and the short probe pulse is used to improve the spatial resolution. The numerical simulations verify that with this scheme the spatial resolution is determined by the duration of the probe pulse, while the BDG reflection spectrum width increases with shorting the duration of the probe pulse. In experiment, a measurement of a 500-m Panda polarization-maintaining fiber with a 20-cm resolution is realized by using a 20-ns pump2 pulse and a 2-ns probe pulse. The results clearly show the 49-cm spatial period of the birefringence variation caused by the uneven stress as winding the fiber around a spool. Besides, longer spatial periods of the birefringence variation are also observed, which could be caused by the residual stress during the fiber drawing and coating processes or the nonuniformity of fiber preform.