Characterization of Distributed Birefringence in Optical Fibers

Abstract

Birefringence is the fundamental physical parameter of optical fibers which characterizes their polarization properties, and it can be classified into phase birefringence and group birefringence. Phase birefringence is the difference in effective index between the two orthogonal linear polarization modes of an optical fiber, while the group birefringence is related to group index representing the polarization mode dispersion. In this chapter, we introduce a distributed phase birefringence measurement method based on Brillouin dynamic grating (BDG), which creates a new horizon for optical fiber evaluation. When two parallel polarized pump waves, with a frequency offset equal to the fiber Brillouin frequency shift, counter-propagate along the fiber, a BDG can be excited through simulated Brillouin scattering (SBS), and another orthogonally polarized probe wave injected into fiber is used to detect the BDG. When the frequency difference between the probe wave and the co-propagating pump wave meets the phase-matching condition, the maximum reflection on probe wave from the BDG can be obtained. The interaction of the excitation and the probing of a BDG involves four optical waves, and the Brillouin-enhanced four-wave mixing model completely describes this coupling process. In the following sections of this chapter, the theoretical operation principles, numerical simulations, and experimental implementation of distributed phase birefringence measurement with BDG are described; some sensing applications of distributed birefringence measurement with BDG are also given including simultaneous distributed temperature and strain measurement, distributed transverse pressure sensing, and distributed hydrostatic pressure sensing.