Distributed modal birefringence measurement in a few-mode fiber based on stimulated Brillouin scattering

We present a novel method to characterize the modal birefringence in a few-mode fiber (FMF), based on stimulated Brillouin scattering. The method exploits two phenomena which can be observed in distributed Brillouin measurements: the dependence of the Brillouin frequency shift (BFS) on the effective refractive index (ERI) of the interacting optical beams, and the spatial oscillations of the Brillouin gain along the FMF deriving from multimodal interference. Using both phenomena, a wide range of ERI separations can be measured, from ≈ 10 -7 to 10 -2 or more. The method has been demonstrated over a two-mode graded-index FMF, using two photonic lanterns to selectively excite the desired spatial modes. The birefringence values obtained using the proposed method have been validated using a different distributed technique, based on the analysis of the fiber's Rayleigh-backscattered trace acquired in the frequency domain.