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

This paper presents a novel technique for the distributed measurement of the modal birefringence in a few-mode fiber (FMF). The method exploits two different phenomena 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 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 measurements have been carried out over a two-mode graded-index FMF, using two photonic lanterns to selectively excite the desired spatial modes. We use the BFS measurements to derive the ERI difference between the LP01 and LP11 mode groups, while the spatial oscillations of the Brillouin gain reveal the birefringence between the vector components (TE01, TM01 and HE21) of the LP11 mode group. The experimental measurements are partly supported by full-vector finite-element-method (FEM) simulations. The reported method may also find application in the field of distributed sensing, by taking advantage of the dependence of modal birefringence from physical parameters such as strain and temperature.