Characterization of Brillouin scattering in a few-mode fiber

Abstract

The few-mode fiber can be used to transmit limited orthogonal modes, which has the advantages of small modular interference and easily controlled modes. The Brillouin scattering sensor based on the few-mode fiber can effectively reduce the cross sensitivity of multi parameter measurement, and realize the measurement of multi physical quantity. In this paper, based on the wave optics theory, the Brillouin scattering spectrum parameters of the step-index few-mode fiber are analyzed, such as frequency shift, line width and peak gain and so on. Firstly, the transmission modes of the few-mode fiber are analyzed. The finite element analysis result shows that there are 5 kinds of transmission modes:LP01, LP11, LP21, LP02 and LP31, and their effective refractive indexes are 1.4664, 1.4652, 1.4637, 1.4630 and 1.4616, respectively. Secondly, the mathematical models of the Brillouin frequency shift, line width and peak gain of different modes in the few-mode fiber are analyzed. Finally, the parameters of Brillouin scattering spectrum with different modes' superposition are also discussed. In the few-mode fiber, due to the different effective refractive index, the light of each mode is propagated along its respective path and interacts with the particles in the fiber, thus producing different Brillouin scattering spectrum. The simulation results show that the frequency shift of the Brillouin scattering spectrum of each mode is in a range of 10.19-10.23 GHz, and the frequency shift increases with the decrease of the mode order. The Brillouin line width of each mode is in a range of 32-34 MHz, and the line width also increaseswith the decrease of the mode order. Moreover, the relative amplitude of the Brillouin scattering gain spectrum increases with the decrease of the mode order. The mathematical models of this paper are used respectively to analyze the Brillouin scattering spectra of other types of step-index few-mode fibers. It is shown that the Brillouin frequency shift, Brillouin line width and peak gain of other types of step-index few-mode fibers also increase with the decrease of the mode order. In a step-index few-mode fiber, intramodal Brillouin scattering spectrum and the intermodal Brillouin scattering spectrum are both in line with the distribution of Lorenz curve. However, the intermodal Brillouin scattering spectrum of modes' superposition leads to the line width broadening of the Brillouin scattering spectrum, and the relative amplitude of the intermodal Brillouin scattering spectrum of modes' superposition being generally smaller than that of intramodal.