Investigation Of Brillouin Scattering Characteristics In Multi-Mode Lithium Niobate Fiber

Stimulated Brillouin scattering（SBS）is an important nonlinear effect in optical fibers that has been applied in multiple fields.The rapid development of communication technology has raised higher demands on communication capacity.The capacity of single-mode fiber（SMF）can no longer meet the needs of global communication.The development of space division multiplexing（SDM）technology has made multi-mode fiber（MMF）a widely studied as a new platform for multi-channel parallel transmission.MMF can transmit many modes and solve the capacity barrier of SMF.The advancement of manufacturing and material technology has led to many micro-devices and nano-materials being used to enhance the SBS effect in various aspects.Lithium niobate（Li Nb O₃,LN）is a promising nonlinear photonic material.Based on its high refractive index,the large refractive index difference between the LN fiber cladding and core can stimulate strong photon-phonon interaction,achieving a large Brillouin gain.Based on the electro-optic effect of LN,LN fibers can be directly modulated by connecting to fiber lasers,reducing losses and improving system integration.Therefore,it is of great importance to study the Brillouin scattering characteristics of different transmission modes in lithium niobate multimode fiber（LN-MMF）.This paper discusses and analyzes the optical nonlinear SBS effect and propagation modes of LN-MMF,and uses COMSOL software to construct a 2D model of LN-MMF.The finite element analysis method is used to numerically simulate the distribution of optical and acoustic fields in LN-MMF,and then the Brillouin characteristics of different optical modes are analyzed.The main achievements of this paper are as follows:A theoretical analysis model of Brillouin scattering in lithium niobate multimode fibers has been established,and the finite element analysis method has been used to solve the optical（acoustic）field distribution of lithium niobate multimode fibers.The refractive index variation law of high-order optical modes is briefly analyzed,and the effective area of acousto-optic,which is one of the influencing factors of the Brillouin peak gain,is studied.Brillouin scattering of the intra-mode（the combination of pump light-signal light mode in the same mode）has been numerically simulated.The Brillouin gain spectrum（BGS）by the interactions of intra-mode is composed of the BGS of the L_0n acoustic mode,and all are the first three-order acoustic modes.The Brillouin frequency shift（BFS）corresponding to the scattering peaks of the four optical modes varies in the same pattern.The Brillouin frequency difference between any two optical modes is in the MHz level and basically consistent.Among the four optical modes,the HE₁₁^（0）0))-HE₁₁^（0）0))optical mode has the highest Brillouin gain for the L₀₁ acoustic mode,reaching 1.205 m^-1?W^-1,which is much higher than Brillouin gain of the traditional silica multimode fiber in the case of intra-mode.For any optical mode,the same distribution of acoustic modes corresponds to the same acoustic velocity.Brillouin scattering of the inter-mode（the combination of pump light-signal light mode in the different mode）)has been numerically simulated.The BGS by the interactions of inter-mode is mainly composed of the L_1n and L_2n acoustic modes,which are both the first and second order acoustic modes.Scattering peak 2（Peak 2）corresponds to a broader range of Brillouin frequency than scattering peak 1（Peak 1）,and the difference in Brillouin frequency between the same acoustic modes is consistent.Among the six optical mode pairs,the HE₁₁^（0）0))-TE₀₁^（）optical mode pair has the highest Brillouin gain for the L₁₁ acoustic mode,reaching0.620 m^-1?W^-1,which is slightly lower than the highest value of Brillouin gain in the case of intra-mode and higher than the Brillouin gain in the traditional silica multimode fiber mode(0.1290 m^-1?W^-1);For any optical mode pair,the acoustic velocity of Peak 2 is greater than that of Peak 1.