Research On Beam Cleanup Based On Stimulated Brillouin Scattering In Multimode Fiber

The process of stimulated Brillouin scattering(SBS) in optical fibers has been widely investigated by many researchers. On one hand, efforts have been reported to mitigate the SBS effect. On the other hand, the SBS effect has been implemented in many fields such as sensors, optical-delays and phase conjugation. Additionally, the ―beam cleanup‖ effect caused by SBS which can transfer a poor-quality pump beam into a high-quality fundamental mode Stokes beam is very attractive. However, the beam cleanup effect is contradicted to the traditional knowledge that the Stokes beam should be conjugated to the pump beam. Since the SBS beam cleanup effect is observed in 1993, many reseachers have been engaged in the work but the mechanism is still unclear until now. This dissertation accomplish a systematic study on the SBS beam cleanup effect in multimode fibers. A theoretical model is established to give the explanation of the mechanism behind the SBS cleanup effect. Then, based on the theory, some essential technologies have been improved and a high quality and high pump conversion efficiency SBS cleanup have been achieved.Firstly, we establish a three-dimensional SBS coupling wave equation which is adaptable to analyze the intermodal SBS process. The equation has the ability to analyze the different effects between different modes in SBS process where the traditional equation failed. The acoustic wave equation is derived from elastodynamics theory which is more suitable for fiber medium. The distinction between the SBS process in multimode fiber and bulk medium is presented by the solustions. The concept of theintermodel gain coefficient is introduced for the first time and is applied to analyzing the differences of the Brillouin gains between different modes.Secondly, the numerical simulation of the coupling wave equation is engaged to demonstrate the distribution curves of Brillouin gain in step-index and graded-index fibers. The effect of fiber parameters on intermodal Brillouin gains is predicted by theoritical results. According to the simulation results, we propose the mechanism of SBS beam cleanup and phase conjugation and demonstrate it by experiments.Finally, the theoritical results are used to guide the experiments that subsequently realized a high quality and high pump conversion efficiency SBS beam cleanup. Factors affecting the pump conversion efficiency have been studied and the application prospects of SBS beam cleanup discussed.