Research On The Key Devices And System Techniques For High Power Fiber Lasers

High power fiber laser is widely used in laser processing,laser medical treatment,laser communication and military defense and other fields due to the advantages of good beam quality,high conversion efficiency,compact structure and convenient thermal management.However,due to the constraints of fiber coupler,gain fiber,nonlinear effects and integration technology,it is difficult for single fiber to obtain high output power.The theoretical and experimental research,such as high power fiber coupler,nonlinear effects and system technology has been carried out.The main contributions of the dissertation are as follows:1)Based on the rate equation theory,the influence of fiber laser output power on the pumping wavelength,the active fiber length and Yb³⁺doping concentration has been studied.Using the fiber coupler and the Yb/Ce co-doped fiber in the forward pumping fiber amplifier,the power output of 4620 W laser output with a slope efficiency of 80.3%was obtained and the beam quality is 1.67.2)Using the beam propagation method,the transmission characteristic of both pumping light and signal laser in the tapered fiber waveguide were studied.And the effect of transmission efficiency for tapering fiber bundle,fiber-end treatment and fiber fusion was also analyzed.The results show that the single arm load capacity of fiber coupler was more than 800 W and the average pump coupling efficiency was94.83%.Furthermore,the backward signal attenuation from pumping fiber at 1080nm was more than 20 dB.3)Nonlinear effects in the high power fiber lasers were theoretically and experimentally investigated.When the phase matching conditions is satisfied in the high power fiber laser,the four-wave mixing between fiber lasers occurs at 1060 and1100 nm,respectively.With gradually increase in the laser intensity,the stimulated Raman scattering wavelength peaking at 1130 nm appears and partially power begins to transfer into this wavelength.The main sources of high-order mode lasers in fiber lasers are analyzed.By reducing the coiling diameter of Yb-doped fiber,the threshold of four-wave mixing effect was upgrated to more than 3.1 kW.And the effects of the pump wavelength,the active fiber length and core diameter on stimulated Raman scattering were studied theoretically.By optimizing the parameters of Yb-doped fiber in high power fiber laser,the threshold of stimulated Raman scattering effect was promoted to more than 3.4 kW.