Study On Power Improvement Of Monolithic Ytterbium-doped Fiber Laser Oscillator

Owing to the advantages of good beam quality,high conversion efficiency,compact structure and convenient thermal management,fiber lasers have been widely used in industrial processing,intelligent manufacturing,national defense and military fields.Compared with the fiber amplifier,the fiber laser oscillator has the advantages of simpler structure,convenient operation,strong anti-reflection ability,which is more popular in industrial applications.However,due to the stimulated Raman scattering(SRS)and transverse mode instability(TMI)in the fiber,the output power of high power all-fiber laser oscillator is limited.Based on this,this thesis takes ytterbium-doped fiber(YDF)laser oscillator as the research object,studies the limiting factors of power scaling in all-fiber laser oscillator theoretically and experimentally,and provides guidance for obtaining higher power of all-fiber laser oscillators.The main contents of this paper are as follows:The development of fiber lasers is reviewed,and the structure of fiber amplifier and fiber laser oscillator is compared.In this thesis,the development of fiber laser oscillator is summarized from space-coupling structure and all-fiber structure,and the limiting factors,SRS and TMI effect in the process of power enhancement of fiber laser oscillator are analyzed.Based on the traditional optical fiber rate equation,a comprehensive physical model of the oscillator including the SRS,fiber bending loss and mode coupling is established.Based on the physical model,the influence of mode coupling coefficient on the output performance of the fiber laser oscillator is analyzed.Combined with the optimization of the gain fiber structure parameters,the thesis analyzes the limited power increase of the core diameter graded gain fiber in the fiber oscillator,and finally analyzes the influence of the fiber fusion quality on the fundamental mode loss of the oscillator.The simulation results show that the axial offset and tilt of the fiber fusion have much greater loss on the fusion than the fiber parameters themselves.The experimental results show that the fusion of different gain fibers results in a large number of high-order mode excitation,and the optical effeciency decreases in the presence of cladding filtering.In addition,a high-power all fiber laser oscillator experimental platform is established based on the domestic confined-doping gain fiber.The experimental results show that the TMI threshold can be increased by backward pumping and bidirectional pumping,and the maximum output of 1224W laser can be achieved when bi-directional pumping scheme is employed.Based on the conventional 25/400?m and 30/600?m YDF,a high-power all fiber laser oscillator is constructed.In the 25/400?m fiber laser oscillator,by eliminating the external feedback,the highest output power of 5070W is achieved and the Raman light is?35dB lower than the signal laser and the beam quality is M~2?1.6.The 5kW oscillator is tested for 5 hours of long-term operation The results show that the output performance of the oscillator is stable and has engineering potential.In a 30/600?m fiber laser oscillator,three schemes of power enhancement are studied:forward pumping,bi-directional pumping and backward pumping.When bi-directional pumping scheme is adopted,the maximum output of 3600W is achieved with no sign of SRS and TMI effects.The experimental results show that the all-fiber laser oscillator based on commercial 30/600?m fiber has the ability to achieve higher power output.