Transverse Mode Instability In Few-mode Fiber

Fiber lasers have beed widely utilized in industrial processing,medical and other industries due to its good beam quality,high energy conversion efficiency,high stability,small size,light weight,etc.However,the emergence of transverse mode instability has become one of the extraordinary limiting factors on the widespread utilization of high power fiber lasers.In order to suppress the nonlinear effect and gain high-power laser,large-mode-area is demanded.The physical characterization of transverse mode instability in few-mode fiber is distinct from single-mode fiber because of its multi-mode guiding character.Therefore,in this paper,based on the transeverse mode instability effect in few-mode fiber amplifier,its characterization and suppression methods are theoretically and experimentally studied.This paper mainly contains:1.It is pointed out that one of the limitation of high power fiber laser power boosting is transeverse mode instability.The physical mechanism of mode instability and its influence on fiber output characteristics are introduced.The theoretical researches on the internal mechanism of mode instability are summarized.The method of mode instability suppression in experimental and theoretical researches are clarified.The significance of studying transeverse mode instability effect in few-mode fiber is illuminated.2.The status quo of theoretical model in mode instability researches at home and abroad is briefly introduced.Based on the classical mode coupling theory,an elementary mathematical model is established for the energy transfer of three modes in few-mode fiber,and the relationship between different physical parameters and transeverse mode instability is analyzed..3.Detailed experimental studies on output power,spectrum,time and frequency domain traces and beam quality of transeverse mode instability effect in 25/400?m and 30/600?m few-mode fiber amplifiers are carried out.The physical characterization of transeverse mode instability in the few-mode fiber is summarized.4.The effects of signal light power,signal beam quality,pump structure and pumping power distribution,and pump/signal light wavelength on mitigating mode instability are studied in different few-mode fiber amplifiers.The influencing factors and suppression effects are discussed with comprehensive comparison.Based on relevant research results,3kW near diffraction limited fiber laser and its stability test within 10 hours is carried out,which is expected to move from laboratory to application.Relevant results provide valuable reference for transeverse mode instability mitigation in few-mode fiber amplifier.