Research On Suppression Of Transverse Mode Instability Effect Of Fiber Laser Based On Pump Optimization

Fiber lasers have the advantages of good beam quality,high electro-optical conversion efficiency,large power-to-volume ratio,strong environmental adaptability,and a wide range of application light,which have a wide application prospect in various fields such as industry,scientific research,and medical treatment.However,the emergence of the transverse mode instability(TMI)effect has limited the development of fiber lasers to higher output power and beam quality,and also limited the application range of fiber lasers.In this paper,we investigated the TMI suppression method for fiber lasers based on pump wavelength optimization.Firstly,the great potential of the method to suppress the TMI effect is illustrated by theoretical simulation,and then the method is experimentally verified at low-power and high-power conditions,respectively.The theory starts from the thermal effect,which is the main factor inducing the TMI effect of fiber lasers,to suppress the thermal effect of fiber lasers as the entry point for suppressing the TMI effect.By analyzing the heat of the fiber laser,it is pointed out that the maximum temperature variation in the gain fiber of the fiber laser when pumped at different wavelengths is highly fitted to the pump absorption coefficient curve,and a method is proposed to deviate the center wavelength of the pump source from the 976 nm ytterbium ion absorption peak to reduce the maximum temperature of the gain fiber and suppress the TMI effect.Experimentally,a relatively low-power fiber laser TMI threshold testbed was first designed to test the TMI threshold of fiber lasers when pumped by different wavelength pump sources.Compared with traditional wavelength-stabilized 976 nm LD,the TMI threshold of the fiber laser was increased by about 2.2 times and 3.46 times when the optimized wavelength-stabilized 981 nm LD and 969 nm LD pump sources were used,respectively.We experimentally verified that the feasibility and great potential of a scheme to optimize the pump wavelength to reduce the pump absorption coefficient to enhance the TMI threshold of fiber lasers.Subsequently,we built a relative high-power fiber laser amplifier based on the Master Oscillator Power-Amplifier(MOPA)structure.and a maximum power output of5030 W was obtained when using the wavelength-stabilized 981 nm laser diodes(LDs)bi-directionally pumped,with the output power limited by the stimualed Raman scattering(SRS)effect.A maximum power output of 4073 W was obtained when using the wavelength-stabilized 969 nm LDs counter-pumping,with the output power limited by the pumping power.The paper also presents the concept of equivalent absorption cross-section to explain the difference in TMI thresholds of fiber lasers when pumped at wavelength-stabilized 976 nm LD and non-wavelength-stabilized 976 nm LD.The TMI effect suppression capability of the optimized new wavelength pump source at high power was verified.