Research On Mode Instability Of High Power Single Mode Fiber Laser

The high power single mode fiber laser has been widely used in many fields such as military science,marking,cutting and soldering,due to its excellent features such as high quality of beam,high power output,good heat dissipating capacity and simple structure.Fiber lasers have high energy density and high cutting precision,so they gradually replace traditional machine tool processing.High-power fiber lasers are first used in defense,and the emergence of laser weapons and laser guidance has greatly enhanced the power of national defense.As a key part in the Made in China2025 plan,many researchers have conducted deep researches on fiber laser.At present,the output power of single mode fiber laser could reach tens of thousands.However,the mode instability phenomenon always occurs during the experiment,destroy the beam quality of fiber lasers,which hindered the further optimization of single mode fiber laser.Recent years,there are different opinions on the causes of the mode instability.In this paper,we carried out the theoretical and experimental researches on the mode instability by a self-made light stripper.The main work of this paper is listed as follows:(1)The theory and experiments of mode instability phenomenon are researched in this paper.The physical origin of mode instability and theoretical analysis of its influencing factors are described..Then explored the influence of the wavelength of pump light on the threshold of mode instability.The result showed that compared with 976 nm pump light,the 915 nm pump light could improve the threshold of mode instability.The effect of coiling radius on mode instability threshold is further investigated.It is found that reducing the coiling radius of gain fiber can improve the mode instability threshold of the system.Further,a bidirectional pump method is used.it was found that the bidirectional pump method could not only improve the threshold of mode instability,but also relieved the pressure of the fusion joint.(2)Based on the fabrication of optical fiber passive device and the analysis of threshold on mode instability,a 915 nm pumping light source and performed by bidirectional pumped method was adopted.The power of the fiber laser could reach to 2kW by designing the coiling radius of Ytterbium-doped fiber.In order to obtain high bean quality laser,the cladding power stripper made by ourselves was used,the attenuation of the stripper is 30 dB,By using a carbon dioxide laser to etch the optical fiber,the inner cladding of the optical fiber is etched to destroy the total reflection structure of the optical fiber,and finally a stripper having a high peeling degree is obtained.The beam quality is limited to 1.19,which means this research has achieved a satisfying result.