Reserch On Yb-doped New-type Multi-clad Active Fiber

Since the 21st century,high power fiber laser has been leading the laser technology innovation with various technical advantages and substitutes for conventional lasers.However,the development of high-power fiber lasers has been stagnating in recent years,which is mainly due to the nonlinear effect caused by high power density and longer fiber length used in laser system,the mode instability induced by thermal effect,and the inherent photodarkening(PD)of fiber materials.Here,to eliminate these limit factors,this dissertation fully demonstrates comprehensive properties of traditional 20/400?m Ytterbium(Yb)-doped double-clad active fiber,regarded as the key component of high power fiber laser.The research results show that,it is difficult to achieve laser output power above 3k W under the limitation of fiber size,difficult to obtain high Yb ion doping concentration and high stimulated Raman scattering effect threshold under the limitation of numerical aperture(NA),and difficult to achieve excellent power stability under the limitation of polymer clad.In this case,we proposed and emphatically demonstrated three new types of Yb-doped multi-clad active fiber including 30/600?m double-clad active fiber,pedestal active fiber,and triple-clad active fiber.The main creative and innovative research results are listed as follows:(1)In order to achieve low PD effect,aluminophosphosilicate(Al₂O₃-P₂O₅-Si O₂,ternary APS)is selected as the matrix material of the designed and fabricated 30/600?m Yb-APS double-clad active fiber.Thanks to Al³⁺/P⁵⁺molar ratio of a little over 1,low core NA is obtained and central dip of refractive index profile is successfully removed.Compared with 20/400 Yb-APS fiber,30/600Yb-APS fiber has larger fiber size,contributing to aggregating more pump power,and effectively improves the fiber nonlinear effect threshold.With a master oscillator power amplifier configuration(MOPA),30/600 Yb-APS fiber presents 5.19k W laser output at 1064.4nm with slope efficiency of85.2%and intense nonlinearity suppression.The Yb-APS fiber-based MOPA system is kept at5.16k W output for over 600 minutes,and no sign of power degradation is observed.It implies excellent laser stability and low PD effect.So,30/600 Yb-APS fiber is highly suitable for developing5k W-level commercial high-power fiber laser.(2)In order to obtain high Yb-doped concentration,we designed and fabricated an Yb/Cerium(Ce)-codoped pedestal active fiber.Yb/Ce-codoped aluminosilicate(Al₂O₃-Si O₂,AS)core and germanosilicate(Ge O₂-Si O₂)pedestal layers to be capable of heavily doping Yb³⁺ions,the molar percent of Yb³⁺is as high as 4450ppm,while the core effective NA is controlled at?0.08.Numerical simulation shows that,five LP-modes can survive and propagate into the fiber core,while,the possibility of obtaining single-mode laser operation in a proposed pedestal fiber by designing low core NA of 0.054 with respect to pedestal layers and choosing small bending diameter of 46.8mm.Absorption coefficient at 915nm is test to be 3.66d B/m,more than 5 times than that of commercial Nufern-20/400-9M Yb/Ce-AS fiber.We applied the fiber test in a fiber amplifier setup,it can be found the heat at fusion junction is severe,while there is no obvious hot spot in the fiber itself.So it can deduce the chelate rare earth ion doping technology could effectively achieve uniform high concentration rare earth ion doping.(3)In order to improve the power stability of laser system,Yb-APS triple-clad active fiber is designed and fabricated.Triple-clad active fiber can effectively mitigate the shortcomings of low-refractive-index polymer clad of double-clad active fiber,such as poor heat resistance and poor light striking resistance.P⁵⁺/Al³⁺molar ratio in the central region of fiber core is specially designed to be?1.08 for compensating P₂O₅evaporation,while the ratio in the outer region is set as?0.92 for obtaining a flatten refractive index profile.Applying a two-stage deposition process,one-time collapse technique,low-temperature multi-time sintering and suitable flow of pure POCl₃during sintering and collapsing process are instrumental in decreasing and offsetting element evaporation.Measured in an all-fiber MOPA configuration,the 7-m-long fiber obtains 1.39k W near-single-mode laser output at 1079.6nm with a slope efficiency of 85.2%and a beam quality M²of 1.36.Output laser spectrum with no sign of nonlinear-related peaks and a narrow 3d B bandwidth of?0.33nm is obtained.The fiber-based laser setup is kept at the maximum power for 1 hour with power degradation less than 0.21%.The results demonstrate Yb-APS triple-clad fiber is a highly competitive candidate for high-power fiber laser.