The Study On Photo-darkening Effect In Ytterbium Doped High Power Fiber Lasers

Due to the high efficiency,high beam quality,excellent thermal management,reliable properties and compact structure,high power fiber lasers have been greatly developed and applied in fields of industrial processing,medical treatment,military,communication,etc.However,the output power exhibits instability,which was induced by photo-darkening effect in Yb-doped fibers and hinders the further development of fiber lasers.This thesis mainly focuses on the photo-darkening effect.Based on the photo-darkening mechanism and the self build excess loss measurement setup,the methods to suppress photo-darkening effect were studied and we finally got high power Yb-doped fiber laser with stable output level.Firstly,the influences of Yb ions characteristic,codoping ions,and fiber structure on fiber properties including numerical aperture,mode area,background loss and absorption efficient were studied.Based on the modified chemical vapor method as well as solution doping,the fabrication process of double cladding Yb-doped fibers was introduced.Furthermore,the gas composition,flow,reaction temperature during the barrier layer and loose layer were also discussed.The soak time and dehydration technology take effect on fiber laser properties.Several methods were proposed to represent the photo-darkening effect.The spectrum measurement setup was built up to measure the photo-darkening induced excess loss.The impacts of Yb ions concentration and inversion level on excess loss were analyzed.The excess loss between visible light and laser wavelength shows linear relation.The excess loss at 633,702 and 810 nm is 40.08,25.69,and 8.73 times of that at 1041 nm,respectively.The methods to mitigate photo-darkening effect were studied.Firstly,through doping cerium,sodium or lithium ions into the fiber core,the excess loss could be effectively suppressed.About 50%loss was decreased when the concentration of Yb:Ce was 4.5:1 and 10%self bleaching in Yb/Al/Ce fiber was observed.When doping sodium or lithium ions,30%excess loss was reduced without any numerical aperture,slope efficiency and background loss deterioration,which has been observed in Yb/Al/Ce fibers.Secondly,the 793 nm was proposed as the photo-bleaching wavelength,which can bleach 68%of the existed loss and was a repeatable process.The bleached loss showed linear relation with 793 nm pump power.When pumping the Yb-doped fiber with 915 nm and 793 nm LDs simultaneously,more than 80%loss was reduced.Besides,the Ce ions doping can accelerate the photo-bleaching process.The bleached excess loss at 702 nm and 810 nm for Yb/Al/Ce fiber was 30%and 37.5%,respectively.At last,the H₂-loading method to suppress photo-darkening effect was discussed.The H₂-loading processed Yb-doped fiber was tested under KW output power level.The power was maintained at 1630 W with 1.69 beam quality for one hour and showed no power or beam quality deterioration,achieving high power and excellent beam quality stability in lower photo-darkening effect Yb-doped fiber.Based on the methods above to suppress the photo-darkening effect,the hypothesis was proposed that there might be several reasons to cause photo-darkening.The methods including codoping with ions and H₂ loading confirm the charge transfer absorption bands mechanism.Through further research on Tm impurity during the formation process of color centers,it was assumed that Tm impurity take effect in photo-darkening.