Research On Photo-darkening Effect Of Ytterbium-doped Fiber

With the rapid development of Yb-doped fiber lasers,photodarkening has become one of the important factors restricting the increase of its power.In order to solve this problem,based on the photodarkening effect of Yb-doped fiber,based on its composition and photodarkening mechanism,by optimizing the matrix composition and doping Ce₂O₃ with different concentrations,Yb-doped aluminum-phosphorous-silicon ternary system optical fiber and Yb/Ce co-doped aluminum-silicon binary system optical fiber were prepared by using an modified chemical vapor deposition combined with chelate gas phase deposition.By testing the excess loss of photon darkening effect,the microscopic mechanism of P₂O₅ restricting photodarkening effect is analyzed,which provides a theoretical basis for high power fiber lasers to suppress photodarkening.The main content of the thesis includes:(1)Based on the modified chemical vapor deposition combined with chelate gas phase deposition technology,the preparation process of Yb-doped active optical fiber preform and fiber drawing process are introduced.The optical properties of the preform are analyzed by testing the element distribution,refractive index distribution,fluorescence performance,absorption coefficient,and fiber core loss of the optical fiber preform.The partial P₂O₅ and SiF₄ volatilization and partial ion migration in the melting and shrinking process lead to less distribution of the central component content of the preform core,resulting in uneven of refractive index distribution.Simultaneously,doping P₂O₅ affects the luminous performance of Yb-doped fiber and the background loss of fiber core.(2)The photodarkening effect of Yb-doped aluminum-phosphorous-silicon ternary fiber was induced by 976nm semiconductor laser.The affect of different P₂O₅ and Al₂O₃ doping concentration ratio on photodarkening was systematically studied.According to the stretching function,the relationship between the excess loss of photodarkening and time is fitted.The experimental results show that when the P/Al ratio is 1.18,the excess loss in equilibrium state is only 1.05dB/m,and the photodarkening rate is 4130min.The study found that short fiber and high-power pump light led to a high level of particle reversal of Yb-doped aluminum-phosphorus-silicon ternary fiber,which increased the excess loss of equilibrium state and accelerated the rate of photodarkening.Yb/Ce aluminum-silicon binary system optical fiber photodarkening excess loss experiment shows that when the ratio of the doping concentration of Ce₂O₃ and Yb₂O₃ is 0.62,it's anti-photodarkening ability is the strongest,and the excess loss in equilibrium reaches 7.53dB/m,but the photodarkening rate reached 42 min.(3)According to the cooperative luminescence spectrum and laser Raman spectrum,Al₂O₃ and P₂O₅ Yb-doped aluminum-phosphorus-silicon ternary system fiber can suppress the Yb³⁺ cluster effect.The probability of energy up-conversion is reduced,and the number of short-wavelength photons induced to form a darkening color center is reduced,thereby achieving the purpose of suppressing the darkening effect of the photon.However,excessive P₂O₅ can generate new defects and reduce photodarkening performance.In the Yb/Ce co-doped aluminum-silicon binary system fiber,Ce³⁺ suppresses the formation of AlOHC by trapping holes,and Ce³⁺⁺ traps electrons to suppress the Yb³⁺ valence state change,which can overcome the photodarkening effect to a certain extent.The essence of Yb-doped fiber in two systems to suppress photodarkening is to reduce the number of photodarkening color centers by co-doping ions,so by further optimizing the Yb-doped fiber core component conditions,highperformance laser fibers with excellent performance can be prepared.This provides a theoretical and experimental basis for the development of domestic high-power lasers.