The Study Of Photodarkening Effect And Its Influence On Mode Instability In Ytterbium Doped High Power Fiber Lasers

Due to the advantages of high conversion efficiency,excellent beam quality and compactness,Yb-doped fiber laser are wildly used in industrial and military fields.However,photodarkening effect in Yb-doped fibers would lead to the degradation of laser performance and reduce the long-term stability of the system.In this work,photodarkening effect was investigated in high power Yb-doped fiber lasers.The thermal load induced by photodarkening and its impact on the output power and lasing mode were explored.The preparation technology of fiber was introduced,including modified chemical vapor deposition process and fiber drawing process.Meanwhile,the key points in the fiber fabrication process were analyzed.As for fiber performance characterization,the basic parameters and testing technology were expounded.The measurement scheme of photodarkening effect was established,which was characterized by photodarkening-induced excess loss.The influence of Yb³⁺ions concentration,inversion and temperature on photodarkening effect was analyzed.It was found that when the temperature increased,the photodarkening rate and the excess loss raised,and the ratio between the excess loss at the visble band and 1?m decreased.The impact of photodarkening effect on the thermal load in short cavity fiber lasers was investigated,and the influence of Yb³⁺ion concentration on photodarkening-induced thermal load was analyzed.For suppressing photodarkening effect,different approaches were applied,and the effect on thermal load were also explored.Photodarkening effect and the resulted thermal load could be effectively suppressed by H₂-loading,while it would introduce additional heat source by co-doping with Ce ions.A theoretical model was established to explore the impact of photodarkening effect on mode instability phenomenon.The results showed that photodarkening effect could decline the mode instability threshold,and counter-pumping or reducing the concentration of Yb³⁺ion could help to alleviate it.In high power system,the connection between photodarkening effect and mode instability was explored.For the pristine fiber,long term operation was conducted in a kilowatt-class fiber amplifier.Photodarkening-induced core laser leakage phenomenon appeared during the operation,and the output power fluctuated quasi-periodically with amplitude exceeded 100 W.After 300 minutes of operation,the mode instability threshold decreased by 19.8%.In view of the excellent performance of H₂-loading in mitigating photodarkening effect and the resulted thermal load,the influence of H₂-loading on high power fiber amplifier was explored.For the H₂-loaded fiber,long term operation was also conducted in a kilowatt-class fiber amplifier,and the output power remained stable with no sign of mode instability.Via H₂-loading,the performance of high power fiber amplifiers was significantly improved.Finally,a 3 k W fiber amplifier was constructed with the H₂-loaded fiber,and it operated steadily at 2600 W for an hour with M² factor of 1.4.532 nm photobleaching was proposed to eliminate the exsiting photodarkening effect,and the excess loss at 702 nm could be erased by 97.5%.532 nm photobleaching showed good application prospects in high power systems.Through 532 nm bleaching,the slope efficiency for the gamma-irradiated fiber recovered from 37.1%to 67.9%with the mode instability threshold increases by more than1.5 times.The effect of photobleaching on quasi-static mode degradation was investigated.Quasi-static mode degradation was observed in the gamma-irradiated fiber.The beam quality deteriorated with the increase of pump power,and the beam profile presented LP₁₁ mode shape with the M² factor increases to 2.30.By532 nm bleaching,the quasi-static mode degradation phenomenon was suppressed and the the beam profile recovered to LP₀₁ mode shape with the M² factor of 1.65.Based on the results of electron paramagnetic resonance measurements,the mechanism of photodarkening and photobleaching effect was discussed and it was verified that Al-Oxygen hole centers played an important role in the process.The aforementioned research results provided technical support for further power scaling of high power fiber laser and its long term operation with high power stability and mode stability.