The Study On The Suppression Of Photodarkening In High Power Fiber

The development of high power fiber lasers has been changing with each passing day and month.With the continuous expansion of the application field,more attention is paid to the output stability of the lasers.However,the photodarkening?PD?effect becomes an important factor restricting the high power and high stability output of fiber lasers.By summarizing the research on PD at home and abroad,it involves the characterization method,mechanism and suppression method.However,the mechanism of PD is still inconclusive,and all the suppression methods have their advantages and disadvantages.Therefore,this paper focuses on the PD effect in high power fiber,guided by the mechanism,based on the characterization method,and conducts in-depth theoretical and experimental research on its suppression method.The main content is as follows.1.The research on the mechanism and characterization methods of PD effect in high power fiberIn terms of mechanism,three models with high recognition were summarized and presented,including the oxygen defect center?ODC?model,charge transfer?CT?transition model and thulium impurity model.At present,the mechanism of PD is still under discussion.Through analysis,the direct cause of PD was color centers,and the formation and type of color centers were not single,but might be the coexistence of multiple processes and multiple types.In terms of characterization methods,the advantages and disadvantages of existing characterization methods were analyzed,and two sets of characterization methods based on full-fiber structure combined lock-in amplifiers were proposed.One is a single wavelength characterization method for single-clad fiber,which could realize the fast measurement of the effect of PD on the transmission power of single-mode fiber.The other is the multi-wavelength characterization method for double-clad fiber,which could realize the rapid measurement of PD-induced excess loss in double-clad fiber.The PD-induced excess loss in equilibrium state could be obtained by the fitting of stretching exponential function,which acted as a criterion to compare and evaluate the PD degree of different ytterbium-doped fibers.2.The suppression effect of alkali metal ions co-doping on PDSodium ions co-doped fiber was prepared by modified chemical vapor deposition?MCVD?combined with solution doping technique.It was found that the sodium ions co-doped fiber had a good effect of mitigating PD effect and had no effect on the numerical aperture?NA?of the fiber core.Moderate doped concentration of sodium had no effect on the slope efficiency of the fiber laser,but high doped concentration could decrease the slope efficiency obviously.Following the consideration of the advantages of lithium ions in atomic radius and relative molecular mass,the mitigation of PD by co-doping of lithium ions was studied.Four groups of lithium ions with different concentration were set.The lithium ions could also effectively reduce the PD-induced excess loss.As the doping concentration of lithium ions increased,the PD-induced excess loss appeared to be decrease initially and then increase.The co-doped fiber of lithium ions could eliminate the PD-induced excess loss at most 28.57%@702 nm and 36.02%@810 nm.In the case of high doping concentration of lithium ions,due to the serious damage of lithium ions to the glass matrix,there is no effect of PD mitigation,and the slope efficiency is seriously deteriorated.To analyze the mechanism of mitigating PD effect by alkali metal ions co-doping,alkali metal ions provided more non-bridging oxygen?NBO?for the fiber core,improved the hypoxia environment in the fiber core with high doping of ytterbium ions,effectively mitigated the formation of dislocation bonds by Yb³⁺ions,and thus reduced the formation of ODC.This mitigation method also verified the ODC model.3.The thermal bleaching?TB?of ytterbium-doped fiber and co-doped fiberFirstly,non-isothermal bleaching of the photodarkened Yb-doped fiber and Yb-Li co-doped fiber was carried out.TB could completely bleach the PD-induced excess loss.It had real time bleaching effect on the excess loss in the visible wavelength.However,the measurement of the excess loss in the near-infrared wavelength was significantly affected by high temperature,and the bleaching effect was shown after cooling to room temperature.Compared with non-isothermal bleaching of Yb-doped fiber and Yb-Li co-doped fiber,the PD-induced excess loss of the two was the same as the change of temperature,which indicated that the lithium ions co-doping had no contribution to reducing the activation energy of TB.The reason we analyzed was related to the mitigation mechanism of alkali metal ions.Alkali metal ions could reduce the formation of ODC by introducing more NBO.Due to the existence of other color colors,lithium ions had no effect on the energy state reduction of color centers and had no influence on the TB process.To verify the above explanation,non-isothermal bleaching at different temperature ramp rates was carried out for Yb-doped fiber and Yb-Li co-doped fiber,and the bleaching activation energy of both was calculated.By comparing the non-isothermal bleaching with the ramp rate of 5?/min and 10?/min,we obtained that the fiber with a higher ramp rate,the higher the temperature required for complete bleaching of the PD-induced excess loss.By fitting the experimental results,bleaching activation energy of Yb-doped fiber and Yb-Li co-doped fiber was 1.288 eV and 1.262 eV respectively.The addition of lithium ions did not significantly alter the activation energy required for fiber bleaching.The observations further confirmed the suppression mechanism of the lithium ions co-doping,which is helpful to understand the principle and relation of the co-doping of ions and TB.4.Mitigation of PD in Yb-doped fiber by hydrogen-loadingBy using H₂ to process the original Yb-doped fiber,it was found that the PD-induced excess loss was reduced by nearly 100%at each wavelength and did not increase with pumping time.Next,the photodarkened Yb-doped fiber was treated with hydrogen-loading.Similarly,the elimination effect of the generated PD-induced excess loss was excellent.In order to obtain more samples without damage,we compared Yb-doped fiber under Gamma irradiation with pump irradiation,so as to study the mitigation effect of hydrogen-loading treatment on the fiber with generated color centers.After the same hydrogen-loading treatment,the spectrum was restored to the original fiber level,and the PD-induced excess loss was completely eliminated.It was found that the suppression effect of hydrogen-loading treatment had obvious degradation phenomenon.By re-measuring the fiber after hydrogen-loading treatment for one month,it was found that the excess loss occurred once again and the degradation of the suppression effect was obvious.To analyze the reason,the suppression mechanism of hydrogen-loading treatment was that under the excitation of pump light,H₂ molecules reacted with the holes on oxygen ligands,so that the hole-related color center was eliminated,therby suppressing PD.Since the hydrogen-loading treatment itself was a non-chemical reaction process,it was only when a pump irradiation was introduced that the H₂ molecule reacted with the hole.With the gradual diffusion of H₂molecules,there was a shortage of reactive H₂ molecules in the fiber core,and a large number of hole-related color centers remained,leading to the degradation of PD suppression.