Investigations Of Photodarkening Suppression Methods In Ytterbium Doped Fibers

Ytterbium(Yb)-doped fiber lasers are requested higher stability of output power,as the output power and application areas continue to increase.However,photodarkening become a main limiting factor when Yb-doped fiber lasers work for a long time.It reduces the output power and leads to the instability of Yb-doped fiber lasers.In this paper,based on the experimental devices for rapid detection of photodarkening,808 nm photo-bleaching and Ce ions influence on photodarkening and thermal bleaching are investigated.Firstly,energy level structure and fluorescence characteristics of Yb3+ ions are introduced in this paper.Then,the modified chemical vapor deposition and solution-doping technique and the performance parameters of the optical fiber are briefly introduced.Moreover,the three photodarkening mechanisms proposed in the literature are analyzed theoretically and the current suppression methods of photodarkening are also summarized.Secondly,the darkened Yb-doped fiber is pumped by an 808 nm laser diode.It is found that photodarkening of Yb-doped fiber can be bleached by an 808 nm laser diode.The excess loss decreases faster,as the output power reaches up to a higher value.The excess loss which is photobleached is linear with the output power of 808 nm laser diode.When the output power of 808 nm laser diode is 3 W,47.9% of excess loss is reduced.Thirdly,a series of Yb/Ce co-doped fibers with different concentrations of Ce ions are prepared and the photodarkening is measured respectively.It is clear that a small number of Ce ions can effectively reduce the excess loss induced by photodarkening.As Ce ion concentration increases,the excess loss gradually decreases.Ce ions co-doping in Yb-doped fiber slightly dwindles the laser properties.Considering the effects of Ce ions on photondarkening and laser properties of Yb-doped fiber,the Yb/Ce concentration ratio of 2: 1 is considered as the optimal concentration.The photondarkening is suppressed by 85.4% and the laser slope efficiency is 68.45%.Fourthly,the three fibers of different Ce concentrations are darkened to the same extent and then are heated.The excess loss caused by heating in Yb / Ce co-doped fiber is less than that of Yb-doped fiber.Ce ions can restore the darkened precursors and inhibit the formation of the color center.Furthermore,Ce ions codoping can decrease the high energy state of the color center.As a result,Yb/Ce co-doped fiber returns to its pristine level at 400 ?.However,Yb-doped fiber is completely bleached at 600 ?.