Photodarkening in Ytterbium doped silica fibers under 488 nm CW irradiation.
University of Southampton, Optoelectronics Research Centre,
Generation of high power 1178 nm laser line from an Yb-doped aluminosilicate fiber laser and subsequent frequency doubling for generating 589 nm laser, can be a smarter alternative to the more complicated approaches utilized for laser guide stars till date. Since high power Yb-doped aluminosilicate fiber lasers suffer from photodarkening, it is of great importance to understand photodarkening thoroughly and mitigate it accordingly.
Non-resonant 488 nm CW laser radiation induced photodarkening in Yb-doped aluminosilicate fibers was investigated as a major part of this project. The 488 nm CW irradiation induced excess background loss, in the visible (VIS) and near infrared (NIR) regions, was observed. Temporal evolution of excess background loss, in VIS and NIR regions, even after stopping the 488 nm irradiation, was also observed, using white light source, 638.7 nm laser diode source, He-Ne source and 1285 nm LED based OTDR accordingly. An Yb-doped phosphosilicate fiber was observed to be highly resistant to photodarkening. Photodarkening results in several aluminosilicate fibers with varied Yb-doping concentration were compared.
Effects of external heating, during and after 488 nm CW irradiation, were investigated. An important observation was the post-irradiation temporal growth in photodarkening which means that photodarkening doesn’t ‘freeze’ immediately after stopping the 488 nm CW irradiation. Variation of this post-irradiation loss dynamics with external heating was also studied.
NIR pump induced photodarkening was also studied to a certain extent and a bluish fluorescence from the fibers was observed, which could be related to Yb ion-pair effect. Effects of hydrogen loading on Yb-doped aluminosilicate fibers, in the context of photodarkening, were also studied briefly.
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