RESUMO
We examine the effect of photodarkening-induced pump light absorption on Yb-doped fiber thermal loading. In these experiments, the fiber is cladding pumped at 915nm, air cooled by natural convection, and monitored with a midinfrared thermal camera. The fiber temperature is found to correlate with progressive photodarkening. The maximum observed fiber temperature was 120 degrees C, recorded at a pump power of 10.5W. The observed increase in fiber temperature can be explained by a model that takes into account the combined effects of the Yb-silica quantum defect, the temperature dependence of the Yb-absorption cross section, and photodarkening-induced loss at 915nm. We hypothesize that the latter effect results in the progressive activation of temperature-dependent photochemical processes (e.g., bleaching) and is an important consideration with regard to the accurate modeling of Yb-doped fiber photodarkening kinetics.
RESUMO
Photodarkening is recognized as a potentially important limiting factor on the lifetime and reliability of many Yb-doped fiber lasers and amplifiers. In particular, a photodarkening process attributed to the formation of photoinduced structural transformations can induce excess loss in the doped glass core of the fiber, resulting in reduced output power efficiency. Yet, quantifiable measurement techniques of this phenomenon have been scarce in the literature to date. Here we present a fast, simple and repeatable method to measure and compare the photodarkening rate caused by the formation of photoinduced structural transformations from Yb-doped single-mode fibers. The method relies on quantifying observations of transmission changes at visible wavelengths as an indicative measure of photodarkening at the signal wavelengths. Preliminary measurement results are presented supporting the utility of the technique for benchmarking the photodarkening behavior of different Yb-doped fibers.
