RESUMO
Transverse photodarkening loss variations in an LMA Yb-DCF are experimentally studied. Photodarkening rate depends on inversion, which is affected by the pump induced increase and signal induced depletion of the inversion. In double-clad fibers, intensity distributions of the pump and signal modes and their overlap with the core are significantly different, leading to transverse differences in inversion within the core. Moreover, practical fiber laser configurations aim at generating and preserving only the fundamental transverse-mode thus creating a high contrast in inversion within the core. Therefore, dramatically different rates of photodarkening across the core of the active fiber can be expected. We demonstrate the existence of transverse mode-induced photodarkening loss variations in an LMA Yb-DCF laser and discuss its implications. Composition-related transverse photodarkening loss variations are measured to be negligible in the studied Yb-DCF.
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.
