Mode-induced transverse photodarkening loss variations in large-mode-area ytterbium doped silica fibers

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.