A single cladding ytterbium doped fibre amplifier pumped at 980 nm that exhibits negligible amount of photodarkening over a long period of time is demonstrated. The output power as a function of time decreased by a very small factor compared to standard single mode ytterbium fibres. To achieve this photodarkening resistant amplifier, a special ytterbium doped fibre has been developed. Codoping with aluminium or other rare-earth such as erbium is shown to decrease the multi-excitation of ytterbium clusters and thus lower photodarkening. Photodarkening was characterized by comparing the amount of excess loss created by core pumping single cladding fibres at high intensity at 980 nm. Photodarkening was found to be directly proportional to the excitation of the ytterbium ions by comparing different pumping scheme and pump wavelength. Core pumping of a single cladding ytterbium doped fibre amplifier at 980 nm represents the worst case scenario for photodarkening. Engineering ytterbium fibres for low photodarkening is therefore critical in pulsed amplification where short length of fibre with high doping level is required as demonstrated with 6 μm core ytterbium fibre amplifier pumped in the core or in the cladding. Photodarkening was correlated to clustering from cooperative luminescence measurement at 500 nm produced by ytterbium clusters that would emit UV radiation under strong pumping.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.