It is still widely believed that ytterbium-doped gain media (e.g. fibers) cannot exhibit quenching phenomena, degrading the laser or amplifier efficiency because their simple energy level structure does not permit that. However, quenching phenomena do occur – not via energy transfer upconversion processes, but via color centers. Such a result has been published about nine years ago, but has not been noticed by many. Some devices are particularly sensitive to such effects, in particular ytterbium-doped fiber lasers operating at 975 nm.
