A fiber laser (or fibre laser in Commonwealth English) is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium and holmium. They are related to doped fiber amplifiers, which provide light amplification without lasing.
Fiber nonlinearities, such as stimulated Raman scattering or four-wave mixing, can also provide gain and thus serve as gain media for a fiber laser.[citation needed]