Ytterbium doped double cladding fibers as gain medium for fiber lasers have received much attention in recent years.The spectroscopic properties of Yb
3+
ions in fibers and the novel structures for high power fiber lasers such as non-symmetrical double clads and large mode area core are briefly described.The development of suitable methods of fabricating silica-based performs doped with rare earth ions for optical fibers are presented.The underlying mechanisms responsible for photodarkening in ytterbium doped fibers and how it can be reduced are also discussed.