Optical fiber lasers and amplifiers used in the amplification of optical signals usually include optical fibers having optically active waveguiding cores doped with rare-earth active ions, such as for example, erbium, ytterbium and thulium. For such applications and others, it is important that the optical properties of the fiber remain stable. However, for some rare-earth doped core compositions, the formation of photoinduced structural transformations which can induce excess loss in the doped glass core of the fibers are observed over time under light illumination or propagation, resulting in reduced output power efficiency of the fibers. This phenomenon is known as photodarkening. It is a very detrimental effect, that decreases for instance the output power of fiber laser or amplifier over time and make them inefficient.
Photodarkening
Photodarkening has already been observed in silica fiber doped with different rare-earth such as thulium (M. M. Broer et al., Opt. Lett. 1993, 18 (10), p. 799-801), cerium (M. M. Broer et al., Opt. Lett. 1991, 16 (18), p. 1391-1393), europium and praseodymium (E. G. Behrens et al., 1990, JOSA B 7 (8), p. 1437-1444). The first experimental evidence of this phenomenon in ytterbium doped silica fiber was described by R. Paschotta et al. in Opt. Commun., 1997, 136 (5-6), p. 375-378.
The present inventors, in their article entitled “Low photodarkening single cladding ytterbium fibre amplifier” published in the Proceedings of SPIE, Vol. 6453, Fiber Lasers IV: Technology, Systems, and Applications; Donald J. Harter, Andreas Tünnermann, Jes Broeng, Clifford Headley III, Editors, 64530H (Feb. 20, 2007), show that photodarkening is reduced in ytterbium fibers co-doped with aluminum or with another rare-earth such as erbium. They attribute this reduction of photodarkening to erbium or aluminium ions that prevents the formation of ytterbium clusters to emit UV radiation that creates color centers.
Gavrilovic et al. in U.S. Pat. No. 6,154,598 propose to prevent photodarkening in ytterbium-doped optical fiber lasers and/or waveguide structures operating at high optical output power level and which contain thulium as an impurity, by introducing another rare-earth ion as co-dopant to the fiber core. Preferably, the co-dopants are selected from terbium, europium and/or neodymium.
Kitabayashi et al. have also demonstrated that high aluminum concentration into ytterbium-doped fibers is effective to suppress photodarkening (“Population Inversion Factor Dependence of Photodarkening of Yb-doped Fibers and its Suppression by Highly Aluminum Doping”, Optical Fiber Communication Conference, 2006 and the 2006 National Fiber Optic Engineers Conference; OFC 2006, 5-10 Mar. 2006).
It is thus apparent to those skilled in the art that there is a need for optical fibers with reduced photodarkening properties, especially for rare-earth-doped optical fiber wherein the rare-earth induces a photodarkening effect. There is also a need for a method which reduces photodarkening in a rare-earth-doped optical fiber wherein the rare-earth induces a photodarkening effect.