Active fibers are optical fibers which have one or more laser-active dopants in the fiber core. In most cases, they are rare-earth-doped fibers, with dopants like ytterbium, erbium or thulium. Due to those dopants, they can be used as laser gain media, also for realizing fiber amplifiers. Only quite rarely, active fibers are made with transition metal ions.
Besides the ability to amplify light (usually with high gain efficiency and often with a high power conversion efficiency and high beam quality of the output), active fibers usually exhibit higher propagation losses, which however are normally of little relevance, since one generally requires only a relatively short fiber. In most cases, they are single-mode or few-mode fibers, and sometimes large mode area fibers or polarization-maintaining fibers.
Laser amplification is not the only possible physical mechanism to obtain gain in a fiber: stimulated Raman scattering (SRS) and the Kerr nonlinearity of the fiber can also be exploited for that purpose. In such cases, one may consider a fiber as active in the sense that it can amplify light.
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