Specialty Optical Fibers Handbook

is exposed to intense pump radiation, the signal degrades over time. This photo-

darkening is likely due to formation of a color center in the glass and appears as

a strong absorption at visible wavelengths, accompanied by a strong absorption

around 975 nm [107]. Although the mechanism for photo-darkening has not

been fully resolved, a number of studies have probed various aspects of the

problem. Similar to other RE ions at high concentration, excited state decay of

Yb can be decomposed into an initial rapid decay, followed by a long fluores-

cence lifetime [108]. The latter is associated with isolated Yb

3þ

ions in the glass

matrix, with lifetimes