We have analyzed the cooperative luminescence of an ytterbium-doped silica fiber in which the ytterbium concentration is quite large, i.e., similar to10(21) cm(-3). The presence of a fast decaying component in a decay profile of F-2(5/2)-->F-2(7/2) transition indicates the formation of ytterbium pairs in the fiber. The ytterbium cooperative luminescence spectrum is characterized by two peaks at 488 nm and at similar to512 nm, respectively. Overall intensity of the visible cooperative up-conversion luminescence is proportional to excitation power at 980 nm. However, as fiber length increases, intensity of the shorter-wavelength side of the cooperative luminescence spectrum becomes smaller than the other portion, which results in a narrower spectral bandwidth. (C) 2002 Elsevier Science B.V. All rights reserved.