Fiber amplifiers with depressed cladding and their uses in er-doped fiber amplifiers for the s-band

Abstract

A fiber amplifier in a depressed cladding or W-profile fiber. The fiber has a core doped with the active material and defined by a core cross-section and a refractive index n. A depressed cladding of index nsurrounds the core and a secondary cladding of index nsurrounds the depressed cladding. The fiber amplifier is pumped to a level of high relative inversion, such that the active material exhibits positive gains in a short wavelength band and high gains in a long wavelength band. The core cross-section, the depressed cladding cross-section and the refractive indices n, n, and nare selected to obtain a roll-off loss curve about a cutoff wavelength λ. The roll-off loss curve yields losses at least comparable to the high gains in the long wavelength band and losses substantially smaller than the positive gains in the short wavelength band.