Patent ID: 6844962

Claim:
A source of light in an S-band of wavelengths comprising: a) a fiber having: 1) a core doped with Erbium and having a core cross-section and a refractive index n o ; 2) a depressed cladding surrounding said core, said depressed cladding having a depressed cladding cross-section and a refractive index n 1 ; 3) a secondary cladding surrounding said depressed cladding, said secondary cladding having a secondary cladding cross-section and a refractive index n 2 ; b) a pump source for pumping said Erbium contained in said core to a high relative inversion D, such that said Erbium exhibits positive gains in said S-band and high gains in a long wavelength band longer than said S-band; wherein said core cross-section, said depressed cladding cross-section, and said refractive indices n o , n 1 , and n 2 are selected to produce losses at least comparable to said high gains in said long wavelength band and losses substantially smaller than said positive gains in said S-band.