Patent ID: 6876490

Claim:
A fiber amplifier comprising: a) a core having a core cross-section and a refractive index n o ; b) an active material doped in said core, wherein said active material is Thulium; c) a depressed cladding surrounding said core, said depressed cladding having a depressed cladding cross-section and a refractive index n 1 ; d) a secondary cladding surrounding said depressed cladding, said secondary cladding having a secondary cladding cross-section and a refractive index n 2 ; and e) a pump source for pumping said active material to a high relative inversion D, such that said active material exhibits positive gains in a short wavelength band and high gains in a long wavelength 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 a roll-off loss curve about a cutoff wavelength λ c , said roll-off loss curve yielding losses at least comparable to said high gains in said long wavelength band and losses substantially smaller than said positive gains in said short wavelength band.