Amplifiers and light sources employing s-band erbium-doped fiber and l-band thulium-doped fiber with distributed suppression of amplified spontaneous emission (ase)

摘要

A fiber amplifier in a depressed cladding or W-profile fiber. The fiber has a core doped with the acitve material and defined by a core cross-section and a refractive index n_o. A depressed cladding of index n₁ surrounds the core and a secondary cladding of index n₂ surrounding the depressed cladding. The fiber amplifier is pumped a level of high relative inversion D, 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_o, n₁, and n₂ are selected to obtain a roll-off loss curve about a cutoff wavelength &lgr;_c. 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. To obtain the desired roll-off loss curve the refractive index n_o in the core is selected such that an effective index n_eff experienced by a mode of radiation which is guided is large. Specifically, the choice is made such that the effective index n_eff experienced by the confined mode maximizes a roll-off slope of the roll-off loss curve before the cutoff wavelength, e.g. the slope of n_eff is in the range of 0.002/10000 nm to 0.008/1000 nm before the cutoff wavelength and the roll-off slope of the roll-off loss curve is greater than the maximum slope of the gain spectrum.