Patent ID: 8902493

Claim:
An optical fiber comprising: a rare earth doped glass comprising silica, ytterbium, phosphorus, and aluminum, wherein the concentration of ytterbium in said rare-earth doped glass is in the range from 0.5 to 15 mol % so that said optical fiber is configured to have a peak absorption greater than about 3000 dB/m at a pump wavelength and a gain greater than about 0.5 dB/cm at an emission wavelength, wherein the concentration of phosphorous in said rare-earth doped glass comprises at least 50 mol % P 2 O 5 so that a saturated value of photo-darkening loss in the optical fiber is less than about 10 dB/m at said emission wavelength, wherein the concentration of aluminum in said rare-earth doped glass is in a range from 0.5 to 15 mol %, and wherein the concentration of boron in said rare-earth doped glass is in a range from 0 to 25 mol %.