Patent ID: 7062137

Claim:
An optical fiber, comprising: a core comprising a host material comprising a concentration of erbium, a concentration of phosphorus, a concentration of ytterbium and a concentration of fluorine for reducing the index of refraction of the host material comprising said concentrations of erbium, phosphorus and ytterbium, said concentration of ytterbium for sensitizing said erbium by absorbing pump light and transferring energy to said erbium, said erbium providing light having a second wavelength different than the wavelength of the pump light; a cladding disposed about said core and having inner and outer perimeters spaced by a thickness therebetween, said cladding having an index of refraction that is less than an index of refraction of said core; and wherein said core has a V-number at said second wavelength of greater than 2.405 and a numerical aperture of less than 0.12 relative to said cladding, and said thickness is larger than a thickness of a region, if present, disposed between said cladding and said core.