| abstract |
An optically-active air-clad fiber ( 30 ) includes a core ( 34, 84 ) that facilitates doping with an ion optically excitable and having a three-level optical transition when pumped at a first end ( 28 ) of an optical cavity ( 46 ) by a multimode pump source ( 72 ) at a pump wavelength ( 64 ) for lasing at a signal wavelength ( 66 ) different than the pump wavelength ( 64 ) at a second end ( 29 ) of the optical cavity ( 46 ), the core ( 34, 84 ) having a refractive index, wherein the core ( 34, 84 ) is transformed from the first end to proximate the second end ( 29 ) thereof such that the optically-active fiber ( 30 ) is multimode at the pump wavelength proximate to the first end ( 28 ), and is single-mode at the signal wavelength proximate to the second end ( 29 ). An air-clad ( 36, 86 ) surrounds at least one portion of the core ( 34, 84 ) and has a lower effective refractive index than the refractive index of the core ( 34, 84 ). |