Active optical fiber and method for fabricating an active optical fiber

A section of active optical fiber comprising

• a core with a first index of refraction, said core being active,
• an inner cladding layer with a second index of refraction for propagating pump radiation, said second index of refraction being smaller than said first index of refraction and
• an outer cladding layer with a third index of refraction around said inner cladding layer, said third index of refraction being smaller than said second index of refraction,
• wherein the diameter of said core and the thickness of said inner cladding change gradually along the length of said section of active optical fiber forming a tapered longitudinal profile enabling a continuous mode conversion process along the length of the section of fiber and the tapered core supporting multimode operation at the thicker end of the section of fiber, wherein said tapered longitudinal profile of said section of active optical fiber is selected from a group of profiles including a power law profile, an exponential profile and a combination of these profiles.