Patent ID: 7760771

Claim:
An active multimode fiber for a gain medium in fiber lasers or fiber amplifiers, comprising a core region and a cladding region surrounding the core region, wherein the core region consists of: (a) a first core section doped with active atoms, and having a first effective refractive index (n 1 ); (b) a second core section undoped with active atoms, surrounding the first core section, and having a third effective refractive index (n 3 ) that is equal or greater than said first effective refractive index (n 1 ); and (c) a barrier layer between the first core section and the second core section, the barrier layer having a second refractive index (n 2 ) that is smaller than the first effective refractive index (n 1 ) of the first core section by an index difference (Δn), and a thickness (d 2 ), and wherein the cladding region surrounds the second core section and has a fourth refractive index (n 4 ) that is smaller than the first effective refractive index (n 1 ) thereby forming a refractive index step in a boundary, and wherein the first effective refractive index (n 1 ) and the third effective refractive index (n 3 ) enable core modes with a first propagation constant (β 1 ) in the first core section, and cladding modes with a second propagation constant (β 2 ) in the second core section, the first propagation constant and the second propagation constant having values approaching each other, and wherein the second core section has a cross-sectional area that is large enough to occupy a multitude of modes; whereupon the index difference (Δn) and the thickness (d 2 ) are such that a fundamental core mode couples less strongly with the cladding modes than higher order core modes.