Patent ID: 7916386

Claim:
High power optical fiber apparatus comprising: an input fiber having a first core region configured to propagate signal light in a single transverse mode and pump light in at least the same, single transverse mode, a high brightness source of said pump light, a multimode, gain-producing optical fiber having a second core region and a cladding region surrounding said second core region, the cross-sectional area of said second core region exceeding approximately 350 μm 2 and exceeding the cross-sectional area of said first core region, said second core region configured to provide gain to said signal light propagating therein when said pump light is absorbed therein, and a mode expander for coupling said signal light and said pump light from said first core region into said second core region and for altering the mode field areas of said signal light and said pump light in said input fiber to match the corresponding mode field areas in said second core region.