Fabrication and characterization of Yb^3+-doped gain-guided index-antiguided fiber with D-shaped inner cladding

Yb3+doped gain-guided index-antiguided fibers based on phosphate glasses are drawn using the rod-in-tube technique. These fibers have a circular core with 200 μm diameter and a D-shaped inner cladding with a 630/480 μm diameter for the longer/shorter axis. The negative index profile is present in these fibers, in which the core index of refraction is 0.019% lower than that of the inner cladding. Moreover, their laser amplifier characteristics are experimentally demonstrated, and the method of further improvement in laser performance is suggested.