A planar wave guide (PWG) having a first end for coupling to a light pump and a second end opposite to the first end and including a first cladding layer; a second cladding layer; and a uniformly doped core layer between the first cladding layer and the second cladding layer, wherein the core layer is tapered having a smaller thickness at the first end and a larger thickness at the second end, and wherein a ratio of the core thickness to thickness of the cladding layers is smaller at the first end and larger at the second end.