Raman fiber laser improvement by using Yb3+-doped fiber

We report here the efficiency improvement in a fiber Raman laser by splicing to it short pieces of 10000-ppm Yb³⁺-doped fibers. The amplified spontaneous emission produced by the doped fiber started the stimulated Raman scattering earlier than the spontaneous scattering in the 350-m silica-fiber cavity. In the best case, when splicing to the cavity 127 cm of doped fiber, the pump threshold decreased from 2.9 down to 1.27 W, representing a 56% improvement. This power was close to the theoretical minimum of 1.25 W for very long fibers. The second stokes signal (1175 nm) was only present when the doped fibers were inserted. At 4-W pump, the non-doped cavity delivered 2 W of first stokes signal (1117 nm) that represent a 50% efficiency compared to ～3 W delivered (～75%) when a 34-cm doped fiber was inserted. This improvement is intended to reduce the cost of fiber Raman lasers because less powerful sources or shorter fibers are necessary to produce more stokes components.