Analysis of the effect of numerical aperture on Pr:ZBLAN upconversion fiber lasers

Theoretical analyses show that the use of high-numerical-aperture (NA) fibers can significantly increase the conversion efficiency of a blue Pr(3+) -doped fluoride fiber upconversion laser and reduce the pump-power threshold. A record high efficiency of ~14% and a low 1017-nm threshold of ~30 mW were achieved experimentally with a fiber with a NA of ~0.39.