Polarization-dependent enhancement of population inversion and of green upconversion in Er:LiNbO3 by Yb codoping

We have investigated the polarization dependence of both the population inversion of the /sup 4/I/sub 13/2/ state and the green upconversion to the /sup 4/S/sub 3/2/ state in Er:LiNbO/sub 3/ optical fiber amplifier when codoped with Yb and pumped around 980 nm. The presence of Yb/sup 3+/ enhances the population of the /sup 4/I/sub 13/2/ state of the Er/sup 3+/ ions when pumped at /spl sigma/-polarization (>2.5 times) and at /spl pi/-polarization (>5.5 times), and introduces a broad useful pump band around 940-960 nm at /spl sigma/-polarization, relaxing fabrication tolerances on semiconductor pump sources. An enhancement of green upconversion is also observed by the Yb codoping. Lifetime measurements indicate that, as in glass substrates, nonradiative energy transfer is responsible for these enhancements.