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 I-4(13/2) state and the green upconversion to the S-4(3/2) state in Er:LiNbO3 when codoped with Yb and pumped around 980 nm. The presence of Yb3+ enhances the population of the I-4(13/2) state of the Er3+ ions when pumped at sigma-polarization (>2.5 times) and at pi-polarization (>5.5 times), and introduces a broad useful pump band around 930-960 nm at 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.