1.5 'mü'm emission and infrared-to-visible frequency upconversion in 'Er POT.+3'/Y'b POT.+3'-doped phosphoniobate glasses - Barbosa, A. J.; Dias Filho, F. A.; Messaddeq, Y.; Ribeiro, S. J. L.; Gonçalves, Rogéria Rocha; Lüthi, S. R.; Gomes, A. S. L. (Journal of Non-Crystalline Solids, 2006)