850 nm Upconversion Lasing in $\mathsf{Er^{+3}}$ Doped Z.B.L.A. Fibers A. Saissy, E. Maurice, G. Monnom, S. Staroske and G. Baxter J. Phys. III France, 5 3 (1995) 297-306 DOI: 10.1051/jp3:1995126