P. Lagueux, M. Chamberland, and R. Vallée, "Blue thulium-doped double-clad fiber amplifier by an up-conversion process: Numerical model,"
in Advanced Solid-State Lasers, M. Fermann and L. Marshall, eds.,
Vol. 68 of Trends in Optics and Photonics Series
(Optical Society of America, 2002), paper MB17.