1. see for instance:A. Cozanet, J. Fleuret, H. Maitre, and M. Rousseau, “Optique et Télécommunications”. (Eyrolle, Paris 1981), p. 217.
2. F. Auzel, “Amplifiers and lasers with optical fibers”, in “Defects in insulating materials”. Ed. O. Kanert and J.-M. Spaeth, World Scientific,(Singapore, 1993), p.39.
3. F. Auzel, “Multiphonon processes, cross-relaxation and up-conversion in ion-activated solids, exemplified by minilaser materials”, in “Radiationless Processes”. Ed. B. Di Bartolo and V. Goldberg (Plenum Press, New York 1980.), p. 213.
4. F. Varsanyi, “Excitation of fluorescence with monochromatic light in rare-earth crystals”, in “Quantum Electronics”. Eds. P. Brivet and N. Bloembergen, (Dunod, Paris 1964.), p. 787.
5. M.J. Weber, Phys. Rev., 138, 54.(1973)