Io V. S. Khomenko, N. N. Mit'kina, and V. V. Kuznetsova, "Anti-Stokes luminescence of the GdOCI:Yb 3+, Ho 3+system,''Vestsi Akad. Nauk BSSR, Ser. Fiz.-Mat. Nauk, No. 6, 89-92 (1976). 2. A. M. Tkachuk, "Spectral and laser properties of concentrated rare earth crystals," in: The Spectroscopy of Crystals [in Russian], Leningrad (1978), pp. 57-71. 3. M. B. Antipenko, B. V. Sinitsyn, and T. V. Uvarova, "Laser switch based on BaY2F,:Ho~+: A three micron generation channel," Kvantovaya Elektron., ~, No. 9, 2019-2022 (1980). 4. I. I. Sergeev, "Temperature dependence of the efficiency of transformation of IR radiation into visible radiation in the YFs:Yb 3+, Er 3+ and YOCI:Yb, Er systems," Preprint No. 125 of the Institute of Physics of the Academy of Sciences of the Belorussian SSR, Minsk (1977). 5. V. A. Lobach, V. Yu. Kara-Ushanov, V. I. Rogovich, and A. S. Vinogradova-Zhabrova, "On the anti-Stokes luminescence of the compositions Y~-x-yYbxEryOF," in: Chemistry of the Solid State, Vol. 3 [in Russian], S. M. Kirov Ural Polytechnic Institute, Sverdlovsk (1979), pp. 109-113. 6. Y. Mira, "Luminescence processes in Yb 3+ sensitized rare-earth phosphors," J. Appl~ Phys., 43, No. 4, 1772-1778 (1972). 7. I. I. Sergeev, "Program for the calculation of the population of the energy levels of rare-earth ions in crystalline matrices," Manuscript presented at the Institute of Physics of the Academy of Sciences of the Belorussian SSR, Minsk (1980); deposited at the Belorussian Scientific-Research Institute for Scientific and Technical Information (BelNIINTI), No. 132, March 4, 1980. F. W. Ostermayer, J. P. van der Ziel, H. M. Marc.s, L. G. van Uitert, and J. E. Geusic, "Frequency upconversion in YFs:Yb 3+, Tm3+, '' Phys. Re,. B, ~, No. 8, 2698-2705 (1971). Sho N. Gifeisman, A. M. Tkachuk, and V. V. Prizmak, "Optical spectra of the Ho 3+ ion in LiYF4 crystals," Opt. Spektrosk., 44, No. i, 120-126 (1978). W. T. Carnall, P. R. Fields, J. Morris.n, and R. Sarup, "Absorption of Tm3+:LaF3," J. Chem. Phys., 52, No. 8, 4045-4059 (1970). R. A. Hewes and J. F. Sarver, "Infrared excitation processes for visible luminescence of Er s+, Ho 3+, and Tm s+ in Yb3+-sensitized rare-earth fluorides," Phys. Rev., 182, No. 2, 427-436 (1969). .