1. V. I. Bredikhin, N. D. Galanin, and V. N. Genkin, Usp. Fiz. Nauk 110(1), 3 (1973) [Sov. Phys. Usp. 16, 299 (1973)].
2. J. M. Worlock, in Laser Handbook, Ed. by F. T. Arecchi and E. O. Sehulz-Dubois (North-Holland, Amsterdam, 1972), Vol. 2, p. 1324.
3. H. Mahr, in Quantum Electronics, Ed. by H. Rabin and C. L. Tang (Academic, New York, 1975), Vol. 1, p. 285.
4. S. I. Shablaev, A. M. Danishevskii, V. K. Subashiev, and A. A. Babashkin, Fiz. Tverd. Tela (Leningrad) 21(4), 1140 (1979) [Sov. Phys. Solid State 21, 662 (1979)].
5. S. I. Shablaev, A. M. Danishevskii, and V. K. Subashiev, Fiz. Tverd. Tela (Leningrad) 26(1), 179 (1984) [Sov. Phys. Solid State 26, 105 (1984)]; Zh. Éksp. Teor. Fiz. 86 (6), 2158 (1984) [Sov. Phys. JETP 59, 1256 (1984)].