1. B. I. Stepanov, E. V. Ivakin, and A. S. Rubanov, Sov. Phys.-Dokl. 16, 46 (1971).
2. S. P. Woerdman, Opt. Commun. 2, 212 (1970).
3. B. Ya. Zeldovich, V. I. Popovichev, V. V. Ragulskii, and F. S. Faisullov, JETP Lett. 15, 109 (1972).
4. O. Yu. Nosach, V. I. Popovichev, V. V. Ragulskii, and F. S. Faisullov, JETP Lett. 16, 435 (1972).
5. R. W. Hellwarth, J. Opt. Soc. Am. 67 (1), 1 (1977).