1. Yu. M. Golubev, V. E. Privalov, S. A. Fridrikhov, and V. A. Khodovoi, Zh. Tekh. Fiz.,38, 1097 (1968).
2. V. E. Privalov and S. A. Fridrikhov, Zh. Tekh. Fiz.,38, 2080 (1968).
3. G. D. Boyd and I. P. Gordon, Bell Syst. Tech. J.,40, 489 (1961).
4. A. A. Fedotov, Avt. Svid. No. 213223, February 19, 1964.
5. B. P. Miretskii, E. P. Ostapchenko, S. G. Sedov, and A. A. Fedotov, Abstracts of Contributions Read at the Conference on Quantum Electronics [in Russian], Vol. 6, Moscow (1967).