1. G. Bret and F. Gires,Compt. Rend.258(to be published).
2. R. W. Hellwarth:Advances in Quantum Electronics(Columbia University Press, New York, 1961), pp 334–341.
3. R. W. Hellwarth, F. J. McClung, and G. F. Smith, (unpublished).
4. F. Gires and G. Mayer (unpublished).