1. See Proc. 1985 FEL Conference Lake Tahoe, CA (USA)
2. G. Dattoli, T. Letardi, J.M.J. Madey, A. Renieri: IEEE J. QE-20, 637 (1984)
3. G. Dattoli, T. Letardi, J.M.J. Madey, A. Renieri: IEEE J. QE-20, 1003 (1984)
4. G. Dattoli, A. Renieri:Laser Handbook,IV, ed. by M.L. Stitch and M. Bass (North-Holland, Amsterdam 1985) p. 1
5. N.A. Vinokurov, A.N. Skrinskii: Preprint INP 77-59 and 77-67 Novosibirsk (1977)