1. S. A. RACKLEY and R. J. BUTCHER, unpublished.
2. B. D. SINCLAIR and M. H. DUNN, unpublished.
3. R. LOUDON, The Quantum Theory of Light, 2nd edn, (Clarendon Press, Oxford, 1983).
4. A. E. SIEGMAN, Lasers (University Science Books, Mill Valley, CA, 1986).
5. P. LORRAIN and D. R. CORSON, Electromagnetic Fields and Waves, 3rd edn (W. H. Freeman, New York, 1988).