1. Carusotto, S. (1988).Physical Review A,38, 3249.
2. Louisell, W. H. (1973).Quantum Statistical Properties of Radiation, Wiley, New York, p. 137.
3. Saavedra, F. A., and Buendia, E. (1990).Physical Review A,42, 5073.
4. Zhang Jialun, Khan, R. D., Ding Sheng, and Shen Wenda (1992).SPIE,1726, to appear.