1. A.E. Siegman, Lasers, Oxford University Press, Oxford, 1986, ch. 13, sect. 8.
2. P.W. Milonni, J.H. Eberly, Lasers, Wiley, New York, 1988, ch. 12, sect. 13.
3. W. Koechner, Solid-State Laser Engineering, 3rd ed., Springer, Berlin, 1992, ch. 4, sect. 4.1.
4. O. Svelto, Principles of Lasers, 3rd ed., Plenum, New York, 1989, ch. 2, sect. 7.3.
5. R.C. Elton, X-Ray Lasers, Academic Press, Boston, 1990, ch. 2, sect. 2.