This paper consists of a theoretical discussion of maser action at optical frequencies with emphasis on the differences between stimulated emission and absorption in coherent and incoherent fields. Use is made of a geometrical representation of the equations of motion of the density matrix to illustrate the conditions for continuous-wave operation, amplification, production of large pulses, and for the appearance of relaxation oscillations.
YOH-HAN PAO, "Quantum Mechanical Description of Maser Action at Optical Frequencies," J. Opt. Soc. Am. 52, 871-874 (1962)