It is typical that the lasing properties of a medium are determined by the temperature and the population at the excited laser level, and are not sensitive to the method of excitation used to achieve it. In this case, the absorption cross-section 
and the emission cross-section
at frequency  can be related to the lasers gain in such a way, that the gain at this frequency can be determined as follows:
D.E.McCumber had postulated these properties and found that the emission and absorption cross-sections are not independent; they are related with Equation (1).