 The spontaneous-emission rate into the lasing resonator mode is given by
Notably, formula_83 is always a positive rate, because one atomic excitation is converted into one photon in the lasing mode.It is the source term of laser radiation and must not be misinterpreted as "noise".The photon-rate equation for a single lasing mode reads
A CW laser is defined by a temporally constant number of photons in the lasing mode, hence formula_85.