 Also in the quantum-optical approaches to the laser linewidth, based on the density-operator master equation, it can be verified that the gain is smaller than the losses.As mentioned above, it is clear from its historical derivation that the original Schawlow-Townes equation is a four-fold approximation of the fundamental laser linewidth.Starting from the fundamental laser linewidth formula_87 derived above, by applying the four approximations (i)-(iv) one then obtains the original Schawlow-Townes equation.