 (i) It is a true CW laser, hence

(ii) It is a true four-level laser, hence

(iii) It has no intrinsic resonator losses, hence

(iv) One photon is coupled into the lasing mode by spontaneous emission during the photon-decay time formula_12, which would happen exactly at the unreachable point of an ideal four-level CW laser with infinite spectral-coherence factor formula_72, photon number formula_76, and output power formula_5, where the gain would equal the losses, hence

I.e., by applying the same four approximations (i)-(iv) to the fundamental laser linewidth formula_87 that were applied in the first derivation, the original Schawlow-Townes equation is obtained.Thus, the fundamental laser linewidth is
whereas the original Schawlow-Townes equation is a four-fold approximation of this fundamental laser linewidth and is merely of historical interest.Following its publication in 1958, the original Schawlow-Townes equation was extended in various ways.