 These extended equations often trade under the same name, the “Schawlow-Townes linewidth”, thereby creating a veritable confusion in the available literature on the laser linewidth, as it is often unclear which particular extension of the original Schawlow-Townes equation the respective authors refer to.Several semi-classical extensions intended to remove one or several of the approximations (i)-(iv) mentioned above, thereby making steps towards the fundamental laser linewidth derived above.The following extensions may add to the fundamental laser linewidth