What is laser intrinsic linewidth?

Laser intrinsic linewidth is due to the laser’s fundamental spontaneous emission white noise that results in Lorentzian spectral line shape (the Schawlow-Townes linewidth). It is a single value spectral purity parameter also often referred to as instantaneous linewidth where technical noise content of a laser source due to, for example, environmental temperature or vibration is unaccounted for. The intrinsic linewidth takes on relatively short time scale measurements and is (unfortunately!) often the figure of merit specified for single frequency laser sources.