 formula_3 is Boltzmann's constant, formula_4 is the temperature, formula_5 is the output power, and formula_6 and formula_7 are the HWHM and FWHM linewidths of the underlying passive microwave resonator, respectively.In 1958, two years before Maiman demonstrated the laser (initially called an "optical maser"), Schawlow and Townes transferred the maser linewidth to the optical regime by replacing the thermal energy formula_8 by the photon energy formula_9, where formula_10 is Planck's constant and formula_11 is the frequency of laser light, thereby approximating that

(iv) one photon is coupled into the lasing mode by spontaneous emission during the photon-decay time formula_12,

resulting in the original Schawlow-Townes approximation of the laser linewidth