 Besides the photon-decay time formula_12, the spectral-coherence properties of the passive resonator mode can be equivalently expressed by the following parameters.The FWHM Lorentzian linewidth formula_44 of the passive resonator mode that appears in the Schawlow-Townes equation is derived from the exponential photon-decay time formula_12 by Fourier transformation,
The "Q"-factor formula_47 is defined as the energy formula_48 stored in the resonator mode over the energy formula_49 lost per oscillation cycle,
where formula_51 is the number of photons in the mode.The coherence time formula_52 and coherence length formula_53 of light emitted from the mode are given by

With the population densities formula_55 and formula_56 of upper and lower laser level, respectively, and the effective cross sections formula_57 and formula_58 of stimulated emission and absorption at the resonance frequency formula_59, respectively, the gain per unit length in the active laser medium at the resonance frequency formula_59 is given by
A value of formula_62 induces amplification, whereas formula_63 induces absorption of light at the resonance frequency formula_59, resulting in an elongated or shortened photon-decay time formula_65 of photons out of the active resonator mode, respectively,
The other four spectral-coherence properties of the active resonator mode are obtained in the same way as for the passive resonator mode.