Laser linewidth represents the fundamental noise of the laser related to the spontaneous emission noise, the output power and the laser cavity Q (quality factor), through the famous Schawlow-Townes formula.  This corresponds to the white segment of the power spectral density of the frequency noise plotted against the Fourier frequency. Frequency noise refers to the total integrated noise in the power spectral density of noise plot.  linewidth measurement via phase/frequency noise measurement and analysis.