How to measure laser linewidth?

Methods to measure laser linewidth vary depending on the intrinsic linewidth of the laser under test (LUT).  Conventional method typically is based on heterodyne technique of mixing two lasers to measure the linewidth of the RF/microwave beat frequency on an electronic signal or spectrum analyzer; the result is the linewidth contributed by both lasers.  Self-heterodyne is another popular method of linewidth measurement where the LUT signal is mixed with its frequency shifted version via an optical delay arm that must be longer than the coherence length of the LUT. Recent advances in traditional homodyne methodology via frequency discrimination have demonstrated the practical capability in ultra-narrow linewidth measurement via phase/frequency noise measurement and analysis.