A novel scheme for hundred-hertz linewidth measurements with self-heterodyne method

We propose a novel scheme for accurately determining hundred-hertz linewidth by delayed self-heterodyne method in which delay time far less than the laser's coherence time. That exceeds the former understanding as to the delayed self-heterodyne technique which requiring a prohibitively long fiber. The self-heterodyne autocorrelation function and power spectrum are evaluated and by numerical analysis we ensure that -3dB of power spectrum is applied to the self-heterodyne linewidth measurements. For laser linewidth less than 100 Hz, linewidth can be measured directly by 10 km fiber, and in more general case linewidth can be deduced from -20 dB or -40 dB of the fitting Lorentzian curve.