A Novel Scheme for Hundred-Hertz Linewidth Measurements with the Self-Heterodyne Method

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