Wideband phase-noise measurement of mode-locked laser pulses by a demodulation technique

A new method is proposed and demonstrated for measuring the phase noise (pulse timing fluctuations) of mode-locked lasers. The instantaneous phases of the pulse intensity are extracted in a time domain, and the phase-noise power spectral density is calculated from the demodulated signals. Compared with the conventional method based on the single-sideband phase-noise measurement, the proposed method has a larger dynamic range and a wider frequency span. The phase noise of a mode-locked Cr:LiSAF laser is measured for 50-mHz-1-MHz Fourier frequencies.