Laser frequency noise characterization by self-heterodyne with both long and short delay.

We propose a novel approach to laser frequency noise characterization by delayed self-heterodyne. Compared with the traditional treatment, our method applies to both long and short delay, corresponding to uncorrelated and correlated self-heterodyne. In the case of long delay, it overcomes the influence of 1/f noise on the intrinsic linewidth extraction from a broadened spectrum, and the results are more accurate than Voigt profile fitting. For short delayed correlated heterodyne, it eliminates artifact peaks at multiples of the reciprocal of delay time introduced by transferring measured RF phase noise to laser phase noise, thus extending the measurement range. In addition, it calibrates the frequency noise overestimation caused by a finite noise floor. This method remains valid when the delay and the coherence time are comparable. Experimental results are presented to demonstrate the effectiveness of the proposed approach in characterizing lasers with intrinsic linewidth ranging from sub-100 Hz to megahertz.