Optical frequency counter based on two mode-locked fiber laser combs

This work demonstrates a semi-automatic optical frequency counter based on two mode-locked fiber laser combs. The mode number of the comb line involved in the optical frequency measurement is determined by operating the two laser combs at three different repetition rates, with two of them similar enough to have the same mode number of the beating comb lines. The determination of the mode number is independent of the frequency fluctuation of the laser under measurement. The whole measurement process was automated, except for the frequency stabilization of the laser combs and the optimization of the beat signal-to-noise ratio.