Multiheterodyne Characterization of Excess Phase Noise in Atmospheric Transfer of a Femtosecond-Laser Frequency Comb

We report an experimental investigation on remote transfer of a femtosecond-laser frequency comb through an open atmospheric link. Optical multiheterodyne is used to measure the excess phase noise and the frequency stability of the transferred comb. The dispersion of air is found to have a minimal impact on the multiheterodyne signal, and the effectiveness of the technique to characterize the behaviors of comb lines under the influence of turbulence is theoretically analyzed. Large phase modulation due to the index fluctuation of the air over a 60-m transmission link is found to cause a significant linewidth broadening. Under low-wind conditions, a fractional frequency stability in the order of 10^-14 has been achieved over several minutes with a 1-s averaging time. A comparison of this work with previous tests based on continuous wave (CW) lasers indicates that pulsed lasers can work as well as CW lasers for remote transfer of optical frequency references through the atmosphere.