Frequency comb-to-comb synchronization between a 1.3-km free-space link via an optical frequency transfer

Optical frequency comb technology has been revolutionizing optical fields of ultra-precision metrology over two decades, connecting the SI unit standards of various optical clocks with utmost accuracy and stability. With an increasing demand for free-space communication and time/frequency transfer applications, the frequency comb is stretching its versatility even the outside of laboratory. Still, the environmental disturbances of free-space degrade the excellent phase noise of the optical frequencies, hindering the reliable utilization of frequency comb at any desirable location. This research performs comb-to-comb synchronization over a 1.3-km free-space optical link (FSO) with frequency stability of 3.80x10(-15) at 0.1 s averaging time throughout the entire comb spectrum. Phase-locked loop compensates the Doppler shift of optical frequencies propagating atmospheric free-space channel, providing optical references for synchronizing distant frequency comb.