High-stability microwave frequency transfer by propagation of an optical frequency comb over 50 km of optical fiber

A high-stability microwave frequency reference is transferred over 50 km of spooled optical fiber by propagation of a 90-nm-wide optical frequency comb centered at 1.56 microm. Environmentally induced fiber phase noise is actively suppressed by returning the optical frequency comb via a separate fiber. The stability of the microwave frequency delivered is measured at the "user" end, eliminating the need for the assumption of correlation between the noise in the forward and return paths. The measured transfer stability is 4.6 x 10(-15)tau(-1), rendering this technique suitable for the transfer of state-of-the-art frequency references.