Phase stable radio distribution over optic cable by phase conjugation using an optical frequency comb

A phase stable radio frequency (RF) distribution technique is proposed and experimentally demonstrated. The system transmits optical frequency comb lines through an optic cable with multiple fibers. At the receiver, RF phase variation is removed by frequency mixing based on phase conjugation. In the proposed system, round-trip transmission is implemented through different fibers, thus the signal degradation due to bidirectional fiber transmission is eliminated. Besides, electrical frequency manipulations are not required because different frequency components can be generated by beating the comb lines at a photodetector, which leads to a simplified structure and alleviated signal distortion. Another advantage of the proposed scheme is that a series of phase stabilized frequencies at In times (n is a positive integer) of the comb spacing can be obtained simply by tuning the electrical filters. In the experiment, phase stable distribution of a 5 GHz signal and a 10 GHz signal over a 1.5 km optic cable is successfully demonstrated in a temperature varying environment. The residual delay variation of the 5 GHz signal is confined within ±1.1 ps, and the phase noise of the signal is well preserved after fiber distribution.