Comparison of highly-stable optical frequency transfer in a single bidirectional and double unidirectional fibers

The stability of the transmitted optical frequency in a single bidirectionally operated fiber was compared with stability in a link consisting of two separate unidirectional fibers. The measurements were performed in a standard closed-loop system canceling the fiber induced phase noise. It was found that phase fluctuations in the higher frequency band (acoustic noise) are well suppressed in both configurations (single bidirectional fiber and separate unidirectional fibers). The long-term drifts however are not fully compensated in a link consisting of two separate unidirectional fibers, but still about two orders of magnitude noise reduction was observed in comparison with not compensated transfer.