Optical Frequency Transfer over 60 km Urban Telecom Fiber Link

This paper demonstrates the coherent optical frequency transfer over a 60 km fiber link between Innovation Academy for Precision Measurement Science and Technology , Chinese Academy of Sciences and Huazhong University of Science and Technology. A 1560 nm ultra-stable laser is used as the transfer laser, and an acousto-optic modulator is employed to actively compensate for the phase noise introduced by optical fibers to achieve noise suppression of 67 dB at 1 Hz Fourier frequency. The end-to-end transfer of the all-fiber structure is adopted by the system, and tracking oscillators are used to suppress the amplitude fluctuation of the heating signals caused by fiber polarization change. In this way, the system can operate without interruption all day. The optical frequency offset introduced by the link is at the order of magnitude of 10 -74 in the four-day measurement, and the frequency instability is measured to he 2.4 X 10(-17) in 1 s, 6. 6 X 10(-20) in 1000 s, and 6. 5 X 10(-)(23) in 65000 s. This result demonstrates the system is applicable for remote optical clock comparison.