Robust Transfer of Optical Frequency over 500km Fiber Link with Instability of 10-21

Our primary objective is to mitigate the adverse effects of temperature fluctuations on the optical frequency transmission system by reducing the length of the interferometer. Following optimization, the phase-temperature coefficient of the optical system is reduced to approximately 1.35 fs/K. By applying a sophisticated temperature control to the remained "out-of-loop" optics fiber, the noise floor of the system has been effectively lowered to 10(-21) level. Based on this performance-enhanced transfer system, we demonstrate coherent transmission of optical frequency through 500-km spooled fiber link. After being actively compensated, the transfer instability of 4.5 x 10(-16) at the averaging time of 1 s and 5.6 x 10(-21) at 10000 s is demonstrated. The frequency uncertainty of received light at remote site relative to that of the origin light at local site is achieved to be 1.15 x 10(-19). This enhanced system configuration is particularly well suited for future long-distance frequency transmission and comparison of the most advanced optical clock signals.