Coherent Optical Frequency Transfer via a 490 km Noisy Fiber Link
Xiang Zhang1,2,3, Xue Deng1,3, Qi Zang1,2,3, Dongdong Jiao1,3, Jing Gao1,2,3, Dan Wang1,2,3, Qian Zhou1,2,3, Jie Liu1,3, Guanjun Xu1,3, Ruifang Dong1,2,3*, Tao Liu1,2,3*, and Shougang Zhang1,2,3
1National Time Service Center, Chinese Academy of Sciences, Xi'an 710600, China 2University of Chinese Academy of Sciences, Beijing 100039, China 3Key Laboratory of Time and Frequency Standards, Chinese Academy of Sciences, Xi'an 710600, China
Abstract：We demonstrate the coherent transfer of an ultrastable optical frequency reference over a 490 km noisy field fiber link. The fiber-induced phase noise power spectrum density per-unit-length at 1 Hz offset frequency can reach up to 510 rad$^2$$\cdot$Hz$^{-1}$$\cdot$km$^{-1}$, which is much higher than the fiber noise observed in previous reports. This extreme level of phase noise is mainly due to the fiber link laying underground along the highway. Appropriate phase-locked loop parameters are chosen to complete the active compensation of fiber noise by measuring the intensity fluctuation of additional phase noise and designing a homemade digital frequency division phase discriminator with a large phase detection range of $2^{12} \pi$ rad. Finally, a noise suppression intensity of approximately 40 dB at 1 Hz is obtained, with fractional frequency instability of $1.1\times10^{-14}$ at 1 s averaging time, and $3.7\times10^{-19}$ at 10000 s. The transfer system will be used for remote atomic clock comparisons and optical frequency distribution over a long-distance communication network established in China.
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2022, Vol. 39 
