In long-haul optical continuous-wave frequency transfer via fiber, remote bidirectional Er$^+$-doped fiber amplifiers are commonly used to mitigate signal attenuation. We demonstrate for the first time the ultrastable transfer of an optical frequency using a remote fiber Brillouin amplifier, placed in a server room along the link. Using it as the only means of remote amplification, on a 660 km loop of installed underground fiber we bridge distances of 250 km and 160 km between amplifications. Over several days of uninterrupted measurement we find an instability of the frequency transfer (Allan deviation of $Λ$-weighted data with 1 s gate time) of around $1\times10^{-19}$ and less for averaging times longer than 3000 s. The modified Allan deviation reaches $3\times10^{-19}$ at an averaging time of 100 s, corresponding to the current noise floor at this averaging time. For averaging times longer than 1000 s the modified Allan deviation is in the $10^{-20}$ range. A conservative value of the overall accuracy is $1\times10^{-19}$.
Phase conjugated master oscillator fiber power amplifier
Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences（中国科学院大学杭州高等研究院）；Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences（中国科学院上海光学精密机械研究所）
Hollow-Core Fiber for Long-Span Optical Frequency Transfer: Improved Instability and Extended Single-Span Reach
National Time Service Center, Chinese Academy of Sciences（中国科学院国家授时中心）；Key Laboratory of Time and Frequency Standards, Chinese Academy of Sciences（中国科学院时间频率标准重点实验室）；China Mobile Research Institute（中国移动研究院）
