Coherent phase transfer via fiber using heterodyne optical phase locking as optical amplification

We demonstrate a coherent phase transfer via a 224 km cascaded fiber link comprising two 112 km links stabilized by two phase-locking loops, respectively. The optical signal is regenerated employing heterodyne optical phase locking (HOPL) after the first 112 km transfer. With a gain of more than 50 dB, the HOPL is capable of tracking the frequency of the incoming carrier with a fluctuation of 0.48 mHz and preserving the instability of the incoming laser to 6 x 10(-20) at 1000 s. The phase noise cancellation of each span is investigated, and the out-loop transfer instability of the 224 km link reaches 7.7 x 10(-19) at 10,000 s. The relation between the transfer instability of each span and that of the whole link is also deduced in the paper, in agreement with experimental results of the 224 km link. (C) 2018 Optical Society of America