Cascaded optical frequency transfer over 500-km fiber link using regenerative amplifier

We demonstrate a 300-km+200-km cascaded coherent phase transfer via fiber link. The transfer is divided into a 300-km span and a 200-km span with independent phase locking loops, aiming to extend the phase control bandwidth of the whole link. The phase noise and transfer instability of the cascaded transmission are investigated and compared with those in the case of a single-span 500-km transfer. We achieve the transfer instabilities of 1.8×10^-14 at 1 s, 8.9×10^-20 at 10⁴ s for the 300-km +200-km cascaded transmission, and 2.7×10^-14 at 1 s for the 500-km single-span transfer.