Microwave frequency transfer over a 500-km cascaded fiber link using tracking filter

This paper demonstrates microwave frequency transfer over a 500-km cascaded fiber link that is composed of 5 spans of spooled fibers. The link noise of each span is actively compensated with an electrical phase compensation system. To ensure that the cascaded link operates continuously and reliably, a clean-up tracking filter is implemented before the fourth span, which suppresses the spurious signal caused by the accumulation of servo spikes span-by-span by at least 30 dB. In addition, the simple-architecture low-noise frequency conversion modules connecting adjacent spans are designed based on the harmonic of frequency divider. Thanks to these designs, frequency transfer instabilities of 1.8 x 10-14 at 1 s and 1.6 x 10-17 at 105 s are obtained over the 500-km cascaded link.