Stable downlink frequency transmission from arbitrary injection point with endless and quick phase error correction

A stable frequency downlink transmission scheme, which delivers the frequency signal back to the central station from an arbitrary injection point along a radio-over-fiber (RoF) loop link, is proposed and demonstrated. The frequency signal at the arbitrary remote point is injected into the RoF loop link in both clockwise and counter-clockwise directions, simultaneously. The phase variation induced by the fiber loop link is obtained in real time with the help of a round-trip assistant frequency signal. The phase error can be exactly cancelled by a series of frequency mixing (i.e., up-conversion and down-conversion) among the signals. In the experiment, a 1.21-GHz frequency signal at an arbitrary remote point is downlink transferred to the central station in a 45-km fiber loop link. The result shows the overlapping Allan deviation (ADEV) of 1.04×10 -12 at 0.1 s, 1.3×10 -13 at 1 s and 1.1×10 -15 at 10 4 s, respectively. The phase error correction operates entirely at the central station, leaving a simple and robust configuration of the remote site. No active adjusting part is integrated, and the all-passive compensation achieves an endless phase error correction range, as well as quick response to fiber delay changes.