Precise time delay sensing combined with stable frequency dissemination on an arbitrary intermediate point along a fiber-optic loop link is presented. Based on -dispersion-induced radio-frequency (RF) phase locking, the whole loop time delay and phase shift are first stabilized. The time and frequency signals are carried on the same optical carrier and are delivered to both the clockwise and anticlockwise directions. On the intermediate point, the instantaneous time delay from the central station to that point can be acquired by measuring the delay difference. The stable frequency standard with twice the angular frequency of the RF reference can be recovered on the intermediate point by mixing the RFs in the two directions. In the 45-km fiber loop experiment, the variation of the sensed time delay is limited in AE50-ps range, and the time deviation of the sensed time delay is measured to be 19.79 ps after 1-s averaging and 0.896 ps after 10 3 -s averaging, as compared to the real delay value. The overlapping Allan deviation of the recovered 2.42-GHz frequency reaches 2:04 Â 10 À13 and 1:71 Â 10 À16 at 1 s and 10 4 s, respectively. The loop delay tunable range is in proportion to the fiber length, giving the potential of constructing a long-distance fiber loop link.
In this work, we present a coherent distributed radio frequency (RF) array, discover and quantitatively describe the strong positive correlation between reconstructed signals for the first time. Eight replicable parallel receivers are connected to the phase-locked common trunk link via eight optical couplers spaced 1 km apart. The forward and backward signals at each receiver, extracted from two ports of optical couplers, are recovered to RF signals separately and then mixed to achieve upward frequency conversion. The link delay jitter is counteracted by wavelength-tuning of the optical carrier. With the long-term stability of point-to-multipoint fiber-optic RF dissemination effectively improved, the coherent distributed array is generated, and further the relative frequency stability between signals at different receivers is studied. The proposed correlation coefficient at 103 s is ∼0.8 and shows a slight downward trend with the increase of averaging time based on our experimental results.
A flexible frequency dissemination scheme is proposed. Frequency signal can be injected from arbitrary point along the RoF loop link and recovered at other intermediate sites. Multiple-to-multiple frequency dissemination network can be expected.
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