Precise Time Delay Sensing and Stable Frequency Dissemination on Arbitrary Intermediate Point Along Fiber-Optic Loop Link With RF Phase Locking Assistance

Precise time delay sensing combined with stable frequency dissemination on an arbitrary intermediate point along a fiber-optic loop link is presented. Based on λ-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 ±50-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 103-s averaging, as compared to the real delay value. The overlapping Allan deviation of the recovered 2.42-GHz frequency reaches 2.04 × 10¹³ and 1.71 × 10¹⁶ at 1 s and 10⁴ 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.