Simultaneous frequency and time delivery on intermediate point of RoF loop link

We present and experimentally demonstrate a technique of simultaneous frequency and time information delivery on arbitrary intermediate point along a fiber loop link based on λ-dispersion-induced radio frequency (RF) phase locking. The equal and stable loop phase shift or time delay, clockwise and anticlockwise, is firstly achieved by altering the optical carrier´s wavelength in the dispersive fiber link as well as a RF phase locking process in the central station. Due to the natural broadband operation, the frequency standard and time signal is carried always on the same wavelength. Given the locked loop phase shift, the stable frequency standard with twice the angular frequency of the RF reference can be recovered on intermediate point by simply mixing the RF signals in the two directions. Simultaneously, the delay difference between two directions from the central station to an arbitrary intermediate point is acquired, and the precise time information on the intermediate point is finally obtained through the above delay difference and the constant total loop delay. A 45-km fiber loop with two intermediate points is experimentally demonstrated. The Overlapping Allan Deviation of the recovered frequency reaches 2.04×10^-13 after 1-s averaging and 1.71×10^-16 after 10⁴-s averaging. The Time Deviation (TDEV) of the delivered time information is measured to be 19.79 ps and 0.896 ps at 1 s and 10³ s, respectively. The loop delay tunable range is in direct proportion to the fiber length, giving the capability of constructing a very long loop link.