Fiber-based radio frequency phase synchronization scheme without a time benchmark

We present a novel, to the best of our knowledge, fiber-optic radio frequency (RF) phase synchronization scheme utilizing frequency division multiplexing, eliminating the need for a time reference. The method begins by generating two homologous RF signals with an integer multiple frequency relationship. These signals are then modulated onto the same laser carrier through intensity modulation. A phase detector with a 2π rad range is used to measure phase variations in the lower frequency signal induced by the fiber link. An optical delay line compensates for the fiber-induced delay, locking the phase between the reference signal and the reflected signal. Consequently, the phase of the higher frequency signal is synchronized due to its harmonic relationship with the lower frequency signal. Phase synchronization over 50 km fiber spools with accuracy of less than 2 ps is demonstrated. This simple and scalable approach is well-suited for distributed applications that require high-precision time and frequency synchronization, such as in distributed phase array radar and radio telescope array.