Low-noise microwave generation based on optical-microwave synchronization

Low-noise microwave signals are of vital importance in fields such as cold atomic optical clocks, photon radars, and remote synchronization at large facilities. Here, we report a compact all-optical-fiber method to generate a low noise microwave signal, in which the fiber loop optical-microwave phase detector is used to coherently transfer the frequency stability of the ultra-stable laser to the microwave. Combining a narrow linewidth optical frequency comb and a fiber loop optical-microwave phase discriminator, a tight phase-lock between 7 GHz dielectric oscillator and optical frequency comb is achieved, the remaining phase noise of the synchronized optical pulse sequence and the microwave signal is –100 dBc/Hz@1 Hz, and the timing jitter is 8.6 fs (1 Hz—1.5 MHz); by building two sets of low-noise microwave generation systems, the measured residual phase noise of the 7 GHz microwave is –90 dBc/Hz@1 Hz, and the corresponding frequency stability is 4.8 × 10^–15@1 s. These results provide a novel idea for generating the low-noise microwaves based on optical coherent frequency division.