Passively stabilized Brillouin fiber laser frequency combs for ultralow-noise microwave generation

Ultralow-noise microwaves are essential in a wide variety of scientific and technological applications, such as metrology, radars, and communications. Here, we propose and demonstrate a scheme for generating an ultralow-noise microwave signal using a Brillouin optical frequency comb (OFC), which is based on the stimulated Brillouin scattering in combination with a frequency-shifted optical injection locking mechanism. The generated two intra-cavity Brillouin lasers are used as the intra-cavity pump for the eventual formation of the OFC and thus the microwave signal via the cascaded four-wave mixing process. Exploiting the cascaded narrowing effect in the Brillouin cavity assisted by the frequency-shifted optical injection locking, the proposed microwave signal source exhibits ultralow phase noise. Experimental results show that the phase noise of the microwave signal is equivalent to below −115 dBc/Hz for a 200-GHz carrier at 10 kHz offset. The system can work in stable operation without the need for any active feedback loop.