Ultra-low phase noise microwave generation with a free-running monolithic femtosecond laser

Phase noise performance of photonic microwave systems, such as optical frequency division (OFD), can surpass state-of-the-art electronic oscillators by several orders of magnitude. However, high-finesse cavities and active stabilization requirements in OFD systems make them complicated and potentially unfit for field deployment. Ultra-low noise mode-locked monolithic lasers offer a viable alternative for a compact and simple photonic microwave system. Here we present a free-running monolithic laser-based 8 GHz microwave generation with ultra-low phase noise performance comparable to laboratory OFD systems. The measured noise performance reached -130 dBc/Hz at 100 Hz, - 150 dBc/Hz at 1 kHz, and -167 dBc/Hz at 10 kHz offsets from the 8-GHz carrier. We also report a sub-Poissonian noise floor of -179 dBc/Hz above 30 kHz (timing noise floor of 32 zs Hz^-1/2), which is ∼12 dB below the noise floor of time-invariant shot noise. In addition to the low phase noise, the system is compact, with a power consumption of less than 9 W, and offers excellent potential for mobile or space-borne applications.