Noise limitations in microwave generation from optical references

Combining a femtosecond laser optical frequency comb (OFC) with the high quality factor (Q) available in optical cavities allows for the realization of microwave signals with phase noise properties surpassing state-of the-art microwave oscillators. Optical Fabry-Perot (FP) resonators can have Q values exceeding 10¹¹, greater than room temperature microwave dielectric resonator oscillators (DRO) at ~10⁵ , and even exceeding that of cryogenic DROs at ~10⁹ . When an optical frequency comb is phase locked to an optical reference, the comb can be thought of as a high fidelity frequency divider, transferring the stability of the optical reference to the microwave domain. With this division in frequency comes a division in phase noise, thus an optical reference with a fractional frequency stability of ~4-10^-16 (and assuming flicker FM) can ideally generate a 10 GHz signal with phase noise of ~110 f³ dBc/Hz. At 1 Hz offset from the carrier, this represents a 40 dB improvement over the best room temperature 10 GHz microwave oscillators, and is below what has been demonstrated with cryogenic 10 GHz oscillators. Work with Er:fiber-based frequency combs has shown a residual phase noise in optical-to-microwave conversion ~118 dBc/Hz at 1 Hz offset from a 11.55 GHz carrier. In recent works, the authors demonstrated absolute phase noise below -104 dBc/Hz at 1 Hz offset from a 10 GHz carrier by comparing two independent systems that employ 1 GHz mode-locked Ti:sapphire laser combs. The results were presented in this paper and this to the authors knowledge represents the lowest close-to-carrier phase noise yet reported on a 10 GHz source.