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, much 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 [1-3]. With this division in frequency comes a division in phase noise, thus an optical reference with a fractional frequency stability of ∼4·10¹⁶ (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 [4], and is below what has been demonstrated with cryogenic 10 GHz oscillators [5]. 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 [2]. Recently, we 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 [3]. To our knowledge, this represents the lowest close-to-carrier phase noise yet reported on a 10 GHz source. A schematic of the microwave generation architecture is shown in Fig. 1(a) and data are presented in Fig. 1(b).