Tunable microwave frequency synthesis with optically-derived spectral purity.

Microwave synthesizers are central to test and measurement systems across applications including wireless communications, radar, spectroscopy, and time and frequency metrology. State-of-the-art microwave sources are fundamentally constrained by trade-offs between frequency tunability and spectral purity. Electro-optic frequency division (eOFD) is an emerging technique for dividing down the purity of optical sources to the microwave domain. Previously reported eOFD-based synthesizers generally have limited tunability due to feedback stabilization requirements. We demonstrate a feed-forward eOFD architecture where frequency tunability is preserved while optical spectral purity is divided without any downstream electronic frequency synthesis. By canceling the phase noise of a microwave source without feedback, this eOFD approach removes loop bandwidth and source noise constraints observed in prior work. Here we show octave-spanning tunability accross the entire X-band (8-16 GHz) with phase noise below -140 dBc/Hz at kilohertz offsets and a noise floor between -155 dBc/Hz and -145 dBc/Hz corresponding to single-femtosecond integrated timing jitter.