Tunable microwave frequency synthesis with optically-derived spectral purity

TL;DR

This paper introduces a feed-forward electro-optic frequency division method that achieves wide tunability and ultra-low phase noise in microwave synthesis, surpassing previous limitations in spectral purity and frequency range.

Contribution

The authors demonstrate a novel feed-forward eOFD architecture that maintains spectral purity while enabling octave-spanning tunability without electronic frequency synthesis.

Findings

Achieved phase noise below -140 dBc/Hz at kilohertz offsets.

Demonstrated octave-spanning tunability including the entire X-band.

Achieved single-femtosecond integrated timing jitter.

Abstract

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, however, 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. Here we demonstrate a feed-forward eOFD architecture in which the frequency tunability of a microwave source is preserved while optical spectral purity is divided through feed-forward cancellation, without any downstream electronic frequency synthesis. By canceling the phase noise of the microwave source without feedback, this eOFD approach removes loop bandwidth and source noise constraints observed in prior eOFD architectures. We achieve octave-spanning tunability, including the entire X-band, with phase noise below -140 dBc/Hz at kilohertz offsets and a high-frequency noise floor between -155 dBc/Hz and -145 dBc/Hz for carrier frequencies from 8 to 16 GHz. This performance corresponds to single-femtosecond integrated timing jitter, enabling, to our knowledge, the first demonstration of coherent, optically referenced microwave synthesis under wide tuning with this level of spectral purity.