Optically Stabilized Reference for Ultralow‐Noise Microwave Generation by Optical Frequency Division

Optical frequency division (OFD) provides the purest microwaves by down‐converting the stability of optical cavity references. State‐of‐the‐art references typically rely on electronic co‐Pound‐Drever‐Hall locking to high‐ Q microresonators, a complex approach that introduces servo bumps and increases footprint. Alternatively, optical co‐self‐injection‐locking (co‐SIL) offers inherent simplicity. Here, we demonstrate a two‐point OFD‐based microwave oscillator that combines a high‐ Q miniature Fabry–Pérot cavity with optical co‐SIL. By combining this low‐relative‐phase‐noise optical reference with an integrated soliton microcomb, the system generates a 50‑GHz microwave with −133 dBc/Hz phase noise at 4‑kHz offset, affording 134 dB noise reduction over the free‑running DFB laser near 1‑kHz offset—a performance competitive with most electronically stabilized systems. We further identify the transverse cavity mode used for co‐self‐injection locking as a factor for common‐mode noise rejection between the two lasers. This work marries the superior noise performance of high‐ Q cavities with the simplicity of optical locking, providing a compact optical reference module that enables low‐noise microwave generation for next‐generation communications, radar and metrology.