Optically locked low-noise photonic microwave oscillator.

The next generation of sensing and communication applications rely on high-frequency microwave generation with low noise. Microwave photonic technology is highly promising; however, its practical applications have so far been limited by the complexity of existing system architectures. Here, we demonstrate an optically locked low-noise photonic microwave oscillator, in which all the optical components are packaged within a small module of 166 mL; low noise microwave generation is achieved at 10.4 GHz with single-sideband phase noise of -54 dBc/Hz at 10 Hz, -141 dBc/Hz at 10 kHz, and -162 dBc/Hz at 10 MHz offset. The above performance arises from a dual-laser self-injection-locking scheme to a single Fabry-Pérot cavity with high Q exceeding 108, with >20 dB common-mode noise suppression. The low-noise nature of this reference is coherently transferred to the X-band through a high-performance thin-film lithium niobate (TFLN) electro-optic comb chip, thereby overcoming long-standing barriers in photonic microwave integration to enable truly field-deployable low-noise microwave generation.