Widely tunable, low phase noise microwave source based on a photonic chip.

Spectrally pure microwave sources are highly desired for several applications, ranging from wireless communication to next generation radar technology and metrology. Additionally, to generate very pure signals at even higher frequencies, these advanced microwave sources have to be compact, low in weight, and low energy consumption to comply with in-field applications. A hybrid optical and electronic cavity, known as an optoelectronic oscillator (OEO), has the potential to leverage the high bandwidth of optics to generate ultrapure high-frequency microwave signals. Here we present a widely tunable, low phase noise microwave source based on a photonic chip. Using on-chip stimulated Brillouin scattering as a narrowband active filter allows single-mode OEO operation and ultrawide frequency tunability with no signal degeneration. Furthermore, we show very low close-to-carrier phase noise. This Letter paves the way to a compact, fully integrated pure microwave source.