Quiet Point Engineering for Low-Noise Microwave Generation with Soliton Microcombs

Microresonator frequency combs allow the direct synthesis of low-noise microwaves by detecting the soliton pulse stream [1]. It has been used to create both low-noise sources of millimeter waves as well as K and X-band microwaves [2]. However, the fundamental limit of the repetition rate noise imposed by thermorefractive noise in such devices is orders of magnitude below the best experimentally demonstrated noise performance. It has been recently observed [3], [4] that particular cavity dispersion profiles enable phase noise reduction by exploiting the interplay between Raman self-frequency shift and dispersive wave (DW) radiation. This particular operating point is often referred to as a quiet point (QP). However, previous studies rely on the fixed-by-design dispersion which restricts the noise reduction capabilities. Fig 1. Mode displacement engineering (a) principle of the low-noise generation via the microresonator dispersion engineering. (b) Engineered dispersion profile with two localized amxs. (c) Fourier spectrum of the corresponding stationary solution. (d,f) Colormap of the soliton group velocity for a single and double mode displacement, respectively. (e,g) Corresponding directional derivative alone detuning direction.