Quiet point engineering for low-noise microwave generation with soliton microcombs

TL;DR

This paper introduces a deterministic method to engineer quiet points in soliton microcombs using coupled resonators, significantly reducing phase noise in microwave signals by controlling mode interactions.

Contribution

It presents a novel approach to engineer quiet points with increased spectral width and position control, enhancing low-noise microwave generation in integrated microcombs.

Findings

Discovered a continuum of quiet points with ultra-low noise within the soliton region.

Achieved increased noise suppression range through controlled optical mode crossings.

Demonstrated potential to reach fundamental thermodynamic noise limits.

Abstract

Low-noise microwave signals can be efficiently generated with microresonator-based dissipative Kerr solitons ('microcombs'). However, the achieved phase noise in integrated microcombs is presently several orders of magnitude above the limit imposed by fundamental thermorefractive noise. One of the major contributors to this additional noise is the pump laser frequency noise transduction to the soliton pulse repetition rate via the Raman self-frequency shift. Quiet points (QPs) allow minimizing the transduction of laser frequency noise to soliton group velocity. While this method has allowed partial reduction of phase noise towards the fundamental thermodynamical limit, it relies on accidental mode crossings and only leads to very narrow regions of laser detuning where cancellation occurs, significantly narrower than the cavity linewidth. Here we present a method to deterministically engineer the QP, both in terms of its spectral width, and position, showing an increased phase noise suppression. This is achieved using coupled high-Q resonators arranged in the Vernier configuration. Investigating a generalized Lugiato-Lefever equation that accounts for the hybridized mode spectral displacement, we discover a continuum of possible QPs within the soliton existence region, characterized by ultra-low noise performance. Furthermore, we discover that by using two controlled optical mode crossings, it is possible to achieve regions where the QPs interact with each other enabling a substantial increase of the noise suppression range. Our work demonstrates a promising way to reach the fundamental limit of low-noise microwave generation in integrated microcombs.