Centi-combs: Low-noise sub-GHz repetition-rate soliton frequency combs from crystalline resonators

TL;DR

This paper reports the creation of low-noise, sub-GHz repetition rate Kerr soliton frequency combs using ultrahigh-Q crystalline magnesium fluoride resonators, advancing the operational range of microcombs.

Contribution

It introduces a new method to generate low-noise, sub-GHz Kerr soliton combs in crystalline resonators, bridging the gap between traditional lasers and microresonator combs.

Findings

Achieved single soliton states with repetition rates from 0.90 GHz to 4.10 GHz.

Demonstrated phase noise of -137 dBc/Hz at 100 kHz offset for near-GHz combs.

Expanded the operational frequency range of Kerr microcombs into the sub-GHz domain.

Abstract

We demonstrate low-noise Kerr soliton frequency combs with repetition rates below 1~GHz in ultrahigh-Q crystalline magnesium fluoride resonators. Single soliton states with repetition rates of 0.90 GHz, 1.19 GHz, 1.59 GHz, 2.48 GHz, and 4.10 GHz are observed with continuous-wave laser excitation. The near-GHz soliton repetition frequency exhibits a single-sideband phase noise of -137 dBc/Hz at a 100 kHz offset, surpassing state-of-the-art microwave generators. These ``centi-combs'' bridge the gap between conventional mode-locked lasers and microresonator frequency combs, providing a new route towards real-time sampling, optical-to-microwave synchronization, and hybrid optical clock networks in a compact form. This work expands the operational range of Kerr soliton microcombs from the terahertz to the sub-gigahertz domain, opening new frontiers for frequency comb technologies.