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

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.

Figures (3)

• Figure 1: Cavity ring-down measurement of a 900 MHz-FSR resonator, yielding an ultrahigh Q factor of $6.4\times10^9$. The data (gray), moving average (blue), and exponential fit (green) are shown.
• Figure 2: (a,b) Top-view photographs of the fabricated $\mathrm{MgF_2}$ crystalline resonators, shown alongside a U.S. dime, a euro coin, and a Japanese Tenpō-Tsūhō coin. Five resonators with diameters of 58 mm, 77 mm, 44 mm, 28 mm, and 17 mm are arranged in a clockwise direction, and the largest resonator yielding a sub-GHz soliton comb. (c–g) Measured optical spectra of single-soliton combs with $\sim\mathrm{sech^2}$ fitting curves. The insets show the electrical spectra of the soliton beat note with a resolution bandwidth of 10 Hz. $\Delta \tau$ denotes the pulse duration. CF: center frequency.
• Figure 3: SSB phase noise of a 1.6 GHz soliton comb, compared with commercial benchtop electronic microwave signal generators.