Engineered mode coupling in high-Q microresonators enables deterministic low-repetition-rate soliton microcombs

TL;DR

The paper tackles the challenge of creating low-repetition-rate soliton microcombs in long, high-Q cavities, where thermal effects hinder stable soliton access. It introduces engineered intermodal coupling by designing Euler bends in racetrack Si3N4 microresonators to couple a nearby auxiliary mode to the pump, enabling passive thermal compensation without external tuning. The authors demonstrate thermally robust, deterministic generation of a 33 GHz soliton microcomb with ~300 nm bandwidth spanning the C- and L-bands while preserving high-Q and a compact footprint. This approach offers a practical route to broadband, low-repetition-rate soliton microcombs for optical communications, microwave photonics, and precision metrology, and can be extended to other thermo-optic platforms.

Abstract

Soliton optical frequency combs have become key enablers for a wide range of applications, including telecommunications, optical atomic clocks, ultrafast distance measurements, dual-comb spectroscopy, and astrophysical spectrometer calibration, many of which benefit from low repetition rates. However, achieving such low-repetition-rate soliton microcombs is nontrivial as long cavities require substantially higher pump power, which induces stronger thermal effects that, in turn, exacerbate thermal instability and complicate access to stable soliton states. The dual-mode pumping scheme, in which a continuous-wave pump couples to both the comb-generating mode and an auxiliary mode, has proven simple and effective for mitigating thermal instability and enabling thermally accessible soliton generation. Yet, in long-cavity devices, the standard bus-to-resonator coupling conditions for these two modes diverge substantially, resulting in insufficient pump coupling to the auxiliary mode, which makes dual-mode pumping particularly challenging for low-repetition-rate microcombs. In this work, we overcome this limitation by coupling the pump to the auxiliary mode via inter-modal coupling, which can be introduced in racetrack microresonators and engineered by tailoring the cavity bend design. We validate this approach in a high-Q (>$10^7$) silicon nitride microresonator and demonstrate thermally accessible, deterministic single-soliton generation at a repetition rate of 33 GHz. This work provides a simple and robust pathway for generating low-repetition-rate soliton microcombs.

Figures (4)

• Figure 1: Proposed scheme of auxiliary mode introduction for low-repetition-rate soliton microcomb generation. (a) Illustration of the desired microresonator supports two modes, where one serves as the fundamental mode for comb generation and the auxiliary mode provides thermal compensation. (b) Auxiliary mode introduction through the conventional point coupler (inset). Green curve: The bus-to-ring coupling efficiency at the critical coupling condition with respect to free spectral range for 10 dB/m and 5 dB/m propagation loss. Red curve: Maximum achievable coupling efficiency for the $\rm TE_{10}$ mode under a point coupler with a bus-to-ring gap of 50 nm. (c) The proposed soliton generation scheme in a device configuration of a racetrack microresonator consists of straight and bent waveguides. The soliton is generated in the fundamental mode (orange), and the auxiliary mode (green) is used for thermal compensation. Dashed red box: mode coupling is introduced through engineering the curved section of the racetrack microresonator.
• Figure 2: Mode coupling engineering in multimode racetrack resonators. Scanning electron microscopy (SEM) image of the fabricated (a) racetrack microresonator, (b) its cross-section, and (c) sidewall. Proposed mode coupling engineering through (d) circular bends with 40 $\rm \mu m$ radius and Euler bends with minimum bending radius ($\rm R_{min}$) of (e) 25 $\rm \mu m$ and (f) 75 $\rm \mu m$ at curved sections. Left: optical microscope. Right: Transmission spectrum. (g, h, i) Corresponding linear characterization. First row: measured and fitted integrated dispersion ($\rm D_{int}/2\pi$). Second row: Resonance frequency deviation ($\rm \Delta f$) from the fitted curve, which reflects the mode coupling strength induced to the TE$_{00}$ mode. Third row: Intrinsic linewidth ($\rm \kappa_0/2\pi$) distribution across a broad wavelength range.
• Figure 3: Linewidth and frequency deviation measurement. (a) Resonance linewidth characterization of racetrack microresonators with different bend designs. (b) Root-mean-square frequency deviation ($\rm \Delta f$) and intrinsic Q measurement in Euler bend with different minimum bending radius ($\rm R_{min}$).
• Figure 4: Low-repetition-rate soliton microcomb generation. (a) Schematic drawing of the experimental setup. AWG: arbitrary waveform generator, ECDL: External Cavity Diode Laser, PC: Polarization Controller, EDFA: Erbium-Doped Fiber Amplifier, LPF: Long-pass filter, PD: Photodetector, OSC: oscilliscope, FBG: Fiber Bragg Grating, OSA: Optical Spectrum Analyzer. (b) Plot of 20 overlaid experimental traces of the output comb power (gray) in the pump forward (blue) and backward tuning (red) over the resonance with the same pump power and tuning speed. The right panel shows the magnified view of the backwards tuning with successive switching of soliton states. (c) The optical spectrum of a broadband single soliton comb with a repetition rate of 33 GHz, where the pump is filtered by a FBG.