Breaking the bandwidth-efficiency trade-off in soliton microcombs via mode coupling

Abstract

Dissipative Kerr solitons in optical microresonators have emerged as a powerful tool for compact and coherent frequency comb generation. Advances in nanofabrication have allowed precise dispersion engineering, unlocking octave-spanning soliton combs that are essential for applications such as optical atomic clocks, frequency synthesis, precision spectroscopy, and astronomical spectrometer calibration. However, a key challenge hindering their practical deployment is the intrinsic bandwidth-efficiency trade-off: achieving broadband soliton generation requires large pump detuning, which suppresses power coupling and limits pump-to-comb conversion efficiencies to only a few percent. Recent efforts using pulsed pumping or coupled-resonator architectures have improved efficiency to several tens of percent, yet their bandwidths remain below one-tenth of an octave, inadequate for applications demanding wide spectral coverage. Here, we overcome this limitation by harnessing mode interactions between spatial modes within a single microresonator. The mode hybridization creates an additional power-transfer channel that supports large pump detuning while maintaining strong pump-to-resonator coupling, enabling broadband soliton formation at substantially reduced pump power. Using this approach, we demonstrate an octave-spanning soliton microcomb with a record pump-to-comb conversion efficiency exceeding 50%. These results resolve the fundamental bandwidth-efficiency dilemma in soliton microcombs and paves the way toward fully-integrated, high-efficiency, ultrabroad comb sources for next-generation photonic systems.

Figures (4)

• Figure 1: Broadband and efficient dissipative Kerr soliton generation enabled by mode coupling. (a) Conventional dissipative Kerr soliton (DKS) generation in a microresonator, where the pump red-detuning slightly from the comb mode. (b) Typical DKS comb spectrum showing substantial residue pump power exceeding the comb peak. Inset: temporal soliton profile over a round-trip time. (c) DKS generation in the mode coupling regime, where an auxiliary mode (TE$_{10}$) hybridizes with the comb mode (TE$_{00}$). Pumping the hybridized resonance larger effective detuning and enhances power transfer via mode hybridization. (d) DKS comb spectrum under mode coupling. Extended detuning broadens the spectrum, while resonant pumping at the hybridized mode leads to efficient pump depletion. Inset: soliton pulse trace with higher peak power and reduced background. (e) Schematic of a multimode microresonator supporting coupling between TE$_{00}$ and TE$_{10}$ modes. (f) Measured integrated dispersion (D$_{int}$) of both modes with respect to the free spectra range (FSR) of the TE$_{00}$ mode at 1556 nm, showing an avoided mode crossing (AMX). (g) Zoomed-in of the dispersion near the AMX region (dashed box in (f)). (h) Measured transmission spectrum near the AMX, where mode coupling shifts both resonances from their unperturbed positions (marked by the vertical blue and red lines for TE$_{00}$ and TE$_{10}$, respectively). (i) Measured DKS comb spectrum at 50 mW pump power showing an octave-spanning bandwidth and nearly complete pump depletion.
• Figure 2: Mode coupling extends soliton existence range and enhances conversion efficiency. Single-soliton existence and pump-to-comb conversion efficiency maps for microcombs under (a) single-mode pumping and (b) coupled-mode pumping schemes. The normalized comb detuning is defined with respect to the unperturbed resonance of the comb mode (i.e., in the absence of coupling). Color shading represents conversion efficiency, and solid contour lines indicate the 70-dB fractional comb bandwidth. Arrows highlight directions toward improving either efficiency or bandwidth. Single-mode pumping reveals a trade-off between bandwidth and efficiency, while coupled-mode pumping introduces a localized regime where both can be simultaneously optimized. (c-d) Single-soliton existence and efficiency map under coupled-mode pumping at constant pump power of 8 for different phase of coupling: (c) $\varphi=\pi$, (d) $\varphi=\frac{11}{18}\pi$ (the measured phase), and (e) $\varphi=0$. $\varphi$ is defined in coupling strength $K_c = |K_c|\exp(i\varphi)$. The blue and red dashed lines trace the intrinsic resonance of comb mode and auxiliary mode, with the auxiliary mode kept red-detuned from the comb mode.
• Figure 3: Accessing single-soliton state with large effective comb detuning at low pump power. (a, b) Measured comb power traces as the pump wavelength is tuned at high (a) and low (b) pump power under the coupled-mode pumping scheme. Blue traces show the comb power evolution during forward tuning pump across the coupled-mode resonance. Color-shaded regions indicate the existence ranges of single-soliton states. At low pump power, two distinct soliton existence regions emerge, corresponding to low-efficiency and high-efficiency branches. Forward tuning at low power (b) accesses only the low-efficiency branch. The high-efficiency branch can be reached by: 1) increasing the pump power beyond the threshold where the two branches merge; 2) forward tuning into the single soliton state at large effective comb detuning; and 3) subsequently reducing the pump power while maintaining the soliton state. The red trace shows the comb power evolution when tuning the pump after reaching the high-efficiency branch from a high-power state. (c, d) Measured pump detuning for soliton combs operating in the low-efficiency (c) and high-efficiency (d) branches identified in (b). Dashed lines mark the position of the unperturbed TE$_{00}$ resonance. (e) Measured soliton comb spectra at low pump power. Blue and red spectra correspond to the detuning cases shown in (c) and (d), respectively. The extended effective comb detuning ($\delta$) in the high-CE branch enables ultra-broadband comb generation.
• Figure 4: Enhancing pump-to-comb conversion efficiency via tailored mode coupling. (a) Transmission spectrum of the microresonator at different temperatures. (b) Measured integrated dispersion (D$_{int}$) of the TE$_{00}$ mode at different temperatures, demonstrating that the mode coupling strength (evident as the coupling-induced resonance shift of the comb mode) can be significantly tuned. (c) Measured pump-to-comb conversion efficiency (CE) as a function of pump power for different initial mode coupling strengths. CE is evaluated at the maximum achievable effective comb detuning under each pumping condition. A maximum CE of approximately 53% is obtained by increasing the mode coupling through temperature tuning to $45\,^\circ\mathrm{C}$.