Thermally accessible broadband soliton microcombs in silicon carbide enabled by dynamic polarization control

Abstract

Optical microcombs generated in high-Q microresonators are promising chip-scale light sources for applications ranging from optical communications to spectroscopy and metrology. However, thermo-optic instabilities remain a major obstacle to reliable soliton access. Self-cooling using auxiliary modes can stabilize the intracavity power, yet part of the power is continuously allocated to thermal compensation rather than comb generation, thereby limiting comb power and bandwidth. Here we propose a thermal compensation scheme based on dynamic polarization control. During soliton initiation, a fraction of the pump is coupled to an orthogonally polarized mode to provide self-cooling and ensure reliable soliton access. After soliton formation, polarization rotation and pump tuning transfer this cooling power to the comb-generating mode, enabling efficient single-soliton operation. Using this approach, we experimentally demonstrate a broadband 108-GHz-FSR single-soliton microcomb spanning over 450 nm, together with approximately 39% improvement in the 20-dB bandwidth and 60% increase in comb power relative to the static self-cooling configuration. This dynamic polarization-based thermal compensation enables efficient use of available laser power and provides a practical route to high-performance soliton microcombs in platforms with strong thermo-optic effects.

Figures (3)

• Figure 1: Broadband single-soliton comb generation via thermal compensation using dynamic polarization control. (a) Schematic of pump polarization control. The TE mode is employed for comb generation, while the TM mode provides self thermal cooling. State 1 corresponds to a purely-TE polarized pump. State 2 and 3 represent obliquely polarized pumping conditions, under which a single soliton can be accessed. From State 2 to State 4, the TM fraction is gradually reduced until the input returns to pure TE polarization. (b) Evolution of comb power ($P$) as a function of pump detuning ($\delta$) and corresponding comb spectra across different states. The procedure is initiated at State 2, where an obliquely polarized pump introduces TM-assisted thermal cooling and enables stable access to the soliton existence range (SER, green shaded regime), mitigating the thermo-optic instabilities that typically obscure the soliton state (dashed line) in the purely TE-polarized case (State 1). After soliton initiation, the cooling power is redirected to the TE comb mode via polarization rotation and pump blue-detuning (no speed requirement), enhancing both output comb power and bandwidth (from State 2 to State 4). The comb spectra corresponding to the maximum comb detuning on the right are schematic representations based on experimental observations, illustrating the continuous broadening bandwidth as cooling power is redirected to the comb mode.
• Figure 2: Characterization of the fabricated SiC microresonator. (a) SEM images of the fabricated SiC microresonator showing the top view (left) and cross section (right). (b) Measured transmission spectra of the fundamental TE₀₀ (green) and TM₀₀ (red) modes across the telecom C-band. (c), (d) Integrated dispersion of TE (anomalous) and TM (normal) modes, with insets showing simulated mode profiles. The small spatial overlap between the modes leads to negligible TE-TM coupling, consistent with the smooth dispersion and absence of avoided mode crossings near the pump wavelength. (e) Zoomed-in transmission spectra around the pump (corresponding to the dashed box in (b)), showing no significant extinction degradation. Lorentzian fitting of the closest TE resonance near the TE-TM mode crossing yields an intrinsic Q of $(3.5 \pm 0.07)\times 10^{6}$, close to the most probable value shown in the intrinsic Q histogram of TE mode (f), confirming that the high Q is preserved despite the nearby TE-TM resonance crossing.
• Figure 3: Experimental demonstrations of soliton comb generation. (a-c) Statistical comb power traces from 100 pump wavelength sweeps at a fixed on-chip pump power of 23 dBm (insertion loss: 4 dB/facet), corresponding to State 2, State 3, and States 1 and 4, respectively. The trace opacity indicates the probability of accessing different soliton numbers, with colored segments highlighting the single-soliton detuning range. Insets show the transmission spectra of the pump resonances captured during the low-power characterization, revealing the mixed TE (green) and TM (red) mode composition of each state. In State 1 (c, dashed traces), no soliton step appears under a purely-TE polarized pump. High-power single-soliton operation is achieved by first using an obliquely polarized pump (a, b), followed by dynamic polarization rotation and pump wavelength tuning to reach State 4 (c, green traces). A larger TM component enhances single-soliton access probability and stability, making it easier for single-soliton initiation. (d-f), Measured single-soliton spectra at the maximum comb detuning for State 2 to State 4, showing a progressive bandwidth broadening as the power coupled to the TM cooling mode is redirected to the TE comb mode. A purely TE-polarized pump ultimately sustains a single-soliton comb spanning over 450 nm.