Octave-spanning, deterministic single soliton generation in 4H-silicon carbide-on-insulator microring resonators

TL;DR

The paper tackles the problem of realizing chip-scale, octave-spanning, self-referenced frequency combs with deterministic single-soliton generation under low power. It introduces dispersion-managed, multi-mode 4H-SiCOI microring resonators that separate comb operation ($TE_{00}$, anomalous dispersion) from laser cooling light ($TE_{10}$ or TM$^{-1}$, normal dispersion), thereby suppressing thermal instabilities. The authors achieve high quality factors ($Q$ up to $5.8\times10^{6}$) with sub-milliwatt Kerr-comb thresholds, and demonstrate deterministic octave-spanning single solitons at on-chip powers as low as $P_{pump}=60~\mathrm{mW}$ with an SER around $5~\mathrm{GHz}$ and a spectral span from $136$ to $307~\mathrm{THz}$ accompanied by two dispersive waves. These results pave the way for turnkey, chip-scale self-referenced frequency combs and have potential applicability to other platforms with large thermo-optic coefficients, advancing integrated metrology and spectroscopy applications.

Abstract

The miniaturization of self-referencing frequency comb systems enables emerging applications in metrology and spectroscopy. One major challenge in realizing the chip-scale self-referencing function is to generate octave-spanning soliton microcombs with low operation power. Accessing soliton states is also not trivial due to the thermal effect. Though an auxiliary laser was utilized to compensate for the thermal effect, deterministic single soliton generation is still elusive, especially for broadband operation. In this work, dispersion management is performed for a 4H-silicon carbide-on-insulator (SiCOI) multi-mode microring resonator, benefiting from the submicron-confinement waveguide layout. The fundamental transverse electric (TE) mode is engineered to anomalous dispersion for two dispersive waves generation over an octave span. While a higher order TE mode is engineered to normal dispersion to accommodate the auxiliary light for thermal compensation. The normal dispersion prevents modulation-instability Kerr comb generation, allowing for a large soliton existence range. We achieve microring resonators with Q up to 5.8 million and sub-milli-watt-threshold Kerr comb generation. Combining the dispersion-managed design and high Q device, we demonstrate the deterministic generation of a single soliton comb spanning beyond an octave with a low on-chip power of 60 mW. Our demonstration paves the way to realize chip-scale, turn-key, self-referenced frequency combs.

Figures (4)

• Figure 1: Design and characterization of SiCOI microring resonators. (a) Schematic cross-section of a SiCOI waveguide defined by waveguide height (${\rm H}$), waveguide width (${\rm W}$), slab thickness (${\rm h}$), and sidewall angle ${\rm \theta}$. (b) Calculated effective mode area (color-shaded contour) and the group velocity dispersion (GVD) (solid-line contour) for the fundamental ${\rm TE_{00}}$ mode (the comb operation mode) of SiCOI waveguides with different cross-section dimensions at 1560 nm. The green contour line represents the zero GVD design for the ${\rm TE_{10}}$ mode (the laser cooling mode). The grey-shaded area is the target design region. (c) Scanning electron microscopy (SEM) image of a fully etched microring resonator waveguide with its cross-sectional view shown in the inset. (d) Histogram of intrinsic resonance linewidth for ten fully-etched microring resonators with the same waveguide dimension (${\rm H = 340}$ nm, ${\rm W = 950}$ nm, ${\rm h = 0}$ nm, and ${\rm \theta = 75^{\circ}}$), showing the most probable value of 55 MHz. (e) Measured (normalized) transmission spectrum for a split resonance showing the highest ${\rm Q}$ of 5.76 million (intrinsic linewidth ${\rm \gamma_{int}}$ of 34 MHz). (f) Intrinsic ${\rm Q}$ value for microring resonators with different waveguide widths (${\rm W}$) and slab thicknesses (${\rm h}$), here ${\rm H = 340}$ nm and ${\rm \theta = 75^{\circ}}$.
• Figure 2: Dispersion engineering for SiCOI microring resonators and octave-spanning Kerr comb generation. (a) An SEM image of a 1-THz-FSR microring resonator with its waveguide cross-sectional view is shown in the inset. (b) Histogram of intrinsic resonance linewidths of ten 1-THz-FSR microring resonators with the waveguide dimension (${\rm H = 340}$ nm, ${\rm W = 950}$ nm, ${\rm h = 100}$ nm, and ${\rm \theta = 75^{\circ}}$). (c) Simulated integrated dispersion of the ${\rm TE_{00}}$ mode for 16.5-µ m-radius bent waveguides with different waveguide slab thicknesses ${\rm h}$. (d) Measured integrated dispersion of microring resonators at a frequency range from 184--200 THz with the waveguide designs shown in (c). (e) Measured threshold power of optical parametric oscillation versus ${\rm Q}$ for same dimension devices with the lowest value of 0.29 mW. The error bars correspond to the uncertainty in estimating fiber-to-chip coupling efficiency. The dashed line corresponds to the estimated threshold power at the critical-coupling condition. (f) Measured (normalized) transmission spectra showing thermal resonance shifts for 1-THz-FSR microring resonators with and without a slab. (g) Measured resonance shifts as a function of coupled power for devices in (f). For both waveguide designs, data points are extracted from the characterization of three microring resonators with similar ${\rm Q}$s and coupling conditions. (h) Measured spectra for the octave-spanning Kerr comb generation in devices with different slab thicknesses in (c).
• Figure 3: Deterministic generation of octave-spanning single soliton comb. (a) Experimental setup for the deterministic soliton generation demonstration. AWG, arbitrary waveform generator. ECDL, external cavity diode laser; ATT, attenuator; PC, polarization rotator; EDFA, erbium-doped fiber amplifier; LPF, long pass filter; PD, photodetector; OSA, optical spectrum analyzer; OSC, oscilloscope. ESA, electronic spectrum analyzer. (b) Transmission spectrum of a multi-mode microring resonator with the waveguide dimension (${\rm H = 340}$ nm, ${\rm W = 950}$ nm, ${\rm h = 100}$ nm, and ${\rm \theta = 75^{\circ}}$). (c) Measured integrated dispersion of the microring resonator for the ${\rm TE_{00}}$ mode and the ${\rm TE_{10}}$ mode. (d, f) Measured output comb power with 100 consecutive pump detuning sweeps using the ${\rm TE_{00}}$ mode (anomalous dispersion) (d) and the ${\rm TE_{10}}$ mode (normal dispersion) (f) as the laser cooling modes. The same pump and auxiliary laser power are used to characterize the same device. (e, g) Measured comb spectra and corresponding low-frequency noise characteristics for the comb generation in (d) and (f), respectively. The black line in (g) is the simulated single soliton comb spectrum envelope. (h) Statistics of soliton generation using ${\rm TE_{00}}$ and ${\rm TE_{10}}$ as the laser cooling mode. The coupled pump power and the cooling laser power are kept at 60 mW and 50 mW, respectively, for the comparison experiments.
• Figure 4: Comparison of soliton generation using the ${\rm TM_{00}}$ mode as the laser cooling mode with different dispersion properties. Measured integrated dispersion (a) and transmission spectra (b) of microring resonators with different slab thicknesses (${\rm h = 100}$ nm, ${\rm h = 150}$ nm) for the ${\rm TM_{00}}$ mode. (c, e) Measured output comb power with 100 consecutive pump detuning sweeps using the anomalous-dispersion ${\rm TM_{00}}$ mode (c) and the normal-dispersion ${\rm TM_{00}}$ mode (e) as the laser cooling mode. The coupled laser power is the same in the laser cooling mode. (d, f) Measured comb spectra and corresponding low-frequency noise characteristics for the comb generation in (c) and (e), respectively. (g) Statistics of soliton generation using ${\rm TM_{00}}$ mode with anomalous and normal ${\rm GVD}$ as the laser cooling mode. The coupled pump power and the cooling laser power are kept at 63 mW and 80 mW, respectively, for the comparison experiments.