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Recent advances in sub-wavelength nanoscale platforms have afforded the control of light from first principles, with impact to ultrafast sciences, optoelectronics and precision measurements. In this talk we will describe recent advances in chip-scale Kerr frequency comb oscillators, where we have achieved sub-100-fs mode-locking, stabilization down to a tooth-to-tooth relative frequency uncertainty of 50 mHz and 2.7×10^{−16}, and single-mode broadband frequency comb generation. Each of these are supported by linear and nonlinear numerical modeling.
In this first chip-scale realization, coherent mode-locking is observed in the normal dispersion regime, verified by phase-resolved ultrafast spectroscopy at sub-100-attojoule sensitivities. The normal dispersion architecture uncovers the mode-locking mechanisms in Kerr frequency combs, matched with first-principles coupled-mode theory. In the second realization, we examine the noise limits in the full stabilization of chip-scale optical frequency combs. The microcomb’s two degrees of freedom, one of the comb lines and the native 18-GHz comb spacing, are simultaneously phase-locked to known optical and microwave references. Active comb spacing stabilization improves long-term stability by six orders of magnitude, reaching a record instrument-limited residual instability of 3.6 mHz per root tau. Comparing 46 nitride frequency comb lines with a benchmark fiber laser frequency comb, we demonstrate the unprecedented microcomb tooth-to-tooth relative frequency uncertainty down to 50 mHz and 2.7×10^{−16}. In the third realization, we report a novel design of Si3N4 microresonator in which single-mode operation, high quality factor, and anomalous dispersion are attained simultaneously. The novel microresonator consists of uniform single-mode waveguides in the semi-circle region, to eliminate bending induced mode coupling, and adiabatically tapered waveguides in the straight region, to avoid excitation of higher order modes. With this microresonator, we demonstrate broadband phase-locked frequency combs. This supports the focus towards chip-scale precision spectroscopy, timing, coherent communications, and astronomical spectrography.
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