High-Q microresonator is perceived as a promising platform for optical frequency comb generation,... more High-Q microresonator is perceived as a promising platform for optical frequency comb generation, via dissipative soliton formation. In order to achieve a higher quality factor and obtain the necessary anomalous dispersion, multi-mode waveguides were previously implemented in Si 3 N 4 microresonators. However, coupling between different transverse mode families in multi-mode waveguides results in periodic disruption of dispersion and quality factor, and consequently causes perturbation to dissipative soliton formation and amplitude modulation to the corresponding spectrum. Careful choice of pump wavelength to avoid the mode crossing region is thus critical in conventional Si 3 N 4 microresonators. Here, we report a novel design of Si 3 N 4 microresonator in which single-mode operation, high quality factor, and anomalous dispersion are attained simultaneously. The novel microresonator is consisted of uniform single-mode waveguides in the semicircle region, to eliminate bending induced mode coupling, and adiabatically tapered waveguides in the straight region, to avoid excitation of higher order modes. The intrinsic quality factor of the microresonator reaches 1.36 × 10 6 while the group velocity dispersion remains to be anomalous at −50 fs 2 /mm. With this novel microresonator, we demonstrate that broadband phase-locked Kerr frequency combs with flat and smooth spectra can be generated by pumping at any resonances in the optical C-band. Optical frequency combs, unique light sources that coherently link optical frequencies with microwave electrical signals, have made a broad impact on frequency metrology, optical clockwork, precision navigation, and high speed communication over the past decades 1,2. Parametric oscillation in ultrahigh Q microresonators 3,4 , facilitated by the high quality factors and the small mode volumes, is an alternative physical process that offers the opportunity of optical frequency comb generation in compact footprints 5. The recent demonstrations of octave spanning parametric oscillation 6,7 , low-phase noise photonic oscillator 8–11 , stabilized optical frequency micro-comb 12–14 , and mode-locked femtosecond pulse train 15–19 have triggered great excitements. In particular, the observation of dissipative soliton formation and soliton-induced Cherenkov radiation 20 offers a reliable route towards self-referenced broadband optical frequency microcomb. Dissipative solitons are localized attractors where the Kerr nonlinearity is compensated by the cavity dispersion and the cavity loss is balanced by the para-metric gain 21. Thus the cavity dispersion and the pump-resonance detuning are two determining parameters in the existence of dissipative solitons in ultrahigh Q microresonators. Generation of microresonator-based optical frequency comb, or Kerr frequency comb, has been studied in various material platforms 10,22–25 , including Si 3 N 4 planar waveguide system that is especially suitable for mon-olithic electronic and feedback integration. For Si 3 N 4 microresonators, dispersion is typically engineered by the design of waveguide geometry. Anomalous dispersion, required for bright dissipative soliton formation, is achieved using multi-mode waveguides in the optical C/L-band wavelength range. Besides, scattering loss in multi-mode waveguides is reduced, leading to higher quality factors and lower comb generation threshold 26. However, coupling between different transverse mode families in the multi-mode waveguides results in periodic disruption of dispersion and quality factor, introducing additional perturbation to the dynamics of Kerr frequency comb and dissipative soliton generation 27–31. Such effect manifests itself as characteristic amplitude modulation in the Kerr frequency comb spectrum or detrimental destabilization of the dissipative cavity soliton, depending on the strength and position of the mode coupling 29,31. Careful choice of pump mode to avoid the
High-Q microresonator is perceived as a promising platform for optical frequency comb generation,... more High-Q microresonator is perceived as a promising platform for optical frequency comb generation, via dissipative soliton formation. In order to achieve a higher quality factor and obtain the necessary anomalous dispersion, multi-mode waveguides were previously implemented in Si 3 N 4 microresonators. However, coupling between different transverse mode families in multi-mode waveguides results in periodic disruption of dispersion and quality factor, and consequently causes perturbation to dissipative soliton formation and amplitude modulation to the corresponding spectrum. Careful choice of pump wavelength to avoid the mode crossing region is thus critical in conventional Si 3 N 4 microresonators. Here, we report a novel design of Si 3 N 4 microresonator in which single-mode operation, high quality factor, and anomalous dispersion are attained simultaneously. The novel microresonator is consisted of uniform single-mode waveguides in the semicircle region, to eliminate bending induced mode coupling, and adiabatically tapered waveguides in the straight region, to avoid excitation of higher order modes. The intrinsic quality factor of the microresonator reaches 1.36 × 10 6 while the group velocity dispersion remains to be anomalous at −50 fs 2 /mm. With this novel microresonator, we demonstrate that broadband phase-locked Kerr frequency combs with flat and smooth spectra can be generated by pumping at any resonances in the optical C-band. Optical frequency combs, unique light sources that coherently link optical frequencies with microwave electrical signals, have made a broad impact on frequency metrology, optical clockwork, precision navigation, and high speed communication over the past decades 1,2. Parametric oscillation in ultrahigh Q microresonators 3,4 , facilitated by the high quality factors and the small mode volumes, is an alternative physical process that offers the opportunity of optical frequency comb generation in compact footprints 5. The recent demonstrations of octave spanning parametric oscillation 6,7 , low-phase noise photonic oscillator 8–11 , stabilized optical frequency micro-comb 12–14 , and mode-locked femtosecond pulse train 15–19 have triggered great excitements. In particular, the observation of dissipative soliton formation and soliton-induced Cherenkov radiation 20 offers a reliable route towards self-referenced broadband optical frequency microcomb. Dissipative solitons are localized attractors where the Kerr nonlinearity is compensated by the cavity dispersion and the cavity loss is balanced by the para-metric gain 21. Thus the cavity dispersion and the pump-resonance detuning are two determining parameters in the existence of dissipative solitons in ultrahigh Q microresonators. Generation of microresonator-based optical frequency comb, or Kerr frequency comb, has been studied in various material platforms 10,22–25 , including Si 3 N 4 planar waveguide system that is especially suitable for mon-olithic electronic and feedback integration. For Si 3 N 4 microresonators, dispersion is typically engineered by the design of waveguide geometry. Anomalous dispersion, required for bright dissipative soliton formation, is achieved using multi-mode waveguides in the optical C/L-band wavelength range. Besides, scattering loss in multi-mode waveguides is reduced, leading to higher quality factors and lower comb generation threshold 26. However, coupling between different transverse mode families in the multi-mode waveguides results in periodic disruption of dispersion and quality factor, introducing additional perturbation to the dynamics of Kerr frequency comb and dissipative soliton generation 27–31. Such effect manifests itself as characteristic amplitude modulation in the Kerr frequency comb spectrum or detrimental destabilization of the dissipative cavity soliton, depending on the strength and position of the mode coupling 29,31. Careful choice of pump mode to avoid the
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Papers by Swift Liu