Smooth and flat phase-locked Kerr frequency comb generation by higher order mode suppression

Area of Science:

• Photonics
• Materials Science
• Nonlinear Optics

Background:

• High-Q microresonators are crucial for generating optical frequency combs via dissipative soliton formation.
• Previous silicon nitride (Si3N4) microresonators used multi-mode waveguides to achieve high quality factor and anomalous dispersion.
• Mode coupling in multi-mode waveguides disrupts dispersion and quality factor, perturbing soliton formation and spectrum.

Purpose of the Study:

• To develop a novel Si3N4 microresonator design that achieves single-mode operation, high quality factor, and anomalous dispersion simultaneously.
• To overcome limitations of previous designs related to mode coupling and dispersion disruption.
• To enable robust optical frequency comb generation independent of pump wavelength selection.

Main Methods:

• Designed a microresonator with uniform single-mode waveguides in curved sections to prevent bending-induced mode coupling.
• Incorporated adiabatically tapered waveguides in straight sections to suppress higher-order mode excitation.
• Fabricated and characterized the Si3N4 microresonator, measuring its quality factor and group velocity dispersion.

Main Results:

• Achieved an intrinsic quality factor of 1.36 × 10^6.
• Maintained anomalous group velocity dispersion of -50 fs²/mm.
• Demonstrated broadband, phase-locked Kerr frequency combs with flat and smooth spectra.
• Showcased comb generation by pumping at any resonance within the optical C-band.

Conclusions:

• The novel Si3N4 microresonator design successfully integrates single-mode operation, high quality factor, and anomalous dispersion.
• This design eliminates the need for careful pump wavelength selection to avoid mode crossings.
• It enables reliable generation of high-quality optical frequency combs across the C-band, broadening their applicability.