Ultraflat Soliton Microcombs in Driven Quadratic-Kerr Nonlinear Microresonators.

Area of Science:

• Nonlinear Optics
• Quantum Optics
• Photonics

Background:

• Soliton microcombs are crucial for various applications, but achieving ultraflat broadband spectra remains challenging.
• Existing methods often require complex dispersion engineering or external modulators.

Purpose of the Study:

• To predict the generation of ultraflat broadband soliton microcombs.
• To explore a novel cavity mechanism for achieving unprecedented spectral flatness.
• To enable octave-spanning, highly efficient, coherent combs without external modulators.

Main Methods:

• Utilizing a driven quadratic-Kerr nonlinear microring resonator.
• Employing phase-matched second-harmonic generation.
• Leveraging opposite group-velocity dispersions (anomalous at fundamental, normal at second harmonic).

Main Results:

• Demonstrated a novel cavity mechanism of symmetric dispersive wave generation.
• Achieved unprecedented spectral flatness due to the unique dispersion properties.
• Observed nearly vanishing comb-line power variations (∼0 dB) over a broad spectral range.
• Characterized a long-rippled-wing bright soliton at the second harmonic.

Conclusions:

• The proposed mechanism offers a pathway to ultraflat broadband soliton microcombs.
• This approach eliminates the need for higher-order dispersion engineering and external modulators.
• Enables applications in high-capacity telecommunications, precision metrology, and astrophysical spectrograph calibration.