We predict the generation of ultraflat broadband soliton microcombs in a driven quadratic-Kerr nonlinear microring resonator via phase-matched second-harmonic generation. The unprecedented spectral flatness arises from a novel cavity mechanism of symmetric dispersive wave generation, enabled by opposite group-velocity dispersions—anomalous at the fundamental frequency and normal at the second harmonic—without requiring higher-order dispersion engineering. This mechanism manifests itself as a characteristic long-rippled-wing bright soliton at the second harmonic, thereby generating the ultraflat spectrum. We develop analytical criteria for predicting the radiated frequencies, and show that, under proper control of relative cavity losses, the resulting combs exhibit nearly vanishing ($\sim 0\mathrm{dB}$) comb-line power variations over a broad spectral range, at variance with platicon microcombs that operate at normal dispersions. Our results offer a pathway to realize octave-spanning, highly efficient, coherent ultraflat combs without needing external phase or intensity modulators, enabling applications such as high-capacity telecommunications, precision metrology, and astrophysical spectrograph calibration.

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• Cherenkov radiation
• Dissipative dynamics
• Frequency combs & self-phase locking
• Optical solitons
• Optical microcavities