  Broadband Kerr combs with a flat comb spectral profile are expected in a
number of applications, such as high-capacity optical communication. Here, we
propose novel concentric dual-ring microresonators (DRMs) for advanced
dispersion engineering to tailor the comb spectral profile. The dispersion can
be flexibly engineered not only by the cross-section of the DRMs, but also by
the gap between concentric dual-ring microresonators, which provides a new path
to geometrically control the spectral profile of the soliton Kerr combs. An
octave-spanning dissipative Kerr soliton comb with superior spectral flatness
has been achieved numerically, covering from the telecommunication band to the
mid-infrared (MIR) band region with a -40 dB bandwidth of 1265 nm (99.82 THz).
Our results are promising to fully understand the nonlinear dynamics in hybrid
modes in DRMs, which helps control broadband comb formation.
