Smooth coherent Kerr frequency combs generation with broadly tunable pump by higher order mode suppression

High-Q microresonator has been suggested a promising platform for optical frequency comb generation, via dissipative soliton formation. To achieve a higher Q and obtain the necessary anomalous dispersion, $Si_3N_4$ microresonators made of multi-mode waveguides were previously implemented. However, coupling between different transverse mode families in the multi-mode waveguides results in periodic disruption of dispersion and quality factor, introducing 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 $Si_3N_4$ microresonators. Here, we report a novel design of $Si_3N_4$ microresonator such that single mode operation, high quality factor, and anomalous dispersion are attained simultaneously. The microresonator is consisted of uniform single mode waveguides in the semi-circle region, to eliminate bending induced mode coupling, and adiabatically tapered waveguides in the straight region, to avoid excitation of higher order modes. The intrinsic Q of the microresonator reaches $1.36 \times 10^6$ while the GVD remains to be anomalous at $-50 fs^2/mm$. We demonstrate, with this novel microresonator, broadband phase-locked Kerr frequency combs with flat and smooth spectra can be generated by pumping at any resonances in the optical C-band.