  Frequency-modulated (FM) laser combs, which offer a periodic
quasi-continuous-wave output and a flat-topped optical spectrum, are emerging
as a promising solution for wavelength-division multiplexing applications,
precision metrology, and ultrafast optical ranging. The generation of FM combs
relies on spatial hole burning, group velocity dispersion (GVD), Kerr
nonlinearity, and four-wave mixing (FWM). While FM combs have been widely
observed in quantum cascade Fabry-Perot (FP) lasers, the requirement for a
low-dispersion FP cavity can be a challenge in platforms where the waveguide
dispersion is mainly determined by the material. Here we report a 60 GHz
quantum-dot (QD) mode-locked laser in which both the amplitude-modulated (AM)
and the FM comb can be generated independently. The high FWM efficiency of -5
dB allows the QD laser to generate an FM comb efficiently. We also demonstrate
that the Kerr nonlinearity can be practically engineered to improve the FM comb
bandwidth without the need for GVD engineering. The maximum 3-dB bandwidth that
our QD platform can deliver is as large as 2.2 THz. This study gives novel
insights into the improvement of FM combs and paves the way for
small-footprint, electrically-pumped, and energy-efficient frequency combs for
silicon photonic integrated circuits (PICs).
