  In this work, a design for a mid-infrared quantum cascade laser (QCL)
frequency comb source that enhances the high frequency response and the comb
characteristics of the device is presented . A state-of-the-art active region
(AR), grown on a heavily n-doped InP:Si substrate, was processed into a buried
heterostructure with a microstrip-like line waveguide. As a result, the
repetition rate frequency $f_{rep}$, around 11.09 GHz, can be locked to an
injected narrow-linewidth radio frequency (RF) signal, over a range of more
than 200 kHz with -10 dBm of injected power, which outperforms normal buried
heterostructure schemes by an order of magnitude. Moreover, under RF injection
at powers higher than 20 dBm, the lasing spectrum is flattened and
significantly broadened, from 24 $cm^{-1}$ to 65 $cm^{-1}$ in bandwidth, while
at the same time the coherence of the comb is maintained and verified.
