  Quantum cascade lasers (QCLs) facilitate compact optical frequency comb
sources that operate in the mid-infrared and terahertz spectral regions, where
many molecules have their fundamental absorption lines. Enhancing the optical
bandwidth of these chip-sized lasers is of paramount importance to address
their application in broadband high-precision spectroscopy. In this work, we
provide a numerical and experimental investigation of the comb spectral width
and show how it can be optimized to obtain its maximum value defined by the
laser gain bandwidth. The interplay of nonoptimal values of the resonant Kerr
nonlinearity and the cavity dispersion can lead to significant narrowing of the
comb spectrum and reveals the best approach for dispersion compensation. The
implementation of high mirror losses is shown to be favourable and results in
proliferation of the comb sidemodes. Ultimately, injection locking of QCLs by
modulating the laser bias around the roundtrip frequency provides a stable
external knob to control the FM comb state and recover the maximum spectral
width of the unlocked laser state.
