  We report on the engineering of broadband quantum cascade lasers (QCLs)
emitting at Terahertz (THz) frequencies, which exploit a heterogeneous active
region scheme and have a current density dynamic range (Jdr) of 3.2,
significantly larger than the state of the art, over a 1.3 THz bandwidth. We
demonstrate that the devised broadband lasers operate as THz optical frequency
comb synthesizers in continuous wave, with a maximum optical output power of 4
mW (0.73 mW in the comb regime). Measurement of the intermode beatnote map
reveals a clear dispersion-compensated frequency comb regime extending over a
continuous 106 mA current range (current density dynamic range of 1.24),
significantly larger than the state of the art reported under similar
geometries, with a corresponding emission bandwidth of 1.05 THz ans a stable
and narrow (4.15 KHz) beatnote detected with a signal-to-noise ratio of 34 dB.
Analysis of the electrical and thermal beatnote tuning reveals a current-tuning
coefficient ranging between 5 MHz/mA and 2.1 MHz/mA and a temperature-tuning
coefficient of -4 MHz/K. The ability to tune the THz QCL combs over their full
dynamic range by temperature and current paves the way for their use as
powerful spectroscopy tool that can provide broad frequency coverage combined
with high precision spectral accuracy.
