  We report a homogeneous quantum cascade laser (QCL) emitting at Terahertz
(THz) frequencies, with a total spectral emission of about 0.6 THz centered
around 3.3 THz, a current density dynamic range of Jdr=1.53, and a continuous
wave output power of 7 mW. The analysis of the intermode beatnote unveils that
the devised laser operates as optical frequency comb (FC) synthesizer over the
whole laser operational regime, with up to 36 optically active laser modes
delivering ~ 200 uW of optical power per comb tooth, a power level unreached so
far in any THz QCL FC. A stable and narrow single beatnote, reaching a minimum
linewidth of 500 Hz, is observed over a current density range of 240 A/cm2, and
even across the negative differential resistance region. We further prove that
the QCL frequency comb can be injection locked with moderate RF power at the
intermode beatnote frequency, covering a locking range of 1.2 MHz. The
demonstration of stable FC operation, in a QCL, over the full current density
dynamic range, and without any external dispersion compensation mechanism,
makes our proposed homogenous THz QCL an ideal tool for metrological
application requiring mode-hop electrical tunability and a tight control of the
frequency and phase jitter.
