  The dynamics of a multimode Quantum Cascade Laser, is studied in a model
based on effective semiconductor Maxwell-Bloch equations, encompassing key
features for the radiationmedium interaction such as an asymmetric, frequency
dependent, gain and refractive index as well as the phase-amplitude coupling
provided by the Henry factor. By considering the role of the free spectral
range and Henry factor, we develop criteria suitable to identify the conditions
which allow to destabilize, close to threshold, the traveling wave emitted by
the laser and lead to chaotic or regular multimode dynamics. In the latter case
our simulations show that the field oscillations are associated to
self-confined structures which travel along the laser cavity, bridging
mode-locking and solitary wave propagation. In addition, we show how a RF
modulation of the bias current leads to active mode-locking yielding
high-contrast, picosecond pulses. Our results compare well with recent
experiments on broad-band THz-QCLs and may help understanding the conditions
for the generation of ultrashort pulses and comb operation in Mid-IR and THz
spectral regions
