  A theoretical study on low-threshold multimode instabilities in quantum
cascade lasers (QCLs) and laser diodes (LDs) is presented. Previously, low
threshold Risken-Nummedal-Graham-Haken (RNGH) instabilities were reported in
several experimental studies of QCLs. They were attributed so far to the
combined effect of the induced grating of carrier population and of a built-in
saturable absorption feature that may be present in the monolithic
single-section cavity of these Fabry-P\'erot lasers. Here we show that
low-threshold RNGH instabilities in QCLs occur due to a carrier coherence
grating induced in the gain medium and not due to intracavity saturable
absorption. We find that QCLs with a few mm long cavity exhibit intermittent
RNGH self-pulsations while regular self-pulsations are possible in short-cavity
QCLs, with the cavity length of 100 um or smaller. We examine a transient
behavior to RNGH self-pulsations in short-cavity QCLs and find features that
resemble cooperative superradiance. Our findings open a practical way of
achieving ultra-short pulse production regimes in the mid-infrared spectral
range. Applying same approach to semiconductor laser diodes (LDs) we explain
the absence of RNGH self-pulsation in LDs based on a quantum well gain media,
while practically established method for reaching the ultrafast coherent
emission regimes in LDs is to incorporate a separately contacted saturable
absorber section in the LD cavity.
