  A comprehensive model for the photon number fluctuations and the current
noise in quantum cascade lasers is presented. It is shown that the photon
intensity noise in quantum cascade lasers exhibits little amplitude squeezing
even when noise in the drive current is suppressed below the shot noise value.
This is in contrast to interband semiconductor diode lasers in which the laser
intensity noise can be squeezed well below the shot noise limit by high
impedance suppression of fluctuations in the drive current. The theoretical
model presented in this paper self-consistently accounts for the suppression of
current noise in electron transport in multiple quantum well structures due to
various electronic correlations. The nature of these electronic correlations is
discussed. Mechanisms responsible for the reduced photon number squeezing in
intersubband lasers are elucidated. Scaling of the laser intensity noise and
the current noise with the number of cascaded gain stages is also described.
Direct current modulation response of quantum cascade lasers is also studied,
and it is shown that contrary to the predictions in the literature of terahertz
modulation bandwidth for these lasers, bandwidth of almost all quantum cascade
lasers that have been reported in the literature is limited by the inverse
photon lifetime inside the laser cavity to tens of gigahertz.
