  We show that mid infrared transmission spectroscopy of a quantum cascade
laser provides clear cut information on changes in charge location at different
bias. Theoretical simulations of the evolution of the gain/absorption spectrum
for the $\lambda \sim$ 7.4 $\mu$m InGaAs/AlInAs/InP quantum cascade laser have
been compared with the experimental findings. Transfer of electrons between the
ground states in the active region and the states in the injector goes in hand
with a decrease of discrete intersubband absorption peaks and an increase of
broad high-energy absorption towards the continuum delocalised states above the
barriers.
