Gain measurements on GaAs-based quantum cascade lasers using atwo-section cavity technique

A two-section cavity device has been used to measure gain spectra and waveguide losses of a GaAs-based quantum cascade laser. The device operates at 8.9 μm and optical confinement is obtained by means of Al-free cladding layers. We investigated the gain characteristics in a spectral window of ~60 meV and up to 200 K. For current densities ranging from 1 to 8 kA/cm², we report a constant gain coefficient of 13 cm/kA at 4 K and 6 cm/kA at 200 K. At low temperatures and for current densities above 8 kA/cm², we observe gain saturation which we attribute to a reduced electron injection in the active region caused by space charge effects. We report a value of 22 cm ^-1 for the waveguide losses in good agreement with previous measurements