Bloch gain in quantum cascade lasers at high temperature

We performed multisection cavity gain measurements at high temperature with quantum cascade lasers that are based on a single quantum well active region and work in a low-inversion regime. A modal gain coefficient g=3.7 +/- 0.9 cm kA(-1) at 330 K is found for the sample with the lowest population inversion. We show that the weak dependence of the threshold current and the high values of the associated T(0) parameter between 260 and 360 K are accompanied by a gain coefficient that depends weakly on temperature.