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This letter presents experimental results of a three-alloy-based short-injector quantum cascade laser (QCL). The investigated 4-mm-long device shows a pulsed threshold current density of 1.24 kA/cm2 , a slope efficiency of 1.4 W/A, a characteristic temperature above 250 K, and a peak average output power above 726 mW at room temperature. A good high-temperature performance is attributed to the diagonal transition design and better depopulation of the lower laser levels at higher temperatures. The laser emission wavelength at room temperature is 8 mum, resulting in a low voltage defect of 71 meV per period for the QCL structure.