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Nonequilibrium Green’s functions model of mid-infrared quantum cascade laser that utilizes double-phonon resonance scheme is analyzed. Good agreement with experimental data is found. For quantities not accessible experimentally, it is shown that electronic distributions in lower active region subbands are nonthermal and have local maximum at higher in-plane energy. In upper subband, carrier distribution is thermal-like. Upper state lifetime is strongly reduced from optical phonon value by interface roughness and alloy scatterings. Net gain originates from population inversion at subbands bottom which is not suppressed by high-k absorption because of nonparabolicity which shifts absorption peak to lower frequencies.