Dependence of the device performance on the number of stages in quantum-cascade lasers

The cascading scheme is a characteristic feature of quantum cascade (QC) lasers. It implies that electrons above threshold generate one photon per active region they successively traverse. This paper presents a study of the cascading behavior as a function of the number N of stacked active regions. Experimental results are presented for devices with N=1, 3, 6, 12, 20, 30, 45, 60, and 75 active stages. The highest optical power and lowest threshold current density are obtained for laser devices with N as high as possible. However, the lowest threshold voltage and the lowest dissipated power at laser threshold are achieved for N=3 and N=22, respectively. We further present the highest power QC lasers so far, which, using N=75 stages, show in pulsed mode peak powers of 1.4, 1.1, and 0.54 W at 50 K, 200 K, and room temperature, respectively. Finally, we also demonstrate the first few-stage (N<10) QC lasers. These QC lasers show strongly reduced operating voltages. A threshold voltage around 1.5 V is achieved for N=3. This makes the lasers very well compliant with conventional laser diode drivers, which in turn will simplify their immediate use in systems and applications.