Scaling behavior of bipolar cascade VCSELs

We present theoretical and experimental results on the scaling properties of bipolar cascade vertical-cavity surface-emitting lasers with respect to active region stacking and out-coupling mirror reflectivity. The measurement results at around 980-nm wavelength confirm the theoretically expected behavior of an almost linear increase in slope efficiency and a reduction in threshold current density with the number of stages. A three-stage laser shows a differential quantum efficiency of 128% in continuous-wave operation at room temperature. For the threshold current density, we obtain a decrease from 3.75 kA/cm/sup 2/ for the conventional VCSEL to about 1.2 kA/cm/sup 2/ for a three-stage device. Such multi-diode cascade VCSELs are highly interesting for optical links, analog systems, and high-power applications.