Influence of design variations on bipolar cascade VCSEL performance

The basic idea of bipolar cascade vertical-cavity surface-emitting lasers (VCSELs) is the monolithic stacking of active regions that are electrically coupled using Esaki tunnel junctions in between. Thus, carriers can be recycled for additional radiative recombination resulting in a higher roundtrip gain for cascade VCSELs compared to the conventional structure. This offers an additional design freedom to improve optical performance of VCSELs. Firstly, it is possible to scale down the threshold current density and increase the differential quantum efficiency simultaneously. Secondly, cascade VCSELs have the potential to boost optical output power. Such devices might be an interesting solution for high-power VCSEL applications, optical links with gain and analog systems. The scalability of device performance with respect to active layer stacking has been successfully demonstrated recently, even with a slope efficiency well exceeding unity in continuous wave operation at room temperature. However, to improve cascade laser performance further, we have investigated various designs to achieve a better understanding of device behavior. In this paper, we present the comparison of electrical and optical properties of three different two-stage diode cascade VCSEL designs. It is observed that current spreading in the cavity and device degradation are important issues that have to be considered in bipolar cascade VCSELs.