Optimization strategies for diode cascade InGaAs VCSELs

We present top surface emitting two- and three-stage bipolar cascade vertical cavity surface emitting lasers (VCSELs) for continuous wave (cw) operation at room temperature. The results are compared with a conventional VCSEL grown under identical growth conditions for reference. The improvement of both threshold current density and differential quantum efficiency can be clearly shown in structures with unchanged top mirror reflectivities (19 pairs). To further increase the differential quantum efficiency, a device with only 14 p-type top mirror pairs is fabricated, yielding an efficiency of 130% for cw room temperature operation. From noise measurements, we extract a modulation efficiency of 8.2 GHz/sqrt(mA}, indicating favorable dynamic properties of these VCSEL.