Near-room-temperature continuous-wave operation of multiple-active-region 1.55 μm vertical-cavity lasers with high differential efficiency

We present completely monolithic, single-step grown, bipolar cascade vertical-cavity surface-emitting lasers at 1.55 $\mu \mathrm{m}$ with a greater-than-unity differential quantum efficiency. A typical device had a threshold current density of 1 ${\mathrm{kA/cm}}^2,$ a threshold voltage of 3.2 V, and demonstrated continuous wave operation up to $\text{8\hspace{0.17em}°}\mathrm{C.}$ Devices smaller than 10 $\mu \mathrm{m}$ in diameter lased single mode. Active regions in our device were epitaxially stacked in three stages. This technique of multiple-active regions enabled the greater-than-unity differential quantum efficiency operation, which is essential in constructing high-efficiency microwave optical links with gain. We report the device characteristics and a model on the scaling properties of active region stacking in multiple-active-region vertical-cavity lasers.