Thermal Properties of Multi-Junction Cascade Vertical Cavity Surface Emitting Lasers

Multi-junction cascade vertical cavity surface emitting lasers (VCSELs) exhibit a noteworthy thermal issue because of their multiple active regions. We have designed and fabricated two six-junction cascade VCSELs with different oxide layers. The light-current-voltage (L-I-V) and divergence angle characteristics, as well as their thermal properties at various ambient temperatures, were analyzed to investigate the thermal problems. Based on the relationship between the emission spectra and power dissipation, we were able to obtain the corresponding thermal resistance values and active region temperatures of different structures afterward. It turned out that the multi-junction cascade VCSELs with multioxide layers have evident advantages of slope efficiency (SE) and optical output power over those with a single-oxide layer due to excellent current and optical confinement. However, owing to the low thermal conductivity of the oxide layers, it is difficult for the heat generated in the active regions to transfer to the surrounding area, resulting in an increase in the thermal resistance of multioxide VCSELs. Meanwhile, the divergence angle of multioxide VCSELs becomes larger because of the strong optical confinement. For better insight, a heat diffusion model of these two devices was established, and the temperature distribution was analyzed. Both the simulation and experimental results agreed fairly well.