Temperature dependence of 1.55μm VCSELs

Abstract

Recent progress in 1.55μm VCSELs using either wafer-fused or epitaxial mirrors has prompted renewed interest in the temperature sensitivity of these devices. Here we model transitions between parabolic bands with and without k-selection, as well as transitions between the first sub-bands in quantum wells without k-selection. The underlying temperature dependence of the cavity resonant photon energy and the bandgap of the semiconductor are found from experimental data, as illustrated in Fig. 1. This also shows the temperature dependence of the photon energy corresponding to the gain peak, as calculated for the case of no k-selection in bulk InGaAsP. Fig.2 plots the corresponding variation of threshold carrier density, nth, assuming charge neutrality. It is clear that the temperature, Tmin, of minimum nth is somewhat lower than the temperature, Tmatch, at which the photon energy of the gain peak matches that of the cavity resonance. This difference is significant because the conventional approach to VCSEL design has been to assume Tmatch determines the optimum operating temperature, whereas the calculations presented here demonstrate that this could lead to non-optimal performance. The difference between Tmin and Tmatch depends to some extent on the model used, and the cases with and without k-selection in bulk and SQW (no k-selection) material are plotted in Fig. 3.