Negative differential gain in 1.3um quantum dot lasers: comparison of self-heating and free carrier effects

A comparative study of the gain spectra of quantum-dot lasers at high carrier densities is reported. The gain spectra of quantum dot lasers under a constant junction temperature (obtained by the use of Fabry-Perot modes as a temperature gauge) and under constant heat-sink temperatures are measured. Negative differential gain, observed for the ensemble of quantum dot ground-states is shown to be mainly due to free carrier effects, where increasing dephasing effects, combined with saturated gain, result in spectral broadening and a reduction in the peak gain.