Enhancement in electroabsorption waveguide modulator slope efficiency at high optical power

An increase in transfer curve slope efficiency for a Franz-Keldysh effect InGaAsP-InP electroabsorption waveguide modulator is observed as the incident optical power is increased from 5.8 to 17 dBm. However, high-frequency RF measurements agree with the gain predicted from the low-power transfer curve at all optical powers. We attribute the increase in dc slope efficiency to a temperature-induced bandgap shrinkage of the electroabsorption material.