Analysis of heterojunction optical waveguides with a modulated region smaller than the guide width

Heterojunction optical waveguide modulators in which the modulated region may be smaller than the guide width are analyzed using two models: the average-field model, which replaces the actual junction field with a uniform field and the abrupt-junction model, which accounts for the linear decrease of the electric field throughout the depletion region. The models are used to calculate the field-induced birefringence in two (Ga,Al)P heterojunction waveguide structures as a function of voltage, including the effects of the change in the depletion width, and the results are compared. Both models predict a mode dependence for the induced birefringence which is especially strong for thin modulated regions.