Improved design for the monolithic integration of a laser and an optical waveguide coupled by an evanescent field

A model is proposed to optimize the design of a laser monolithically integrated with a transparent waveguide by an evanescent field. A five-layer structure is shown to be so sensitive to the growth parameters that the threshold current and coupled into the waveguide are basically uncontrollable. The model makes it clear that separate-confinement structures, in which the active layer is attached to a thicker waveguide, are more suitable. The increase of threshold current in these configurations can be efficiently offset by depositing high-reflectivity coatings on the facets of the devices. Moreover, these coatings are shown to further reduce the sensitivity of the devices to technological fluctuations. The use of a single quantum well embedded into a single-mode guide is proposed to obtain very low threshold currents while maintaining a low sensitivity to growth fluctuations.>