An optical device is disclosed which includes a single-mode waveguide (700) which supports a first optical mode in a first region and a second optical mode in a second region. The waveguide includes a guiding layer (703) having at least one wing (750) extended outwardly from the guiding layer (703). The guiding layer (703) may desirably have a rib waveguide (706, 707) cross sectional shape at the wings. The wings (750) decrease in width along the length of the guiding layer to convert a rib waveguide mode at the wings to a channel waveguide mode.