The invention provides a waveguide with a waveguide core having longitudinal sidewall surfaces, a longitudinal top surface, and a longitudinal bottom surface that is disposed on a substrate. An interface layer is disposed on at least one longitudinal sidewall surface of the waveguide core. A waveguide cladding layer is disposed on at least the waveguide core sidewall and top surfaces, over the interface layer. The waveguide of the invention can be produced by forming a waveguide undercladding layer on a substrate, and then forming a waveguide core on the undercladding layer. An interface layer is then formed on at least a longitudinal sidewall surface of the waveguide core, and an upper cladding layer is formed on a longitudinal top surface and on longitudinal sidewall surfaces of the waveguide core, over the interface layer.