A refractive index grating has at least one configuration of heating elements on or adjacent to a wave-guiding structure. Regions are introduced into the wave-guiding structure and/or into an adjoining substrate, which regions have a higher thermo-optical coefficient or a lower thermal conductivity compared to the wave-guiding structure or the substrate. Substantially each heating element is assigned at least one of the regions with a higher thermo-optical coefficient and/or at least one region with a lower thermal conductivity. There is also provided a mode coupler with a refractive index grating of this type.