Method for modifying the splitting or combining ratio of a first multimode interference (MMI) coupler (100), which first coupler is arranged to convey light from one or several input waveguides to one or several output waveguides, wherein a film (103a) of a material is arranged over the first coupler, wherein the film is strained so that a force is applied by the film to the surface of the first coupler, and so that the refractive index profile in the material of the first coupler changes as a consequence of the force, and wherein the splitting or combining ratio is modified as a consequence of the changed refractive index profile.