Abstract: According to the present invention, the method for designing a diffraction grating structure (1), the grating period (d) of the structure comprising at least two grating lines each consisting of a pair of adjacent pillars (2) and grooves (3), comprises the steps of—determining desired diffraction efficiencies ?d of the diffraction orders, and—dimensioning the pillars (2) and grooves (3) so that when calculating for each pillar, on the basis of the effective refractive index neff for the fundamental wave mode propagating along that pillar, the phase shift ? experienced by light propagated through the grating structure, the differences in the calculated phase shifts between adjacent pillars corresponds to the phase profile ?r required by the desired diffraction efficiencies.