A monolithic double-grating phase mask comprising three short-pitch grating sections of spatial frequencies k(1) and k(2) collocated at one side of a substrate produces a large-period interferogram without higher harmonics to print in a photoresist film a latent grating of small spatial frequency equal to twice k(2)-k(1). When incorporated in a write-on-the-fly scheme, the elements permit the fabrication of unlimitedly long gratings.