A grated waveguide is partitioned into two or more collateral gratings or sub-gratings, such as sidewall gratings, and the gratings are phase offset from each other to obtain a desired perturbation strength of the grated waveguide. Other waveguide parameters, such as the effective indices of modes and the difference in grating strength between modes, are minimally affected by the phase offset. A phase offset is differentiated from a more commonly used phase shift in that a phase shift can be defined as a longitudinal spatial translation of sequential or overlayed gratings relative to each other, whereas a phase offset can be defined as a longitudinal spatial translation of collateral gratings or sub-gratings relative to each other. The phase offset between gratings is also varied along the length of the waveguide to obtain an apodized grating strength.