The present invention is directed to a system and method for designing efficient multi-channel FBG gratings using a pre-compensated phase mask for diffracting light for side-writing the grating on an optical fiber core. A desired phase function of the FBG is generated, specifically tailored to an effective spacing between the phase mask and the optical fiber core. From the phase function a phase mask is pre-compensated to offset diffraction effects associated with each longitudinal position of the FBG receiving light from two corresponding longitudinal positions of the phase mask substantially symmetrically spaced longitudinally relative to each particular longitudinal position of the FBG. The two corresponding longitudinal positions of the phase mask are spaced longitudinally from each other by a spacing determined by the effective spacing between the phase mask and fiber core and by the first order diffraction angle of light through the phase mask.