We show by rigorous coupled-wave analysis and experimental measurements the optimal values of phase retardation and duty cycle of the phase mask to minimize zero and even diffracted orders. The values are more precise than those usually used according to the scalar theory of diffraction. Under an oblique incidence, we optimize the phase mask with three parameters: phase retardation, duty cycle, and refractive index, which result in a perfect sinusoidal interference field with high diffraction efficiency for writing the fiber Bragg grating (FBG).
Yue Qiu, Yunlong Sheng, and Claude Beaulieu, "Optimal Phase Mask for Fiber Bragg Grating Fabrication," J. Lightwave Technol. 17, 2366- (1999)