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A new achromatic phase retarder based on a metal-multilayer dielectric grating structure is designed using the rigorous coupled wave analysis method and the genetic algorithm.The optimized phase retarder can maintain phase retardation around 90° from 900 nm to 1200 nm,and the maximum deviation is less than 4.5％ while the diffraction efficiencies of TE and TM waves are both higher than 95％.Numerical analysis shows the designed phase retarder has a high fabrication tolerance of groove depth,duty cycle and incident angle.This achromatic phase retarder is simple in design and stable in performance,and can bc widely used in optical systems.