Reflection from a semi-infinite rugate filter

A theoretical approach was applied to compute the reflectance spectrums of semi-infinite rugate filters with a sinusoidal refractive-index profile. Regardless of the linear polarization state of the incident plane wave, the reflectance spectrum of a sufficiently thick rugate filter contains a top-hat feature - indicative of the Bragg regime - with ripples on both flanks. The corresponding reflectance spectrum of a semi-infinite rugate filter also contains the top-hat feature, but is devoid of ripples.