Wideband four-level transmission gratings with flattened spectral efficiency.

Four-level transmission-type surface relief grating profiles with nearly flat efficiency over a spectral octave are designed by rigorous electromagnetic diffraction theory. Parametric optimization of the relief depths and transition points of the profile steps of these leads to efficiencies in the range 50-60% over the entire octave if the ratio of the grating period and the mean spectral wavelength is greater than ~ 3.