Diffraction efficiency gain, blazing, and apodizing of a symmetric square reflection grating in an étalon

First-order diffraction efficiency of 50% into a single beam was obtained from a weak symmetric square metal diffraction grating operated in an étalon at the 632.8-nm He-Ne laser wavelength. Without the étalon the diffraction efficiency was just 2.7% in each of the two first-order beams. First-order diffraction efficiency gain as well as blazing and apodizing of the grating occur because of the resonant paraxial angular spectrum of the étalon.