Precise characterization of diffraction grating groove patterns

Non-equidistant and non-parallel grooves of a diffraction grating distort the diffracted wavefront. For a fixed wavelength, a grating with an arbitrary groove pattern is equivalent to a grating with perfect grooves having an effective surface profile different from the geometric one. It is shown that the previously proposed method for measuring non-equidistance can be used for simultaneous measurement of non-parallelism, and the measurement error is determined without any a priori information and fitting parameters. The demonstrated error (RMS) for the diffraction gratings with an aperture of 320 × 230 mm manufactured by us was 2 nm for the effective profile, 4 · 10^-7 for non-equidistance, and 4.5 · 10^-7 for the groove inclination angle when measuring with a spatial resolution of 0.27 mm. Rms of the wavefront distortions of the beam reflected from these gratings was 10 ± 2 nm.