Technique for measuring the groove density of a diffraction grating with elimination of the eccentricity effect

For absolute measurement of groove density of diffraction gratings, we suggest a simple method that utilizes two parallel beams, a rotary beam splitter and single-beam long trace profiler. We use the fact that two parallel beams are focused on a point in the focal plane of a Fourier transform lens. Especially, with elimination of the eccentricity effect, the groove density of a diffraction grating was measured for higher precision than ever before. The eccentricity effect is a shift of the interaction points due to the off-centered rotating mount of grating, which is the essential error source in previous measurements. The method was applied to measure the groove density of a varied line spacing plane grating with a central groove density of 400 line/mm. The groove density error was measured to be 0.01 line/mm rms, with a period measurement error of 0.3 nm rms.