Interferometric translation-rotation encoder using diffraction gratings

A new type of translation-rotation encoder using diffraction gratings is proposed and its advantages upon previously developed ones are pointed out. It makes use of two transparent dielectric gratings with the same groove spacing, each of them being joint to each of the moving parts whose relative displacement has to be detected and measured. The first grating is lighted under -1 order Littrow incidence, and the wavelength-to-groove spacing ratio is chosen in such a way that only 2 diffracted orders propagate both in reflection and transmission. The two transmitted orders fall on the second grating, and it is shown that they produce interference fringes with highest possible visibility in a large range of groove profile parameters and other experimental conditions. Thus this mounting allows producing high accuracy encoders by using cheap photoresist or plastic gratings, which opens the way to industrial applications in high-precision mechanics, information processing, etc.