Double Diffraction in the Fresnel Region

Abstract

Double diffraction pattern of two identical gratings placed in the Fresnel region is sensitive to the relative displacement of the gratings, and this phenomenon can be used as a method of displacement measurement. Experimental studies and computer analyses on the contrast revealed that a very high contrast (over 90% modulation depth) can be obtained for the zeroth order beam when the separation of the gratings satisfy the Fourier image conditions. The contrast is also affected by the finite radius of coherent component of the incident laser beam, which was shown by the simultaneous measurement of the spatial coherence factor of the source.