High-density interferometric wide-area measurement of grating pitch and homogeneity

Abstract

High-accuracy characterization of optical grating pitch is increasingly important for the quality control of diffractive optical elements, particularly in the emerging augmented reality optics. To address the limitations of conventional measurement methods, such as AFM and diffractometry, we present a snap-shot interferometric method utilizing polarization camera, that provides high-density wide-area pitch data in rapid acquisition, eliminating the need for sample scanning. Phase sensitive interferometer utilizes a polarizer and wave-plates with polarizing beam splitter to control the polarization of the beams in the test and reference arms. A polarization camera records 4 interferograms with consecutive phase shifts of 90° without any moving components. The interferograms allow for calculation of the phase map of the beam diffracted from the sample. When measured with calibrated rotary table in Littrow configuration for diffraction orders m = ±1, the phase maps can be used to calculate the traceable accurate pitch information across the field of view.