3D Measurements and Characterizations of Refractive Index Distributions of Volume Holographic Gratings Using Low-Coherence Holotomography

Abstract

Holographic Optical Elements (HOEs) employ refractive index (RI) modulation to manipulate light, establishing their widespread application in augmented and virtual reality technologies. Despite significant strides in HOEs' fabrication, a comprehensive 3D examination and measurement of these elements remain largely unexplored. This study targets this void by conducting an in-depth exploration of the 3D RI distributions of volume holographic gratings (VHGs), a quintessential representative of HOEs. Direct tomographic measurements of the 3D RI distributions within millimeter-scale VHGs are performed at a spatial resolution of 156 nm x 156 nm x 1.2 mu m and an RI precision of 2x10-4. This level of precision facilitates a systematic high-resolution characterization of VHGs. The findings present a substantial evolution from traditional measurement techniques and promise to considerably advance the evaluation of intricate HOE structures and refine the corresponding fabrication processes. Holotomography is utilized for the measurement of 3D refractive index tomograms of volume holographic gratings. This method offers precise quantification of refractive index distributions and the detection of defects within these gratings. It enables the evaluation of voids, debris, and the grating formation. This study underscores the potential of holotomography for both the analysis and fabrication of volume holographic gratings. image