# Quasioptics for corneal sensing at 220 - 330 GHz: design, evaluation, and ex-vivo cornea parameter extraction

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## Abstract

The design, simulation, and characterization of a quasioptical system for submillimeter-wave quantification of corneal thickness and water content are presented. The optics operate in the 220-330 GHz band and are comprised of two, custom aspheric, biconvex lenses in a gaussian beam telescope configuration. The design produced broad band wavefront curvature matching to 7.5 mm radius of curvature target surfaces thus emulating a plane-wave-on-planar-media condition and enabling application of stratified medium theory to data analysis. Aspheric lens coefficients were optimized with geometric ray tracing subject to optical path length penalties and physical optics simulations showed aspheric designs achieved wavefront coupling to spherical surfaces, superior to equivalent, canonical hyperbolic lenses. The fabricated lens system was characterized in a planar near-field scanner system and demonstrated good agreement to physical-optics simulations. An average central corneal thickness of 652 <formula><tex>$\mu$</tex></formula>m and free water content volume of 47 % were extracted from ex vivo sheep corneal via complex s-parameters and agree with literature values. Extracted contact lens thickness and water content agreed with independently validated values. Moreover, the quasioptical system enabled observation of dynamic changes in artificial tear-film, thickness, and water content. This work elucidates two major findings related to submillimeter-wave wavefront matching on spherical surfaces, with wavelength order radii of curvature: (1) An asphere whose sag coefficients are optimized via field phase variation on a spherical surface produces coupling superior to a plano-hyperbolic lens. (2) For most feasible apertures, the Gaussian beam waist is located in the aperture near field, suggesting consideration for operating in the beam near field.

Original language English 9264654 135 - 149 15 IEEE Transactions on Terahertz Science and Technology 11 2 https://doi.org/10.1109/TTHZ.2020.3039454 Published - Mar 2021 A1 Journal article-refereed

## Keywords

• Cornea
• millimeter waves
• quasioptics