Quasioptical System for Corneal Sensing at 220-330 GHz: Design, Evaluation, and Ex Vivo Cornea Parameter Extraction

• Published in: IEEE transactions on terahertz science and technology Vol. 11; no. 2; pp. 135 - 149
• Main Authors: Tamminen, Aleksi, Palli, Samu-Ville, Ala-Laurinaho, Juha, Salkola, Mika, Raisanen, Antti V., Taylor, Zachary D.
• Format: Journal Article
• Language: English
• Published: Piscataway IEEE 01.03.2021; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Apertures; Broadband; Coefficient of variation; Configuration management; Contact lenses; Cornea; Coupling; Couplings; Data analysis; Design parameters; Gaussian beams (optics); Lenses; Lighting; Mathematical analysis; Moisture content; Near fields; Optical surface waves; quasioptics; Radius of curvature; Ray tracing; Reflectivity; Simulation; Spherical waves; Submillimeter waves; Surface matching; Surface waves; Thickness; Wave fronts
• DOI: 10.1109/TTHZ.2020.3039454
• ISSN: 2156-342X

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 produces broadband 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 μ m and free water content volume of 47% were extracted from ex vivo sheep corneas 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.