Submillimeter-Wave Cornea Phantom Sensing Over an Extended Depth of Field With an Axicon-Generated Bessel Beam

Abstract

The feasibility of a 220–330 GHz zero-order axicon-generated Bessel beam for corneal water content was explored. Simulation and experimental data from the 25° cone angle hyperbolic-axicon lens illuminating metallic spherical targets demonstrate a monotonically decreasing, band integrated, backscatter intensity for increasing radius of curvature from 7–11 mm, when lens reflector and optical axis are aligned. Furthermore, for radii > = 9.5 mm, maximum signal was obtained with a 1-mm transverse displacement between lens and reflector optical axes arising from spatial correlation between main lobe and out-of-phase sidelobes. Thickness and permittivity parameter estimation experiments were performed on an 8-mm radius of curvature, 1-mm-thick fused quartz dome over a 10-mm axial span. Extracted thickness and permittivity varied by less than ∼25 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m and 0.2, respectively, after correction for superluminal velocity. Estimated water permittivity and thickness of water backed gelatin phantoms showed significantly more variation due to a time varying radius of curvature. To the best of our knowledge, this is the first work that describes axicon-generated Bessel beam measurements of layered spheres with varying radii of curvature, in the submillimeter range.