Optical path length measurement of spherical dielectric shells with axicon-generated Bessel beams; applications to corneal sensing

A custom aspheric axicon lens for corneal water content sensing, operating from 220 to 330 GHz, was evaluated with spherical, dielectric shell targets. The lens consisted of a hyperbolic back surface and conical front surface with a 25-degree axicon angle. An 8 mm radius of curvature, 1 mm thick, air backed dome was placed in the beam path and S 11 acquired at numerous axial offsets between dome and axicon. The data was normalized by equivalent radius of curvature metallic spherical reflectors. Fits to reflectivity data ranged from ε = 4.1 and t = 0.89 mm and ε = 4.16 and t = 0.885 for lens-tip to shell-apex distances of 34 mm and 25 mm respectively. The results suggest that Bessel beam illumination may relax axial alignment constraints in THz imaging of cornea.