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 S11 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.