Design and characterization of a hyperbolic-elliptical lens pair in a rapid beam steering system for THz spectral imaging of the cornea

Abstract

Terahertz (THz) time-domain spectroscopy has been investigated for assessment of the hydration levels in the cornea, and changes in corneal topography and the intraocular pressure. Previous efforts at THz imaging of the cornea have employed off-axis parabolic mirrors to achieve normal incidence along the spherical surface. However, this comes at the cost of an asymmetric field-of-view (FOV) and a long scan time because it requires raster-scanning the collimated beam across the large mirror diameter. In this paper, we propose a solution by designing a pair of aspheric lenses that can provide a larger symmetric spherical FOV (9.6 mm) and reduce the scan time by two orders of magnitude using a novel beam-steering approach. Using Nelder-Mead optimization, a hyperbolic-elliptical lens was designed to achieve normal incidence and phase-front matching between the focused THz beam and the target curvature. The lenses were machined from a slab of high-density polyethylene and characterized in comparison to ray-tracing simulations by imaging several targets of similar sizes to the cornea. Our experimental results showed excellent agreement in the increased symmetric FOV and confirmed the reduction in scan time to about 3-4 seconds. In the future, this lens design process can be extended for imaging the sclera of the eye and other curved biological surfaces, such as nose and fingers.