Design and characterization of a single-pixel imaging system using double hyperbolic-elliptical objective lens for THz spherical scanning of cornea with a large field-of-view

Corneal topographic imaging using terahertz (THz) radiation is a novel diagnostic tool for several ophthalmological conditions. While techniques such as OCT are excellent at measuring corneal thickness, they do not provide hydration information. However, because THz spectroscopy is highly sensitive to water absorption, it is an ideal candidate for topographical mapping of tissue hydration gradients. For imaging a corneal sphere with an 8 mm radius of curvature, previous studies have used a collocated system and raster-scanned a collimated THz beam along the aperture of an off axis parabolic mirror (OAPM) to focus the beam with normal incidence onto a hemispherical target. Aside from alignment difficulty, an OAPM provides an asymmetric field-of-view (FOV) and scanning the collimated beam over the large aperture takes several minutes. Here, we propose a new double hyperbolic-elliptical-lens imaging system to achieve a larger and symmetrical FOV in a significantly shorter scan time. Using Nelder-Mead optimization and ray-Tracing simulations to determine the aspheric surfaces of the lenses, a large FOV of 9 mm can be achieved on an 8 mm radius target with a high degree of phase-front matching ensuring normal incidence on the entire curved surface of the target. Additionally, we demonstrate a telecentric beam-steering system using a heliostat configuration which greatly reduces the imaging time to a few seconds (?4 seconds). The aspheric lenses were tested in a THz time-domain spectroscopy system and the imaging performance characteristics, such as FOV, axial resolution and spot size, were determined and compared to simulations using reflective spherical targets.