The optical behavior of a terahertz imaging system employing a train of four identical off-axis parabolic mirrors with oblique incidence angle illumination is investigated in this work. The aperture filling and aberrations of a single off-axis parabolic mirror when illuminated by a Gaussian terahertz beam at its focus point is measured and simulated. The amplitude of E-field in transverse electric (TE) and transverse magnetic (TM) polarizations at target plane reveals a significant cross polarization, even when there is zero cross polarization at the source beam, amplitude of which is ∼ 33% of TE polarization. The investigation of the E-field on the detector plane reveals that this ratio is ∼ 1.5% at the detector plane, and the cross polarized E-field at the target plane is rotated back to co polarization. Although its amplitude is negligible, the TM distribution at detector plane is bimodal and tilted about the optical axis.@en

A system concept for online alignment verification of millimeter-wave, corneal reflectometry is presented. The system utilizes beam scanning to generate magnitude-only reflectivity maps of the cornea at 650 GHz and compares these images to a precomputed/measured template map to confirm/reject sufficient alignment. A system utilizing five off-axis parabolic mirrors, a thin film beam splitter, and two-axis galvanometric mirror was designed, simulated, and evaluated with geometric and physical optics. Simulation results informed the construction of a demonstrator system which was tested with a reference reflector. Similarity metrics computed with the aligned template and 26 misaligned positions, distributed on a 0.5 mm x 0.5 mm x 0.5 mm mesh, demonstrated sufficient misalignment detection sensitivity in 23 out of 26 positions. The results show that positional accuracy on the order of 0.5 mm is possible using 0.462 mm wavelength radiation due to the perturbation of coupling efficiency via beam distortion and beam walk-off.

@en

An efficient method for rapid, non-contact scanning of human cornea is presented. The optics utilize two, 101.6-mm diameter off-axis parabolic mirrors fed by a goniometrically scanned, planar mirror. An evaluation system at 650 GHz was built and demonstrated ~ 1.5 mm beam radius on target and ~ 30 ^o x 30 ^o field of view. The system was inspired by confocal laser scanning principles and represents a system design where image acquisition time is limited by SNR, not mechanical translation.

@en

Reflection-mode terahertz (THz) imaging of corneal tissue water content (CTWC) is a proposed method for early accurate detection and study of corneal diseases. Despite promising results from ex vivo and in vivo cornea studies, interpretation of the reflectivity data is confounded by the contact between corneal tissue and dielectric windows used to flatten the imaging field. Herein, we present an optical design for noncontact THz imaging of cornea. A beam-scanning methodology performs angular normal incidence sweeps of a focused beam over the corneal surface while keeping the source, detector, and patient stationary. A quasi-optical analysis method is developed to analyze the theoretical resolution and imaging field intensity profile. These results are compared to the electric field distribution computed with a physical optics analysis code. Imaging experiments validate the optical theories behind the design and suggest that quasi-optical methods are sufficient for designing of THz corneal imaging systems. Successful imaging operations support the feasibility of noncontact in vivo imaging. We believe that this optical system design will enable the first, clinically relevant, in vivo exploration of CTWC using THz technology.@en