Millimeter-wave techniques for the detection of corneal water content

This thesis concerns the study of millimeter-wave and THz reflectometry for corneal sensing. Human cornea is a lossy thin film at millimeter wave frequencies. It sits on top of a material of known properties: water and its physical thickness can be measured with ultrasound and optical techniques. Two key features render it a great match to millimeter wave imaging. First, the thickness ranges from 0.4 mm up to 0.7 mm falling right in the middle of the submillimeter range: 0.1 mm to 3 mm. Second the permittivity at millimeter-wave and THz frequencies is a strong function of water content. Therefore, the water content of the cornea and even the water gradient can be studied by illuminating cornea with millimeter waves and solving the inverse scattering problem. An effective medium theory and multilayer T-matrices is proposed as a model for the cornea reflection coefficient. A Montecarlo sensitivity analysis is also performed to investigate the possibility of solving the inverse scattering problem with presence of additive white Gaussian noise and to extract the water gradient and thickness of cornea with a particle swarm optimization algorithm. The analysis reveals that corneal parameter extraction might suffer from ambiguity and an additional thickness measurement might be needed. This technique has also been implemented in two experiments with two different millimeter-wave systems. First, a Gaussian beam telescope was used to perform corneal phantom reflectivity measurements in the WR 3.4 waveguide band (220-330 GHz). The Gaussian beam telescope was chosen because it allows to create a Gaussian beam with a certain beam waist and radius of curvature evolution. The phantoms were aligned to the optics, their reflectivity was measured, and their thickness and water gradient were also extracted. Another WR-3.4 quasioptical system was built, which used an axicon-hyperbolic lens as focusing objective. This system is less sensitive to distance deviation than the Gaussian beam telescope as the axicon has a long depth of field. The corneal phantom reflectometry experiment was repeated. The phantom thickness and water extraction results were performed. Other quasioptical systems are studied to integrate millimeter-wave reflectometry with an adjunct system to assist the corneal alignment in real-time. A first solution is to use a fast-scanning submillimeter wave adjunct imaging system that creates coupling coefficient maps. Anterior segment optical coherence tomography is also explored as a possible adjunct system by integrating it with dual reflector objective