Absolute thickness and free water content gradients in gelatinbased corneal phantoms with physiologically accurate radii of curvature, and aqueous backing were extracted via coherent submillimeter wave reflectometry at 220 -330 GHz. Fourier-domain based calibration methods, utilizing temporal and spatial gating, were developed and yielded peak-topeak amplitude and phase clutter of 10 -3 and 0.1°, respectively for signal to noise ratios between 40 dB and 50 dB. Twelve phantoms were fabricated. Calibration methods enabled quantification of target sphericity that strongly correlated with optical coherence tomography-based sphericity metrics via image segmentation. Extracted free water volume fraction varied less than 5 % in the 5 phantoms whose fabrication yielded the most spherical geometry. Submillimeter wave-based thickness accuracy was better than 111 μm (~λ/9) with average of 65 μm (~λ/17) and standard deviation of 44 μm (~λ/25) for phantoms with physiologically relevant geometry. MonteCarlo simulations of measurement noise and uncertainty limits agree with experimental data analysis and indicates a lower thickness accuracy limit of 33 μm, and water content sensitivities of 0.5 % and 11.8 % for the anterior and posterior segments respectively. Numerical analysis suggests measurement fidelity was SNR limited and identified optical path length ambiguities within the cornea where a continuum of thickness/water gradient pairs produce statistically insignificant differences in complex reflection coefficient for finite SNR. This is the first known submillimeterwave measurement technique able to extract both the thickness and water content gradients from a soft-tissue phantom, with a water backing, without the need for ancillary measurements.

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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 5 off-axis parabolic mirrors, a thin film beam splitter, and 2-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.