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 and 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 non-contact 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 quasioptical 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 quasioptical methods are sufficient for designing of THz corneal imaging systems. Successful imaging operations support the feasibility of non-contact imaging. We believe that this optical system design will enable the first, clinically relevant, exploration of CTWC using THz technology.
Passive imaging cameras at millimeter and submillimeter wavelengths are currently entering a new era with the development of large format arrays of direct detectors. Several of these arrays are being developed with bare absorbing meshes without any antenna coupling (lens or horn) structures. The design of such arrays is typically done resorting to geometrical considerations or basic broadside plane wave incidence analysis. This paper presents a spectral technique for the analysis of such focal plane arrays in reception using Fourier Optics, which is valid also for moderately skewed incident angles. The analysis constitutes a step improvement with respect to previously used methods by providing an accurate and efficient way to estimate the point-source angular response and the throughput from a distributed incoherent source of an absorbing mesh in the focal plane of a quasi-optical component (e.g. a parabolic reflector or lens). The proposed technique is validated with full-wave simulations. After presenting the analysis, the paper compares the performance of arrays of bare absorber in the focal plane of a quasi-optical component to those of corresponding antenna based arrays. It is found that absorbers lead to a comparable trade-off, in terms of spill-over and focusing efficiency, only for very tight samplings. For larger samplings, the focusing efficiency of absorbers is significantly lower than the one for antennas.
We present a freely accessible graphical user interface for analysing antenna-fed Quasi-Optical systems in reception. This analysis is presented here for four widely used canonical Quasi-Optical components: parabolic reflectors, elliptical, extended hemispherical, and hyperbolic lenses. The employed methods are Geometrical Optics and Fourier Optics. Specifically, Quasi-Optical components are illuminated by incident plane waves. By using a Geometrical Optics based propagation code, the scattered fields are evaluated at an equivalent sphere centred on the primary focus of the component. The Fourier Optics methodology is then used to represent the scattered fields over the focal plane as Plane Wave Spectrum. A field correlation between this spectrum and the antenna feed radiating without the Quasi-Optical component is implemented to evaluate the induced open-circuit voltage on the feed in reception. By performing a field matching between these two spectral fields, feed designers can optimize the broadside and/or steering aperture efficiencies of Quasi-Optical systems in a fast manner. The tool is packaged into a MATLAB graphical user interface, which reports the efficiency terms, directivity and gain patterns of antenna-coupled Quasi-Optical systems. The described tool is validated via full-wave simulations with excellent agreement.
Passive imaging cameras at sub-millimeter wavelengths with large format focal plane arrays are being developed as the next generation of security screening systems. In this contribution, a dual-band focal plane array (FPA) for security imagers at submillimeter wave frequencies is presented. The detectors are based on bolometric superconducting kinetic inductance resonators, which allows the development of large FPAs at medium cooled temperatures. Two frequency selective absorber (FSA) sets coupled to superconductive resonator lines are designed to implement a dual color security imager. The performance of the dual band imager is evaluated using spectral analysis approach that combines Fourier optics with a Floquet mode field representation. The geometry of the unit cells is based on a Jerusalem cross configuration and the designed FSAs show a stable angular response and a rejection 1 to 3 of the undesired bandwidth. The detectors in the dual band FPA are distributed over a hexagonal grid to maximize their physical size and then improve their sensitivity. The effective point spread function of the imager coupled to a black body point source over a wide frequency band (1:6) was demonstrated experimentally with excellent agreement to the one estimated by using the proposed spectral technique.Index Terms-Sub-mm wavelengths, focal plane array, frequency selective absorber, incoherent source.
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