Confocal Ellipsoidal Reflector System for a Mechanically Scanned Active Terahertz Imager

Abstract

Abstract-We present the design of a reflector system that can rapidly scan and refocus a terahertz beam for high-resolution standoff imaging applications. The proposed optical system utilizes a confocal Gregorian geometry with a small mechanical rotating mirror and an axial displacement of the feed. For operation at submillimeter wavelengths and standoff ranges of many meters, the imaging targets are electrically very close to the antenna aperture. Therefore the main reflector surface must be an ellipse, instead of a parabola, in order to achieve the best imaging performance. Here we demonstrate how a simple design equivalence can be used to generalize the design of a Gregorian reflector system based on a paraboloidal main reflector to one with an ellipsoidal main reflector. The system parameters are determined by minimizing the optical path length error, and the results are validated with numerical simulations from the commercial antenna software package GRASP. The system is able to scan the beam over 0.5 m in cross-range at a 25 m standoff range with less than 1% increase of the half-power beam-width.

Optomechanical System Design for Dual-Mode Stand-Off Submillimeter Wavelength Imagers

“…Several sensors can be used to protect the same building or venue and they have to work at different distances from the target and with different resolutions. Since quasi-optical systems operating in the Fresnel or near-field region are needed to meet the requirements of resolution and operating distances in submillimeter wavelength imagers for security applications [13], the operational imaging distance is defined by the surface of the primary reflector (see Fig.1). In particular, an elliptical reflector with a focus on the image plane is commonly used as the primary mirror to achieve diffraction limited beams at the center of the FOV [13].…”

“…Since quasi-optical systems operating in the Fresnel or near-field region are needed to meet the requirements of resolution and operating distances in submillimeter wavelength imagers for security applications [13], the operational imaging distance is defined by the surface of the primary reflector (see Fig.1). In particular, an elliptical reflector with a focus on the image plane is commonly used as the primary mirror to achieve diffraction limited beams at the center of the FOV [13]. Even if, to certain extent, it is possible to achieve mechanical refocusing [14], these systems cannot operate in different environments requiring significantly different focusing ranges and similar cross-range resolutions, because the use of the same mirror aperture while refocusing will lead to larger beams at larger distances.…”

“…A reflector solution that was found to be efficient for security imagers based on mechanical scanners is the confocal architecture [13], [19]. The latter is shown in Fig.…”

“…In the current configuration, the quadratic loss introduced by the confocal system [13] has to be optimized while the beam-scan occurs in both the symmetric and offset planes at the same time. In particular, the system is illuminated by the FPA in its offset plane and the scanner displaces the beams in the symmetric plane.…”

Time-Delay Multiplexing of Two Beams in a Terahertz Imaging Radar

“…The fields shown in Fig. 8 exhibit low distortion because of the confocal reflector geometry between the main antenna and the second parabola of the quasi-optical waveguide [15].…”