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

Abstract

Abstract-In this contribution, the practical trade-offs for designing submillimeter wavelength imagers based on optomechanical systems combined with focal plane arrays are presented. The architecture of these systems differs for operation at short and long ranges. General formulas to derive the effective field of view of diffraction limited quasi-optical systems in these two scenarios are shown. These formulas can be used to evaluate the performance of a specific optical system implementation.As an application example, we present the design of an optomechanical system that can operate at both ranges in a modular approach. The presented implementation achieves an effective field of view which is 70% of the canonical one. The proposed solution consists of a linear focal plane array of eight active transceivers combined with a raster scan technique. The system for short range scenario is a side-fed dual-reflector Dragonian architecture because of its good scanning performance when illuminated by a focal plane array. Thanks to the small system aperture, the scanner is arranged after the primary mirror, without causing additional scan loss. The Dragonian system is then used to illuminate a confocal dual-reflector architecture to magnify its aperture and be used in the long range scenario. The scanner in this case is before the main aperture and it has to be considered in the performance optimization of the optical system since it adds phase aberration loss.

Wide Field of View Inversely Magnified Dual-Lens for Near-Field Submillimeter Wavelength Imagers

“…In [12], a comparison table of the performance of the quasioptical systems of the state-of-the-art imagers for security applications at sub-millimeter wavelengths was presented. The discussed scenario is well beyond previous implemented systems.…”

“…Indeed, standard optical systems using conical surfaces cannot achieve these requirements. The effective FOV of a canonical symmetric elliptical reflector with the required HPBW is only 1 m wide at a 2 m range (see Fig 4(a) in [12]). The best reflector configuration, that leads to an effective FOV of 70% of a canonical reflector, was shown in [13] to be a Dragonian dual-reflector system [14].…”

“…3. In contrast with standard magnified quasioptical systems [12], it consists of a primary lens dimensioned to achieve the required HPBW and a larger secondary lens. The use of a large secondary optic allows the optimization of the path lengths of the center and edge detectors differently because these feeds illuminate different parts of the lens.…”

THz Imaging Using Uncooled Wideband Direct Detection Focal Plane Arrays

“…This process of mechanical scanning is referred to as jiggling. For small FPAs, i.e., when N t = N pix N det 1, the main limitation in achieving high imaging speeds is the implementation of the scanner [36]. However, as it is shown in [36, Table I], even for relatively small arrays (< 100 elements), near real-time imaging speeds have been achieved.…”

Coherent Fourier Optics Model for the Synthesis of Large Format Lens-Based Focal Plane Arrays

“…Various solutions have been proposed in the past to improve the scanning performance of quasi-optical systems either using Gaussian horn feeds combined with shaped reflector or lens antennas [19]− [22] (with most of the cases over sizing the radiating aperture) and/or determining an optimum focal surface [23], [24], where the array elements are placed [17]; or by using array clusters of feeds to achieve a conjugate field match condition with the focal plane field [25]− [27]. This work considers a relatively simple FPA architecture based on a lens array.…”