Reception Power Pattern of Distributed Absorbers in Focal Plane Arrays: A Fourier Optics Analysis

Passive imaging cameras at millimeter and sub-millimeter 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, this 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 tradeoff, 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.