Generating uniform irradiance in the Fresnel region by quasi-optical beam shaping of a millimeter-wave source

Terahertz metasurface for near-field beam conversion

A uniform illumination over a screen is crucial for terahertz imaging. As such, conversion from a Gaussian beam to a flattop beam becomes necessary. Most of the current beam conversion techniques rely on bulky multi-lens systems for collimated input and operate in the far-field. We present a single metasurface lens to efficiently convert a quasi-Gaussian beam from the near-field region of a WR-3.4 horn antenna to a flattop beam. The design process is divided into three sections to minimize simulation time, and the conventional Gerchberg-Saxton (GS) algorithm is supplemented with the Kirchhoff-Fresnel diffraction equation. Experimental validation confirms that a flattop beam with an efficiency of 80% has been achieved at 275 GHz. Such high-efficiency conversion is desirable for practical terahertz systems and the design approach can be generally used for beam shaping in the near-field.

Design and 3D printing of the Powell lens for sub-terahertz imaging

The aim behind this work is to design and manufacture a beam shaping lens for active terahertz imaging systems that boosts their performance in terms of sensitivity and image quality. The proposed beam shaper is based on an adaptation of the original optical Powell lens, where a collimated Gaussian beam is converted into a uniform flattop intensity beam. The design model for such a lens was introduced, and its parameters were optimized by a simulation study conducted using COMSOL Multiphysics software. The lens was then fabricated using a carefully chosen material [polylactic acid (PLA)] through a 3D printing process. The manufactured lens was implemented in an experimental setup to validate its performance using a continuous-wave sub-terahertz source around 100 GHz. Experimental results demonstrated a high-quality flattop beam maintained along the propagation path, which is highly recommended for terahertz and millimeter-wave active imaging systems to produce high-quality images.

Improving the quality of active millimeter wave standoff imaging by incorporating the cross-polarized scattering wave

In this paper, we study the feasibility of incorporating the cross-polarized scattered wave in active standoff millimeter-wave imaging in order to improve the edge detection and background suppression for metallic objects. By analyzing the scattering from a perfectly conducting (PEC) patch of a simple geometrical shape we show that the edge diffraction is the major source of cross-polarized scattering. A similar scattering behavior is also observed for a PEC patch placed on a dielectric medium. Hence, the cross-polarized scattered field conveys valuable information about the edges of the object. In addition, the cross-polarized scattering can be utilized to resolve the object from an unstructured reflective background. To put these ideas to the test, a standoff imaging system composed of a continuous-wave (CW) semiconductor source, a focal plane array detector (camera), and collimating and objective lenses at 95 GHz is utilized to image the co- and cross-polarized reflection from metallic patches both in the presence and in the absence of a background medium. In agreement with theory, the experiments reveal that the edges of the object can be enhanced and reflections from a smooth background medium can be suppressed by using the cross-polarized scattering. In this regard, the conducted experiments on the metallic patches placed on the human body also yield promising results.

Variable-diameter beam-shaping system design with high zoom ratio containing aspheric optical components

Recently, the optical design of refractive beam-shaping systems has been extensively studied, although such study remains focused on two optical surfaces. Designing a beam-shaping system with variable output beam sizes and prescribed irradiance profiles remains a challenging but rewarding task. Here, we present a design framework, including calculation of the initial system and optimization process, to achieve variable-diameter beam-shaping systems with high zoom ratios. We introduce the whole process of designing a compact 8× zoom system of superior optical performance by transforming a Gaussian beam into flat-top beams with different magnifications. We also present a design of a zoom beam-shaping system transforming a Gaussian beam into variable beams with inverse Gaussian distributions to demonstrate the robustness and efficiency of the proposed method.

Freeform reflector antenna design method to improve transmission efficiency and control output intensity distribution

In this paper, a reflector antenna design method is proposed to improve transmission efficiency and control output intensity distribution. Primary and secondary mirrors are constructed segment by segment using a numerical method according to the law of energy conservation, the vector theory of reflection, and Fermat's principle that the optical path length of each ray must be equal. To verify the effectiveness and practicability of this method, an example of an antenna with the prescribed intensity distribution of the output beam is given. The transmission efficiency of the antenna system designed by this method reaches 100% in theory. The effects of output intensity distribution and deviation on transmission efficiency were also analyzed. We believe that the results show that the proposed method is a highly efficient approach to the design of a freeform reflector antenna to improve transmission efficiency and control the output intensity distribution.