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

In this paper, we report on the design, simulation and measurement of a flat-top beam shaper (FBS) lens for a millimeter-wave (mm-wave) antenna coupled source. The beam shaper is designed to transform the pseudo-Gaussian beam of the mm-wave source into a flat-top beam of sharp roll-off at a specific distance in the Fresnel region. Firstly, relying on the geometrical optics principles, we propose an analytical formulation for the design of lens profiles. Next, a simple optimization method based on the full-wave simulation of the lens and the mm-wave antenna is suggested to tune the analytically extracted lens profile. The optimized lens profile is then studied to assess its performance sensitivity to various deviations. Finally, a prototype of the proposed FBS lens is fabricated and measured. As an example study, the radiated beam of a 100GHz conical horn antenna is shaped into a flat-top beam of radius 40 cm with a sharp skirt and low ripples at the distance of 3 meters. The radius of the generated flat-top beam is also shown to be easily tunable by displacing the FBS lens with respect to the horn antenna. The proposed procedure for the design of FBS lens results in a high-quality flat-top beam, which is promising for the applications where a uniform irradiance within the field-of-view with negligible spillover is of high importance. Terahertz (THz) and mm-wave imaging systems are potential candidates in this regard.