Multifocal bootlace lens design concepts: a review

Reflectors and lenses provide two optical mechanisms to magnify small apertures. There are a number of basic differences between the two structures. While the reflector system is less complex, it may, occupy more volume for the same features and suffers from blockage losses. The bootlace or Rotman lenses are more complex, have three multiple-element radiating structures, and offer more degrees of freedom that help in realizing multiple focal points. The three radiating structures are the feed array, the receiving array on one side of the lens, and the radiating array on the other side of the lens. The traditional bootlace lens is two dimensional, with feed points, receiving and transmitting arrays all lying on a plane. While this provides for easy stripline or printed circuit fabrications, there is no physical restriction from extending the formulations to three-dimensional structures. The process of optimizing the lens, aimed at increasing the number of focal points and minimizing the phase errors at other points, has produced lens designs with two to five focal points. The paper reviews some of these designs.