Most papers on interstellar laser sailing acknowledge the fact that large optics are required to direct the laser/maser emissions from the Solar System based power station against the distant starship sail. The type of optic usually assumed, the O'Meara para-lens, was introduced by Forward in 1984. Shown schematically in Figure A.2, this device is essentially a large Fresnel lens made of concentric rings of low-mass, transparent material. The para-lens is constructed in such a way that there
are concentric voids between the rings; support spars are used to give the lens structural integrity.
Although many authors have mentioned the para-lens, few have attempted a detailed diffraction analysis. In a preliminary 1989 analysis, Mallove and Matloff sugested that a reflective optic would function better. Taylor et al. (2003) have published a diffraction analysis of a sample 500 m diameter O'Meara para-lens designed to focus 500-nm laser light against a 500 m diameter sail at a distance of 2 light years. The results of this effort indicate that reflective optics are both more efficient and easier to engineer than the para-lens for this application.
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