Optical systems for large-aperture phased laser array including diffractive optics

Abstract

Directed energy propulsion for interstellar travel has been proposed as an ideal method for reaching appreciable speeds relative to the speed of light: 0.2c. However, the amount of energy required necessitates a large aperture, on the order of kilometers, while mitigation of atmospheric perturbations requires a discretization of the aperture into many individual laser elements. The use of fiber lasers for these elements obligates mode-matching the fiber to the desired 10 cm aperture for a collimated beam. Various collimation systems were designed and compared. A 3-lens system with one achromat and two aspheric lenses, with two of the lenses used as a Keplerian telescope to achieve a system-shortening effect was analyzed. A similar system made with a plano-convex lens replacing the large-aperture aspheric lens with two additional compensating lenses was compared. A single diffractive optic operating at F/8 was likewise considered. The optical performance of these systems was compared, as was the cost-effectiveness. Scalability to millions of elements was required, so cost-per-system was a crucial consideration factor. Possible manufacturing processes for a diffractive system were investigated, and stamping processes for replication were analyzed to determine the possibility of replication of such an optic reliably, cheaply, and with acceptable results.