Laser beam projection for starshot launch

The Breakthrough Starshot Initiative aims to send a nanocraft to the Proxima Centauri system. Light sails 4 m in diameter will be accelerated by a 100 GW laser beam projected from a segmented aperture of several kilometer diameter. Adaptive optics to maintain a coherent beam require wavefront and phase sensing across the entire aperture. One projection system considered here comprises millions of 1 m², 20 kW block of lasers each passing through beam expanding and beam directing units with a steering flat to project a 2 m x 2 m square diffraction limited beam. These units are packed close together for ~70% filled aperture. The beam expanders allow for mechanical and servicing access between the laser blocks. The off-axis Mersenne beam expanders both project the outgoing laser beam and receive incoming light from a beacon or the spacecraft for wavefront detection. A dichroic mirror separates the outgoing and incoming light. A wavefront sensor measures the atmospheric aberration across each 2 m aperture, and an adaptive deformable mirror is used to correct the projected beam to the diffraction limit over each 2 m aperture. Our approach is compatible with fixed frequency or tunable lasers. For fixed frequency lasers, a dichroic coating that transmits 99.99% at 1060 nm and reflects 90% of the redshifted return beam from the spacecraft accelerated beyond 70 km/s for P polarization. The 2m square beams will be steered over 2.5° to aim the beam during the 10 minute launch interval. Atmospheric wavefront tilts will be measured and the beam wavefront will be adaptively corrected by modulating laser phases. A preliminary trade study will compare performance and material and mass production costs for these two options.