A novel design for beamforming the Breakthrough Starshot laser array

Abstract

We outline a laser array beam forming concept that is capable of meeting the laser power requirements of Breakthrough Starshot, a concept aimed at propelling spacecraft to extremely high velocities via laser radiation pressure. Our laser beam forming design offers solutions to the required laser power, optical radiance, beam steering, beam focusing, and atmospheric turbulence compensation specifications. The design consists of a large array of hexagonally close-packed optical modules. Each module contains a collimating lens illuminated by a single fiber from a coherent array of fiber amplifiers. Beam steering is accomplished by transverse micromotion of the fiber amplifier tip illuminating each collimating optic, coupled with a phase shift applied to each module by an electrooptic modulator. Overall array focus is performed by adjusting the transverse and longitudinal fiber positions of each module. Thus, by simply manipulating the position of the fiber feeds of each module and applying the appropriate phase shifts, the beam shape of the entire array can be conditioned to meet the focusing and tracking requirements of the Breakthrough Starshot concept. The same hardware can compensate for the atmospheric aberrations of piston, tilt, and quadratic curvature. For our strawman design, we calculate the overall Strehl ratio reduction of the entire array resulting from incomplete array fill factors, errors in the array phasing, finite laser bandwidth and atmospheric turbulence to be 0.36.