Vision-Based Alignment Control for Grating Tiling in Petawatt-Class Laser System

Large-sized diffraction gratings are required for the compressor of chirped-pulse-amplification systems. However, producing single large grating is of great difficulty, which makes grating tiling an alterative solution. This paper presents a vision-based alignment control method for grating tiling in petawatt-class laser system. Tiling errors with six degrees of freedom are grouped into three ones, and a decoupled vision control method is proposed. The piston error is measured from the far-field focal-spot image. To determine the piston error, the energy ratio between the main and secondary spots in the image is computed, and then a mapping formulation is determined to compute the piston from the energy ratio. To guarantee precision and reliability, a closed-loop piston control system with clustering error filtering and output verification is designed. The angular error is composed of two parts, i.e., tip and tilt errors, which is measured from the near-field interference fringe image. To reduce the influence of airflow on the angular measurement, a novel image quality estimation method is designed using the average crest-to-trough difference. Then, Fourier fringe pattern analysis is applied to the left and right parts of the image to reconstruct the wavefront, based on which a closed-loop grating angular control system is designed. Finally, experiments are well conducted in a tritiled gratings to verify the performance and precision of the proposed grating tiling system.