Single-shot measurement of the near-field and focal spot profiles of a 351 nm laser beam for SGII-upgraded facility with multiple-focal-plane constraint coherent modulation imaging.

Inertial confinement fusion (ICF) places an urgent demand for precise measurement of 351 nm (3ω) laser beam parameters when performing physical experiments on high-power laser facilities. The near-field and focal spot distributions are the utmost important parameters to characterize the quality of the laser beam. Coherent modulation imaging (CMI) is a promising technique for online laser beam measurement, however, it fails to reconstruct the near-field and focal spot profiles when it is used to measure the beam quality of a 351 nm laser beam for SGII-upgrade facility. To solve this problem, a novel CMI reconstruction algorithm is proposed in this work, and the performance of the algorithm in 3ω laser beam measurement can be obviously improved. By adopting multiple-virtual-focal-plane constraint in the proposed algorithm, the near-field and focal spot profiles of the 3ω laser beam can be successfully reconstructed. Experiments have been conducted on SGII-upgrade facility to verify the feasibility of the proposed method.