Small-scale self-focusing assessment using PSD Shannon entropy of a 3ω process in a high-power laser facility

A high-power laser facility produces a high power-density laser to drive the target compression. When the facility works at extremely high fluences, the laser may induce small-scale self-focusing, which damages the optics. To ensure safe operation, we propose a method using power spectral density (PSD) entropy to judge the occurrence and assess the development of small-scale self-focusing in the final optics assembly of a high-power laser facility. In this method, Shannon entropy statistics is adopted to analyze the PSD change in the output near-field image. The results show that, as the operating fluence increases, the PSD Shannon entropy curve displays an evident turning point when the small-scale self-focusing begins to develop. PSD Shannon entropy is superior to the intensity entropy method in detecting small-scale self-focusing in this process. It also supplements Shannon entropy applications.