Prejudgement of hot-images' damage based on gradient direction matching method in high power laser system

摘要

Hot-images' formation process from damage sites to locations after nonlinear mediums is analyzed under high flux theoretically and numerically. Analysis shows that peak intensities of hot images scarcely change with damage depth whereas there can be a variation of locations which is approximately the thickness of the nonlinear medium. Considering that hot images appear periodically on alternating mediums, a gradient direction matching method with high signal-to-noise ratio is proposed to prejudge hot images, which is calculated efficiently with FFT method. It calculates damage sites' properties inversely with the extracted diffraction intensities from the detected ring's center. Accuracies of the inversed diffraction length and damage size are experimentally studied with the single diffraction ring. As intensity is disturbed by random noise, accuracy of damage size has an error of about tens of microns. Images are usually full of random noise. Experimental result shows that this algorithm can accurately find the diffraction center under this circumstance. Rings are generally overlapped with each other, so the resolution is studied when two rings are overlapped with different percentages. Experimental result shows that the minimum spacing between two damage sites is 1.374 mm when diffraction length is 245 mm. A method using a fan-shaped template is put forward to improve the resolution of the overlapped diffraction rings when the diffraction intensities are disturbed seriously. Results show that this algorithm can prejudge hot images quickly prior to the initiation of a full-system shot.