Abstract
The schlieren method of measuring far-field focal spots offers many advantages at the Shenguang III laser facility such as low cost and automatic laser-path collimation. However, current methods of far-field focal spot measurement often suffer from low precision and efficiency when the final focal spot is merged manually, thereby reducing the accuracy of reconstruction. In this paper, we introduce an improved schlieren method to construct the high dynamic-range image of far-field focal spots and improve the reconstruction accuracy and efficiency. First, a detection method based on weak light beam sampling and magnification imaging was designed; images of the main and side lobes of the focused laser irradiance in the far field were obtained using two scientific CCD cameras. Second, using a self-correlation template matching algorithm, a circle the same size as the schlieren ball was dug from the main lobe cutting image and used to change the relative region of the main lobe cutting image within a 100×100 pixel region. The position that had the largest correlation coefficient between the side lobe cutting image and the main lobe cutting image when a circle was dug was identified as the best matching point. Finally, the least squares method was used to fit the center of the side lobe schlieren small ball, and the error was less than 1 pixel. The experimental results show that this method enables the accurate, high-dynamic-range measurement of a far-field focal spot and automatic image reconstruction. Because the best matching point is obtained through image processing rather than traditional reconstruction methods based on manual splicing, this method is less sensitive to the efficiency of focal-spot reconstruction and thus offers better experimental precision.
Highlights
The output parameters of a large laser facility, including the distribution in the time domain, the distribution in the spatial domain, the distribution in the frequency domain, the temporal distribution of the wave parameters, and the distribution in the far field, which differ for different experimental requirements, are closely related to the input energy
Because the main lobe image is spliced in the side lobe image doi:10.1371/journal.pone.0171415.g009
The merged image of the inner ring is filled with main lobe image data, and the merged image of the outer ring is filled with side lobe image data
Summary
The output parameters of a large laser facility, including the distribution in the time domain, the distribution in the spatial domain, the distribution in the frequency domain, the temporal distribution of the wave parameters, and the distribution in the far field, which differ for different experimental requirements, are closely related to the input energy. The distribution in the far field plays a key role in the measurement of the far-field focal spot. There are two purposes of measuring the precise focal spot in a laser facility [2]. The first purpose is to determine the influence of the optical components on the far-field focal spot under different input power conditions [3]. The second purpose is to provide a basis for improving the quality of the optical components once the spatial distribution of the focal spot has been obtained [4]. The size and shape of the focal spot can only be improved when the position of the optical element is adjusted or when the structure of the laser path is modified by the designer
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