The far-field intensity is detected from far-field image to estimate the piston distance between two gratings. The image processing algorithm includes projections along the horizontal and vertical directions, search for each focal spot's centre, feature extraction and intensity computation. Each focal spot's centre can be found with the projections. A self-growing method is used for feature extraction, where the threshold value depends on the gray value of each focal spot's centre. For each focal spot, the sum of gray values within the relevant domain is taken to be its energy intensity. Furthermore, the energy ratio of left and main focal spots (or main and right focal spots) is computed. A formula that expresses the piston distance between two gratings as the function of the energy ratio is fitted with several measured points. Based on this formula, the piston distance is obtained for a new energy-ratio. Finally, the proposed method is verified with a series of experiments.
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 visionbased alignment control method for grating tiling in petawattclass 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 farfield 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.
In this paper, we created some microstructures on the surface of silicon by irradiating a silicon wafer for soalr cell with trains of short pulses in the presence of an ambient gas.The laser pulse produced by TEA CO2 laser is high power and shorter than a microsecond in duration. We found that the morphology of the structures is highly dependent on the species of ambient gas and the number of laser pulses used. I note that surfaces covered with these microstructures have striking optical properties: structures absorb approximately 90% of incident light with wavelengths between 250 nm and 0.85μm.The remarkable and potentially useful optical properties of the structures should be beneficial in increasing the efficiency of crystalline silicon soalr cell.
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