A phase-only spatial light modulator is used to provide and compensate for the spatial periodic modulation (SPM) of the near-field beam at the near infrared at 1053nm wavelength with an improved iterative weight-based method. The transmission characteristics of the incident beam has been changed by a spatial light modulator (SLM) to shape the spatial intensity of the output beam. The propagation and reverse propagation of the light in free space are two important processes in the iterative process. The based theory is the beam angular spectrum transmit formula (ASTF) and the principle of the iterative weight-based method. We have made two improvements to the originally proposed iterative weight-based method. We select the appropriate parameter by choosing the minimum value of the output beam contrast degree and use the MATLAB built-in angle function to acquire the corresponding phase of the light wave function. The required phase that compensates for the intensity distribution of the incident SPM beam is iterated by this algorithm, which can decrease the magnitude of the SPM of the intensity on the observation plane. The experimental results show that the phase-type SPM of the near-field beam is subject to a certain restriction. We have also analyzed some factors that make the results imperfect. The experiment results verifies the possible applicability of this iterative weight-based method to compensate for the SPM of the near-field beam.
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.