Propagation Characteristics Of IF Band High-power Lasers

ICF(Inertial Confinement Fusion) needs high temperatures and densities relied on high-power solid-state laser driver to achieve the fusion reaction conditions, so it requires output good beam quality. In order to ensure high harmonic conversion efficiency and getting through the pin-hole, the beam must have good near-field distribution uniformity and wavefront. In the similar NIF device, large aperture electro-optical switch KDP crystal acting as a key component of the four-pass amplifier due to its material properties and processing methods will introduce a lot of middle and high frequency information. Combined with wave control methods in the device, it can be analyzed that some of its frequency can get through the pin-hole and can’t be corrected by deformable mirror. The middle frequency wavefront has a great impact on near-field intensity uniformity and far-field focusing ability, thus limits the capabilities of the system load. In order to analyze the effects of the middle frequency wavefront, the paper carries out the following jobs:First, to study the curved modulation in the near-field data collected by CCD in the experiments, filtering analysis on the typical Nd:glass and KDP crystal is done to find out that there are residual perturbation which have a period of20mm on the KDP crystal. According to the wavefront space band division and laser device system characteristics compare the data of the33mm high-pass filtering on the optical components with NIF standard curve and have a general judgment on the modulation carried by KDP crystal contributing to the final output of the system.Second, a calculation model which is based on FFT theory an optics wavefront data of SG-II is established and get different near-field distributions in three cases:with middle and high frequencies introduced by KDP crystal; without KDP crystal; with the increased middle and high frequencies modulation. It can be found that because the KDP crystal has a much higher power spectral density the other optical elements in middle-band frequency, a similar periodic distribution of intensity in near-field can be found and the contrast of the near-field will be significantly increased and the uniformity will get worse when the perturbation increases.Third, introduce the’spatial coordinate transformation method’ which is usually used in shortwave and short focal length system to calculate far-field focal-spot under the large aperture and low sampling rate case. Simulate the far-field focal-spots in different spatial filters in different cases and get the middle-frequency perturbation which can get through the pin-hole impact on the size of the focal-spot.At last, in the paraxial approximation, wave equation in nonlinear media can be derived based on Maxwell’s equations. Calculate the fastest growth frequencies in different propagation interval according to B-T theory. Establish the phase defects mathematical expression and analyze its propagation in nonlinear media, calculate the amplitude growth of different defects’size, thus a basic understanding on localized wavefront defects in ICF laser device can be got.