High-power Laser Driver Beam Focusing Properties

The characteristic of beam focusing is a very important issue of the ICF laser system. Beam focusing phenomenon appears in the multiple spatial filter system and the target system of the ICF laser system, and the research already done is mostly about the light spot of the focal plane. However, when indirect drive mode is used in ICF laser system, the laser beam walks thought the pinhole of the target to the wall of the cylindrical cavity target, so the characteristic of the light spot at the focal plane and position around which should be tuned to satisfy the requirement of the physics experiment. The heat effect and error in system installing and debugging will both result in the defocus of the spatial filter pinhole with the focal plane, which finally cause the actual filter being different with the designed binary filter.To establish a more general analytical model of the beam focusing process, and to commit a painstaking analysis of the characteristic of the light spot of the focal plane and non-focal plane and the control method contribute to the upgrading of the performance of the ICF laser system. This paper is mainly about:1 The mathematical model of the beam focusing process has been ameliorated, and a brand new beam focusing analysis method, the angular spectrum plus ray tracing method, is proposed.2 According to the method mentioned above, the spectrum response function is researched, and the result shows that non-focal spatial filter is a soft-edge filter. When the defocus distance is too far, the light diffraction grows so severe that the response function comes to be disordered, and the middle and the high frequency element of the light field will be reinforced. The critical distance is determined by the power and spectral distribution of the light field and the aperture of the pinhole. Using typical pinhole parameter and wavefront distortion, the critical distance is about 10 times of the depth of focus.3 The effect of wavefront PV value on triple frequency light spot is studied using spectral analytical method, and the result shows that the wavefront PV is one of the most important reason why triple frequency light spot grows large and split to pieces, the analysis could commendably explain the experimental phenomenon, further more, the basic law of the relation of wavefront PV to the triple frequency light spot splitting phenomenon is proposed.4 The relation of the characteristic of the focal plane and non-focal position light field to the wavefront distortion is analyzed based on beam focusing model, the influence of the defocus distance on the light spot characteristic is studied, and the result could be a reference to the target parameter design.5 The control arithmetic of the AO system is studied based on the theory mentioned above, and the control capability of the AO system over the light spot characteristic is researched, which contribute to the wavefront control system design of the future high-power solid state lasers.