Near-field character and improvement technology of induced spatial incoherence

Abstract

Induced spatial incoherence technology is a beam-smoothing method with its own unique advantages for laser driven inertial confinement fusion. However, simply using the induced spatial incoherent method will induce a strong near-field intensity spatial modulation, which will threaten the safety of the operation and severely limit the maximum output capability of the device. This is also one of the main technical obstacles to applying induced spatial incoherence to a high-power laser device used for fusion. In this paper, a technique of smoothing the near-field spatial intensity modulation caused by induced spatial incoherence is introduced. By using a two-lens filter system, a homogeneous and stable near-field intensity distribution can be obtained on the premise of reserving the innate advantages of induced spatial incoherence (better far-field smoothing characteristics), thereby avoiding the damage to devices and limitation to output capacity in high power laser system using induced spatial incoherence. Based on the theoretical modeling and numerical analysis, using modulation degree, softening factor, and transmittance as evaluation parameters, the near-field light characters with three kinds of filter apertures, such as square, round, and Gaussian, are compared and analyzed. Finally, in a typical optimization result there are used 16×16 induced spatial incoherent divisions and a square aperture with 0.8 times diffraction limit width. In this case, the near-field intensity distribution is uniform, and at the same time, good smoothing effect on far-field and a high energy utilization rate are ensured. On this basis, according to the actual application of the device, the influence of the collimation error on the near-field intensity distribution is further analyzed. The results show that as long as the collimation error is less than 0.1 times the diffraction limit, the near-field quality will not be affected. The simulation analysis of the focal spot obtained by induced spatial incoherence shows that the addition of the filtering system can further improve the low frequency uniformity of the focal spot.