Propagation Properties Of Laser Beam Through The Four-cylindrical-lens Slit Spatial Filter

The spatial filter is one of the most important devices in large laser facility, and one of the main functions is for cleaning up the middle and high spatial frequencies. In conventional pinhole spatial filter, the laser beam is focused into a focal spot, raising a series of problems such as the pinhole closure, large scale, and the requirement for high vacuum, which limits the development of high-power lasers. In 2012, A. C. Erlandson at the Lawrence Livermore National Laboratory proposed a four-cylindrical-lens slit spatial filter, which can focus the laser beam into a focal line to enlarge the focal area and decrease the focal intensity. However, the main functions of the four-cylindrical-lens slit spatial filter were not been studied theoretically or experimentally.In this thesis, based on the Rayleigh-Sommerfeld diffraction integral and the commercial Matlab software, the beam propagation characteristics in the four-cylindrical-lens slit spatial filter is theoretically studied, and the mode of the image relay of slit spatial filter is obtained. Unlike the conventional pinhole spatial filter, the front(or back) focal plane of the slit spatial filter system is not only related with focal length, but also with the distance between the x- and y-directional cylindrical lenses. Compared with the conventional pinhole spatial filter, the ability on spatial filtering of the four-cylindrical-lens slit spatial filter has been studied in this thesis, and the result shows that the characteristics of slit spatial filter is consistent with that of the conventional pinhole spatial filter. Meanwhile, the use of the slit spatial filter can decrease the focal intensity in high-power lasers by about 3 orders of magnitudes so that the pinhole closure can be optimized and the scale of the spatial filter can be greatly decreased.Based on the B-T theory and the Rayleigh-Sommerfeld diffraction integral, the small-scale self-focusing problem is discussed on the condition of beam aperture of 350mm×350mm, in which the B-integral values are set as 1.8 rad, 2.0 rad and 2.4 rad, respectively. The simulation shows the filtering abilities of the conventional spatial filter and the four-cylindrical-lens slit spatial filter are equivalent, and the near-field modulation and near-field contrast of the filtered beams from the conventional spatial filter and the four-cylindrical-lens slit spatial filter can be both reduced to less than 1.4 and 0.13, respectively. The intensity on inner edge of slit is about 109 W/cm2,which is decreased by two orders of magnitude than that of the pinhole spatial filter. The overall length of spatial filter in large lasers can be reduced by 25% with four-cylindrical-lens slit spatial filter instead of the conventional pinhole spatial filter, which may has potential applications in high power lasers.