| High-power laser system is an important equipment for laser fusion. Spatial filter (SF)is one of the key units equipped in high-power lasers, which can be used to effectivelyimprove beam quality by filtering out the middle and high spatial frequencies. Thus the fastgrowing nonlinear effects and small-scale self-focusing can be controlled and the damageto optical components in the optical paths can be eased. Besides, spaitial filter plays animportant role in image relay and aperture matching, etc. Despite the positive sides ofspatial filter, there are some negative issues with its application in high-power lasers,including pinhole closure, air breakdown and contamination of lenses, etc. To avoid theseproblems, spatial filter with long focal length is required to be placed in high vaccumchamber, which will increase the scale of laser systems and raise the cost of laser system,maintenance, etc.To solve these problems, slit SF is applied in this thesis to replace the conventionalpinhole SF in high-power lasers. With the use of slit SF, the focal intensity in filter can bedecreased by2to3orders of magnitude, thus the overall length of the filter can be greatlyshortened and the vaccum degree required in the filter can be decreased by about one orderof magnitude while remaining the output beam quality, which is benefitial to buildingcompact lasers with high reliability. Besides, the new-type optical systems for main laseramplifier with the use of slit SF is discussed, which is helpful for future high-power lasers.The main results and progresses in the thesis are as follows:1.Two kinds of slit SFs, the crossed four lenses and three-lens configuration slit SFsare proposed in this thesis. Including the four-lens and the two-lens configuration slit SFsproposed by Lawrence Livermore National Laboratory, there are four types of slit SFs intotal. By comparing the slit SFs with one another on structure and characteristics, thethree-lens and the two-lens slit SFs are considered to be the possible alternative to replace the conventional pinhole SF used in high-power lasers, which possess the merits ofcompactness and inhibition of pinhole closure.2. The process of beam propagation through slit SF are studied with the combinationof Rayleigh-Sommerfeld diffraction integral, lens transformation and multi-Gaussionfunction, and the problem on image relay function of slit SF is theoretically solved. Withintroducing the front and back focal plane of system, iamge relay is further understood inslit SF. With calculation, simulation and experiment, the results of three-lens and thetwo-lnes slit SFs are in good agreement with conventional pinhole SF on spatial filtering,image relay and aperture matching. For the beam aperture, the aperture of output beam forthe two-lens slit SF will become different from that of the incident one, and the outputspectrum will also be affected. The relationship between the beam ratio and spectrum areobtained with simulation and experiment in the thesis.3.The three-lens and the two-lens slit SF are used to replace the transport SF inNational Ignition Facility (NIF) system, and the simulation shows that the introducing ofslit SF can decrease the scale of NIF system and inhibit pinhole closure while remainingthe system performance. The application and reliability of the laser system can be muchimproved with the use of slit SF. According to the simulation, the overall length of thefilter can be shortened to28m from the original60m with the three-lens slit SF, and thefocal intensity can be decreasd by about3orders of magnitude. Thus the pinhole closurecan be eased and even avoided, the vaccum required in spatial filter can be reduced byabout one order of magnitude, and the cutoff frequency can be reduced to30timesdiffraction limit from the original39times diffraction limit for better output beam qualityor increased to42times diffraction limit for higher energy transmittance. For the two-lensslit SF, the overall length of the SF can be shortened to39m and the focal intensity can bedecreased by about2orders of magnitude. Besides, the structure of the two-lens slit SF issimpler than the other slit SFs, which makes it more possible to be applied in present lasersystems.4. The conceptual design on main laser optical system of NIF has been discussedwith the two-lens slit SF. For a single beam line for one laser pulse or two laser pulses,new-type main laser optical systems with small scale and low cost are obtained, which provides new ideas for the design of future high-power lasers. According to the analysis,the main laser optical system for single beam line is only100m, which is far shorter thanthe180-meter-long overall length of NIF main laser optical system. |
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