Investigation On Flat-top Laser Beam Shaping Using Liquid Crystal Spatial Light Modulator

With the development of laser technology,lasers are used more and more widely in many fields.But the Gaussian intensity distribution of the laser beam limits the use of the laser in many cases,for example,uniform intensity distribution(flat-top)of laser beam is required as a light source in laser radar,laser projection printing and laser inertial confinement fusion etc.On the basis of the liquid crystal spatial light modulator,algorithm for solving phase distribution to generate flat-top beam was improved in this thesis,flat-top beam shaping by changing the phase distribution of laser beam was achieved successfully in the experiment;Simulation results agreed well with experimental results.The thesis contents are as follows:(1)Brief introduction of methods on beam shaping,both advantages and disadvantages are summarized.The phase modulation principle of liquid crystal spatial light modulator(LCSLM)and the principle of diffraction theory were investigated.Beam shaping by LCSLM scheme was decided as the main work in this thesis.(2)Principles of global algorithm and local algorithm were studied.Because the global algorithm is time-consuming,we decided to use local algorithm as the basic algorithm to calculate phase distribution for beam shaping.Then we investigated local algorithm like GS algorithm,MRAF algorithm and Adaptive-Additive algorithm which is an algorithm developed from the GS algorithm,they are all iterative algorithm.MATLAB was used to do the simulation work,the advantages and disadvantages of these algorithms were analyzed.(3)We proposed an improved algorithm based on MRAF algorithm.When the MRAFalgorithm is used to calculate the distribution of phase,the output beam's stray light is too high in noise region,it's very difficult to obtain a good flat-top beam in the beam shaping experiment.The improved algorithm overcomes the difficulties that MRAF algorithm is hard to achieve in the experiment.Using improved algorithm,a flat-top beam with energy utilization rate of 76% and uniformity error of 3.5% were obtained.By comparing the above three algorithms,its superiority in the realization of flat-top beam shaping is obvious.(4)Abeam shaping system based on liquid crystal spatial light modulator was built.Therequired phase distribution was input into liquid crystal spatial light modulator,an optical stop was used to filter noise intensity,and a blazed grating was used to separate the flat-top beam and zero-order diffracted light spots,a flat-top beam on the output plane was obtained.Experimental results show that the uniformity error of the flat-top beam is 4.7%,and the energy utilization rate ofthe flat-top beam is 71%,the experimental results agree well withthe simulation results,it shows that the improved algorithm based on MRAF algorithm can give us the relatively better phase distribution for generating flat-top beam.