Research On High Quality Bessel Beam Array Generation Method Based On Femtosecond Laser Shaping Technology

Compared with electron beam exposure,focused ion beam etching,nanoimprint and other micro-nano processing technologies,femtosecond laser direct writing technology has the advantages of no mask,high degree of freedom,controllable morphology,short processing period,etc.However,the traditional femtosecond laser source is Gaussian beam,the focal depth of the focusing spot is short,point tapping is usually used to process the pore structure with high depth-diameter ratio,and the processing quality is poor.Due to its long focal depth,Bessel beam is widely used in machining the pore structure with high depth-diameter ratio.For the large area and periodic pore micro-nano structure,the parallel machining method of Bessel beam array can improve the machining efficiency.However,the uniformity of Bessel beam arrays generated by traditional methods is poor,and the signal-to-noise ratio is low,which seriously affects the machining quality.In this thesis,two high quality Bessel beam array generation methods are proposed to achieve high efficiency and high quality preparation of large area and periodic pore micro-nano structures.The main research content of this thesis is as follows:1.Gaussian light with a short focal depth is shaped into Bessel beam with a long focal depth to achieve a single machining of high depth-diameter ratio pore structure.zero-order Bessel beam is generated by using the computational hologram method to simulate the phase of axicon.However,there are annular sidelobe distribution outside the center spot of zero-order Bessel beam.When the beam energy is large and the distance between adjacent holes is close,the sidelobe energy will damage the processing quality of adjacent holes.To solve this problem,the side-lobe energy suppression of zero-order Bessel beams is realized by using the method of superposition of different order Bessel beams.The effectiveness of image subtraction for side-lobe suppression is verified by simulation.2.Two computational holography methods are proposed,which can generate high quality parallel and divergent Bessel beam arrays respectively.The multi-axicon phase parallel splicer method is proposed to effectively reduce the background noise of the optical field and improve the energy utilization rate by improving the "aperture utilization rate" of the window.The simulation results show that the uniformity of the generated 3×3 parallel Bessel beam array is 98.94%,and the diffraction efficiency is78.12%;The multi-lens and axicon phase superposition method is proposed,the number and position of Bessel beams are regulated by the multi-lens and axicon phase superposition.The lens phase plays a focusing role on the Bessel beam,and the Bessel beam array with the diameter of the center spot reduced can be obtained near the rear focal plane of the lens.The simulation results show that the uniformity of the generated3×3 divergent Bessel beam array is 97.95%,and the diffraction efficiency is 79.23%.Compared with traditional methods,the uniformity of parallel and divergent Bessel beam arrays produced by the proposed method is increased by 2.97% and 4.70%,and the diffraction efficiency is increased by 48.22% and 54.75%,respectively.The images of 120 mm,130mm and 140 mm along the transmission direction of Bessel beam array are collected through experiments,which is highly consistent with the simulation.