| With the development of femtosecond laser technology,femtosecond laser plays an increasingly important role in the field of material processing.Femtosecond laser has ultra-short laser pulse and ultra-high energy density,which can be used in the field of material processing to achieve nanoscale machining accuracy,it also can achieve ultra-fine machining of three-dimensional structure.The traditional femtosecond laser micromachining generally adopts the single-point scanning processing method,which has low processing efficiency and slow processing process.In order to improve the efficiency of laser micromachining,researchers put forward a method of parallel laser micromachining by splitting laser beams.Spatial light modulator(SLM)has been widely used in femtosecond laser micromachining because it can regulate the laser-focused light field more conveniently and flexibly.So how to use the spatial light modulator to regulate the light field accurately and flexibly has become a hot topic of research.To design the optical field with spatial light modulator,the corresponding algorithm is needed to calculate the phase information of the required optical field.Most of the traditional phase iterative algorithms are designed for scalar optical fields.It is impossible to control the vector light field flexibly and accurately according to the specific experimental equipment.Therefore,the traditional phase iteration algorithm has great application limitations.Based on the vector diffraction integral theory of objective lens focusing system,the vector Fourier transform relation of objective lens focusing system with a high numerical aperture is derived.Based on this vector Fourier transform,a vector G-S iterative optimization algorithm is designed for the focusing system of vector optical field with high numerical aperture objective lens.This algorithm fully considers the vector characteristics of the focusing system.In the process of iterative optimization,some operations related to the distribution of focused field are introduced.Therefore,a3-D focal spots array with high uniformity can be designed and produced,and the number,shape,size,polarization Angle and spatial position of the focal spot array are precisely adjustable.Furthermore,on the basis of the pure phase vector G-S algorithm,a method to precisely design the vector compact focusing field by modulating the polarization stateof incident light is proposed.The polarization state of the incident light needed for the designed focal field is calculated iteratively,and the polarization state of the incident light beam is modulated by the controllable polarization element.The expected optical field distribution can be generated in the focal field by focusing through the objective lens.The simulation results show that the algorithm can also be used to generate 3d focal spot arrays whose number,shape,size,polarization angle and spatial position of each focal spot can be precisely adjusted.In the simulation,it is found that in the focal field generated by the pure polarization regulation mode,a focal mirror field that is symmetric with the focal design field in the focal plane and rotated 180 degrees around the Z-axis relative to the focal design field.The characteristics of the focal field of the mirror image are studied by a large number of simulation analyses under different conditions.A femtosecond laser processing system based on a spatial light modulator is built to verify the feasibility of the proposed algorithm in parallel laser processing.Taking the gold film as an example,the feasibility of the focal spot array generated by this algorithm in the laser etching process is verified by several experiments.The feasibility of this algorithm in two-photon polymerization processing is verified by several experiments of using photoresist to process 3-D structures.The proposed technique is expected to be used in laser material processing,optical microscopic imaging,optical storage and optical micromanipulation. |
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