Femtosecond Laser Beam Shaping With A Spatial Light Modulator

Laser processing is a non-contact processing method with the advantages of high energy density,high processing precision and small heat-affected zone.Because of the unique advantages of ultra-short pulse width and ultra-high peak power,femtosecond lasers are widely used in various fields and materials micromachining processes.However,the laser beam generally follows the Gaussian distribution,which limits the adaptability of laser technology in complex processing scene.Beam shaping technology came into being.This paper mainly studies several shaping methods by using the Spatial Light Modulator(SLM),which are used to control the light field distribution in the focal region,to adapt to specific processing scenarios.First of all,the related theoretical basis is introduced,including the theoretical model and propagation characteristics of the Gaussian beam,simulation principle of phase-modulated Gaussian beam focusing,the grating diffraction theory and the function of spatial light modulator.Then,according to the experimental requirements,the experimental system was designed and built,and the three shaping methods were studied in turn,and the following conclusions were obtained:Firstly,the shaping of the annular spot is achieved.By changing the period of the diffraction axicon phase hologram and the single pulse energy,the control of the ring spot diameter and width is achieved.The annular spot with a radius of 5.40?m to 26.34?m and a width of 2.53?m to 5.18?m is obtained,which is basically consistent with the theoretical calculation results.The phase hologram of the prism or the Fresnel lens is superimposed to remove the zero-order light.Secondly,the shaping of long focal depth is achieved.By analyzing the discrete effects caused by interface refraction and objective lens correction collar,a variable period phase hologram based on the diffraction axicon is designed.The focal axial intensity distribution is expressed in the form of micropores processed on the surface of polyimide film(PI),and its distribution is consistent with the simulation results.The defocusing process is performed using a 40-fold mirror of 0.6NA.When the minimum ring period of the ring-shaped blazed grating is 200?m and the inner-outward direction is increased by 400?m,a 50.90?m uniform depth of focus can be obtained,which is 8.28 times length than that obtained by direct processing using an objective lens.In addition,the composite defocusing process is performed by the SLM and the correction collar to further homogenize the entire depth of focus region.Thirdly,the shaping of the waveguide with a circular cross section is realized.The depth of focus of objective lens with different NA values is analysed as well as the effect of correction collar on the cross section of waveguide writing inside a transparent material.The results show that processing with small NA objective lens and the aberration caused by interface refraction will both increase the aspect ratio of the written waveguide section.Using a Fresnel cylindrical mirror phase hologram with a centre period of 2mm,the waveguide with a nearly circular cross section was successfully written inside the fused silica sample.The aspect ratio increased from 0.182 before beam shaping to 0.875 with beam shaping.