| Femtosecond laser can produce instantaneous power of GW or even TW on the condition of the very low single pulse energy for ultrashort pulse.This results in a remarkable nonlinear phenomenon in the interaction between femtosecond laser and material.In recent years,femtosecond laser micromachining has become a hot spot in the field of micr o/nano fabrication by two-photon induced photopolymerization.It has gradually developed into a new type of micro/nano machining technology,which is composed of ultrafast laser technology,photochemical material technology,micro technology,ultra precision positioning technology and CAD/CAM technology.Femtosecond laser two photon micromachining usually adopts the direct writing method,which scans the polymer material in a predetermined direction.For a given machining system and processed material,there are the important factors that affect the machining precision and surface quality,such as aberration due to different refractive index of air and processed material,the ellipsoidal distribution of the spot and the size of the scanning step.In this paper,some researches are carried out in the field of aberration compensation,three-dimensional shaping technology and optimization method of scanning step.The main contents are as follows:1.Through the mechanical and electrical modif ication of the exposure control system of femtosecond laser micromachining experiment system,the motion range of the 3D scanning platform is extended,and the processing of object diversity and multi axis linkage under the three-dimensional spatial continuous scanning are achieved.2.According to the mechanism of femtosecond laser micromachining and the theory of light propagation in different medium,the influence of the refractive index of the air and the processed material on the intensity distribution of the spot is analyzed and the numerical simulation is carried out.The aberration expressions are derived and the curves of optical system parameters,working depth and aberration are obtained by wave-front aberration function.The results show that the aberration increases with the increase of the depth of processing,the numerical aperture of the objective lens and the refractive index of the medium.Zernike polynomials cycle is used to compensate for the aberrations to obtain post-compensation system point spread function.Take it for example,the femtosecond laser pulses is used for machining points in two-photon photo-chromic materials,a numerical simulation is conducted for the compensation function.Simulation results show that the aberration can be corrected effec tively by the established model of compensation.When the primary aberration is compensated,the attenuation of the fluorescence signal intensity at the processing points has been significantly improved and the working depth is close to 600 ?m.After secondary aberration compensation,the signal intensity does not change as the working depth is changed.Kepler telescope system is proposed to be used for aberration compensation and the compensation effect is analyzed based on the experiments.3.Based on the Fresnel diffraction theory and the normalized intensity distribution function of the transverse direction and axial direction of the laser beam,the focal spot shape of femtosecond laser micromachining is simulated.On the basis of the theoretical analys is and numerical simulation of the axial direction and transverse direction shaping,the mathematical model for three-dimensional laser beam shaping is established.The global optimization algor ithm and genetic algor ithm are used to optimize the key parameters of beam shaping element(phase plate).According to the results of the optimization design and numerical simulation,a kind of four-ring complex amplitude transmittance phase plate is fabricated and verified experimentally.The experimental results show that,after adding the phase plate to the laser beam,the transverse direction and axial direction dimensions of the laser beam become smaller,and the compression ratio of the beam is basically consistent with the theoretical calculation.The principle and the realization method of the beam shaping technology to improve the asymmetric shape of the spot of the processing spot are analyzed in detail,such as the use of slit aperture or increase cylinder lenses or increase pre focusing lens.4.According to the distribution function of light intens ity and the free radical concentration theory,the mathematical model of coverage of solidification unit in femtosecond laser micromachining is established,and the effect of coverage on the machining quality and efficiency of micro devices is analyze.Using the principle of exposure equivalence,the analytic expression of the relationship between the surface quality characteristic parameters of micro devices and the scanning step we is obtained,and the numeric al simulation and exper iment are carried out.The results show that the scanning step has little inf luence on the surface quality of the line when it is much smaller than the size of the solidification unit.However,when the scanning step is gradually increasing,the smoothness of line surface is reduced rapidly,and the surface quality becomes worse drastically.For stereo device processing with different slope,a kind three dimensional continuous variable scanning method is proposed and the calculation expression of the scanning step are given.From the experimental example of fabricating spherical structure,it is indicated that compared with the fixed step scanning method,the continuous variable step scanning method is used to improve the surface quality of micro devices. |
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