Research On Microstructures In Lithium Niobate Crystals Induced By Laser Pulses

Recently,femtosecond laser micromachining has been studied intensively as one of the most efficient techniques for direct three-dimensional?3D?microfabrication of transparent optical materials.However,the low repetition rate of the traditional laser source limites the machining efficiency greatly,which can be solved by the high repetition rate fiber femtosecond laser.In addition,though much more studies were concentrated on the MgO: LiNbO₃,there were quite few literatures about using high repetition rate femtosecond laser induced X-,Y-and Z-cut MgO: LiNbO₃.On this basis,this paper focuses on the interaction between high repetition rate femtoseocnd laser and three orientations MgO: LiNbO₃ crystals,and the influence of the average power,expose time and crystal orientation upon the ablation morphology has been discussed.The contents are listed below:1.The characteristics of femtosecond laser micromachining and the research progress of femtosecond laser induced the microstructure of lithium niobate crystal were summarized.The internal mechanism,photo ionization and avalanche lonization,between femtoseocnd laser and transparent materials were analyzed.The influence of repetition upon the heat effect of laser processing transparent material was discussed,and a developed thermal conduction model was established.2.Using high repetition rate femtosecond laser,the static ablation experiment on lithium niobate crystal surface was carried out,and the ablation morphologies are analyzed.The experiment discussed the influence of the incident average power and scanning speed on the results.The analysis found that ablation area increases as the incident average power increasing and scanning speed decreasing.The measured laser-induced damage threshold of X-,Y-and Z-cut MgO: LN crystals are 0.82 mJ/cm2,0.74 mJ/cm2 and 0.76 mJ/cm2,respectively.3.Straight lines were writed on X-,Y-and Z-cut MgO: LN crystals surface and within the crystals,respectively.The influence of laser parameters on line width and the reasonable parameter for induced waveguide is confirmed.The surface temperature distribution of writing line was simulated numerically,and the influence of scanning speed on periodic horizontal stripes within the scribed line was discussed.4.The special structures in static ablation experiment and scribed line experiment were researched.In Z-cut MgO:LiNbO₃,the angles of cracks agree well with the crystalline angle of the rhombohedron unit cell of LiNbO₃ crystal,and the observed micro-gratings with period about 1.3 mm and 2.3 mm closely match the free space wavelength of the laser?1040 nm?.Besides,the reason for no a cavity in Y-cut crystal is that the refractive index mismatch.