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题名	超短脉冲激光加工电介质的若干现象及机理
作者	王承伟
学位类别	博士
答辩日期	2016
授予单位	中国科学院上海光学精密机械研究所
导师	赵全忠
关键词	微加工，超短脉冲激光，等离子体，非线性传输，电介质
其他题名	Several phenomena and mechanisms in machining dielectrics by ultrashort pulsed laser
中文摘要	超短脉冲激光加工具有无接触和冷加工的优点，适用于加工高硬度或易脆的蓝宝石、玻璃等电介质。为了提高超短脉冲激光加工的性能，切割厚度的饱和现象、裂纹以及偏振依赖的加工形貌等有待研究。我们结合散射效应、应力分析、逆韧致辐射和脉冲前沿倾斜，以及电子密度演化、超短脉冲激光的线性和非线性传输，建立相应的模型，解释了这些现象。 1. 后续的超短激光脉冲加工蓝宝石时，会被之前的超短激光脉冲形成的微结构散射。我们结合散射和超短脉冲激光的非线性传输建立模型。当光强超过损伤阈值的一半后，该点处的光电场设为0，对后续位置的光强没有贡献。该模型不仅解释了损伤的壳层内部存在椭球形的未损伤区域，而且解释了切割厚度的饱和现象。 2. 当不同偏振的超短脉冲激光加工中心对称的LiF晶体时，加工长度随偏振变化。当偏振方向从<100>晶向转到<110>晶向，超短激光加工区域的长度减小。这是由不同偏振下逆韧致辐射系数不同引起。逆韧致辐射和碰撞产生的等离子体的密度依赖于偏振。电子复合时释放能量给晶格，晶格温度升高。由于释放的能量与电子密度成正比，因此晶格温度随偏振变化。如果某处的晶格温度超过熔点，该处的电介质被加工。模拟的加工起点和终点，与实验值的相对误差在3.0%以内。 3. 带脉冲前沿倾斜的超短脉冲激光加工石英，当激光偏振从S偏振变化到P偏振，加工长度增大。带脉冲前沿倾斜的超短脉冲激光通过强场电离和雪崩电离产生电子等离子体，等离子体在脉冲前沿和波矢构成的平面内倾斜。S偏振下激光的反射率大于P偏振下的反射率，而且共振吸收发生在P偏振而非S偏振，因此S偏振下的有效光强小于P偏振。 4. 超短脉冲激光聚焦到电介质内部后，诱导出微结构，微结构发出黑体辐射用光谱仪和ICCD测量。用Planck公式拟合黑体辐射谱，得到加工区域的瞬时温度。在热扩散控制下，加工区域的温度以指数形式衰减。
英文摘要	Because ultrashort laser machining is noncontact and precise, it is suitable for cutting hard or fragile dielectrics, like sapphire and glass. To improve the performance of ultrashort laser machining, it should be studied that saturation phenomenon of cutting thickness, cracks and polarization dependent machining morphology. We bulid physical models to explain the phenomena. The models combine scattering, stress analysis, inverse bremsstrahlung, pulse front tilt, with the evolution of plasma density, linear and nonlinear propagation of ultrashort pulsed laser. 1. The successive ultrashort laser pulses would be scattered by the machined regions resulting from the previous laser pulses. The scattering effect is combined with nonlinear propagation of ultrashort laser to build a model. When the laser intensity is over half the damage threshold, the electric field is set to 0 and doesn’t contribute to the laser intensity at latter positions. The simulated laser intensity not only offer explanation to the elliptical undamaged shell around the damaged core, but also give reason to the saturation of cutting thickness. 2. When the centrosymmetric LiF crystal was irradiated by femtosecond laser at different polarizations, the machining length varied with polarizations. When the polarization direction turns from the crystal orientation <100> to <110>, the machining length decreases. This is attributed to the fact that the inverse bremsstrahlung coefficient changed with polarizations. The density of plasma caused by inverse bremsstrahlung and collision is dependent on polarizations. The energy which was released by electron recombination was transferred to crystal lattice. The temperature of crystal lattice relies on polarizations. The dielectrics were machined if the temperature of lattice overpassed the melting point. Moreover, the relevant errors between the simulated starting point and ending point and the experimental values are within 3.0%. 3. When fused silica was irradiated by the femtosecond laser pulses with pulse-front tilt, the machining length in the s-polarized state was less than that in the p-polarized state. The density of plasma, which was produced by ultrashort laser with pulse-front tilt via strong field ionization and avanlanche ionization, inclines along the plane composed by the direction of pulse front and wavevector. The reflection at s-polarizaton is more than that at p-polarization. Furthermore, the resonant absorption happens at p-polarization instead of s-polarization. Therefore, the effective laser intensity at s-polarization is less than that at p-polarization. 4. After one ultrashort laser pulse was focused into the dielectrics, microstructure was induced. The microstructure in the dielectrics emitted black-body irradiation, which was collected by a spectrometer and an intensified charge coupled device (ICCD). The black-body irradiation spectra was fitted by the Planck formula. And the temperature of dielectrics, which was controlled by heat diffusion, decreased exponentially.
语种	中文
内容类型	学位论文
源URL	[http://ir.siom.ac.cn/handle/181231/16000]
专题	上海光学精密机械研究所_学位论文
推荐引用方式 GB/T 7714	王承伟. 超短脉冲激光加工电介质的若干现象及机理[D]. 中国科学院上海光学精密机械研究所. 2016.

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