Ultra-fast imaging study on femtosecond laser processes

超快激光材料加工已成为激光材料加工中具有吸引力的新方法。然而，这些过程还不够清晰，无法理解这些未来。本研究开发了一套超快速成像系统，用于研究金属、金属薄膜、半导体和电介质等多种材料的飞秒激光加工。所开发的系统是一种泵-探测系统。二次谐波发生器的输出同时包含基波光和二次谐波光：基波光作为泵浦光，次谐波光作为探测脉冲。通过光延迟线改变两个脉冲之间的时间间隔，用CCD相机拍摄探针脉冲照射后的图像。在不同时间间隔拍摄的一系列图像揭示了超快激光加工的时间依赖性。系统的观测几何可以在侧面观测和表面观测两种模式之间转换。在侧视模式下观察到的图像显示激光诱导羽流在激光照射后从2到9 ns传播到空气中。从羽流前端位置随脉冲数在一定时间内的变化可知，初始阶段的烧蚀速率是恒定的，玻璃基板的烧蚀速率最大，铝基板的烧蚀速率最小。在表面观察模式下观察到的图像表明，在泵浦脉冲和在照射位置出现可观察到的变化之间存在显着的时间延迟。此外，还建立了激光瞬态应力场的时间分辨观测方法，对快速激光加工进行了详细的研究。该方法利用纳秒脉冲激光作为探测脉冲，光弹性方法作为应力场可视化的原理。利用该系统，成功地实现了激光辐照后环氧树脂块体在100 ~ 2000 ns范围内的激光诱导应力场的可视化。

Materials processing by ultra-fast lasers have become new, attractive methods in laser materials processing. The processes, are, however, not clear enough to understand these futures.In this study, an ultra-fast imaging system has been developed for a study on femtosecond laser processes of several kinds of materials, such as metal, metal film, semi-conductor and dielectrics. The system developed is a kind of pump-probe systems. Out put of a second harmonic generator contains both the fundamental and the second-harmonic wavelength light : the fundamental light is used as a pump pulse and the second harmonics light as a probe pulse. Time interval between these two pulses is changed by optical delay lines and images illuminated by probe pulses are taken by a CCD camera. Series of images taken at different time interval reveal time dependence of ultra-fast laser processing. The observation geometry of the system can change between two modes: side-looking and surface looking modes.Observed images in side-looking mode show laser-induced plumes propagating into air from 2 to 9 ns after laser irradiation. From the change of the position of the front of the plume at constant time with pulse number suggest that the ablation rates at initial stage are constant and are greatest for glass substrate and smallest for aluminum. Observed images in the surface-looking mode suggest that there is a significant time delay between the pump pulse and the appearance of observable change at the irradiated position.In addition, a time-resolved observation method of laser-induced transient stress field has been developed to study the fast laser processing in details. The method utilizes a nano-second pulse laser as a probe pulse and photoelasticity methods as a principle of visualization of the stress field. Using this system, laser-induced stress fields in epoxy-resin blocks from 100 to 2000 ns after laser irradiation are successfully visualized.