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Influence of pulse width and speckle formation on the ablation thresholds in water by means of pulsed mid-IR laser radiation

机译:脉冲中红外激光辐射对脉冲宽度和斑点形成对水中消融阈值的影响

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Abstract: The evaporation of water by means of fiber guided pulsed mid- infrared laser radiation was investigated. The influence of the pulse duration and speckle formation on the ablation threshold was determined. Optical on-line monitoring of refractive index changes at the distal fiber tip has been used to determine the cavitation threshold. Temporal and spatial speckle pattern at the distal application fiber tip were measured by imaging the fiber tip to a reference plane. An IR- CCD camera and fast IR-photodiodes were used to monitor the intensity modulation. The measured cavitation thresholds show a strong dependence on the pulse energy and the pulse duration. Overheating of water at the evaporation threshold could be calculated for the Q-switched pulse mode. Regarding speckle, spatial peak intensities up to a factor of 1.5 and 2.6 over the average intensity were evaluated for the Q- switched and the free running mode, respectively. Speckle modulation plays an important role in the free running mode to achieve the boiling point of water with the threshold radiant exposure determined. Furthermore, transient intensity fluctuations were measured across the beam profile at the fiber tip. !11
机译:摘要:研究了纤维引导脉冲中红外激光辐射对水的蒸发。确定了脉冲持续时间和斑点形成对消融阈值的影响。远端光纤尖端的折射率变化的光学在线监测已用于确定空化阈值。通过将纤维末梢成像到参考平面来测量远端应用纤维末梢的时间和空间斑点图案。使用IR-CCD照相机和快速IR光电二极管来监测强度调制。测得的气蚀阈值显示出对脉冲能量和脉冲持续时间的强烈依赖性。对于Q开关脉冲模式,可以计算出蒸发阈值处的水过热。关于散斑,分别针对Q切换模式和自由运行模式评估了比平均强度高1.5到2.6倍的空间峰值强度。散斑调制在自由运行模式中起着重要作用,该自由模式在确定阈值辐射暴露的情况下达到水的沸点。此外,在光纤尖端的光束轮廓上测量了瞬态强度波动。 !11

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