Reich Stefan, Schäffer Sebastian, Lueck Martin, Wickert Matthias, Osterholz Jens
Fraunhofer Institute for High-Speed Dynamics, Ernst-Mach-Institut, EMI, Ernst-Zermelo Straße 4, 79104, Freiburg, Germany.
Sci Rep. 2021 Nov 19;11(1):22619. doi: 10.1038/s41598-021-02112-6.
When laser beams propagate through media with non-vanishing absorption, the media is heated resulting in a change of the refractive index, which can lead to thermal lensing and thermal blooming. However, experimental details about both phenomena for propagations in water are lacking, especially for high-power lasers in the kilowatt range. We show that significant thermal lensing occurs only for high input powers before the onset of convective flow, while for low input powers, no strong thermal lens arises. After the onset of water flow, thermal blooming occurs at low input powers comparable to that known for propagations over kilometres in the air. However, for high input powers a thermal blooming on a qualitatively higher level is shown. By wavefront sensing, the change of refractive index distribution in water is investigated. This clearly shows the fast development of a strong thermal lens for high input powers and the onset of convection. Furthermore, a qualitatively good agreement of the accompanying simulations is observed. It is found that the absorption coefficient is linear with a value of [Formula: see text] at least up to 7.5 kW, i.e. 8 [Formula: see text]. However, the directed transmission into an aperture is only constant before any thermal lensing of blooming occurs.
当激光束在具有非零吸收的介质中传播时，介质会被加热，导致折射率发生变化，这可能会引起热透镜效应和热晕效应。然而，关于这两种现象在水中传播的实验细节尚缺，特别是对于千瓦级的高功率激光。我们表明，仅在对流开始前的高输入功率下才会出现显著的热透镜效应，而在低输入功率下，不会出现强烈的热透镜。水流开始后，在与空气中千米传播已知情况相当的低输入功率下会出现热晕效应。然而，对于高输入功率，会出现定性上更高水平的热晕效应。通过波前传感，研究了水中折射率分布的变化。这清楚地显示了高输入功率下强热透镜的快速形成以及对流的开始。此外，观察到伴随模拟在定性上有很好的一致性。发现吸收系数至少在高达7.5千瓦（即8[公式：见原文]）时呈线性。然而，在热透镜或热晕效应发生之前，进入孔径的定向传输才是恒定的。
