Laser beam self-focusing in the atmosphere
Laser beam self-focusing in the atmosphere
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DOI：
10.1103/physrevlett.102.233902
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发表时间：
2009-06
期刊：
影响因子：
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通讯作者：
A. Rubenchik;M. Fedoruk;S. Turitsyn
中科院分区：
文献类型：
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作者：
A. Rubenchik;M. Fedoruk;S. Turitsyn
地球以外丰富的太阳能使太空发电具有吸引力。最初的建议是基于微波的能量传输。然而，激光科学的最新进展刺激了对使用基于激光的轨道系统的可行性的研究，其中激光辐射被利用来将转换的太阳能传输到地面[1，2]。注意，即使对于衍射极限光束，也必须在空间中具有精确的聚焦光学器件和大型地面接收器，以促进有效的功率传输和收集。在这里，我们建议利用大气中的自聚焦效应，以帮助提供强大的激光束。通常，对于具有超过自聚焦的临界功率的功率的光束，发生不受控制的光束偏转和光束分裂。在这项工作中，我们证明了当自聚焦长度与大气高度相当时，灾难性的自聚焦可以被大大抑制，并且整个光束的平滑压缩是可能的。为了说明这个想法，不失一般性，我们考虑一个激光束垂直传播通过地球的大气层从空间到地面。非线性折射率变化与密度变化成正比。引入海平面上的密度为ρ0，在任意高度上对应的非线性折射率为，n2（z）=n2（0）ρ/ρ0;ρ=ρ 0 e −z/h。在这里，地面上的折射率为n2（0）=5.6×−19 cm 2/W。当光束功率超过临界值Pcr=0.93λ02/2π n 0 n2时，均匀介质中的自聚焦开始。我们在模拟中改变的两个关键参数是反射镜半径和光束功率。图1显示了反射镜半径R 0 = 1 m和几种光束功率下地面上的强度分布。与线性传播相比，可以观察到强的（五倍的）光束压缩，而没有任何折射的指示。图2给出了不同反射镜半径下大气中光束半径演变的模拟结果。
The abundance of solar energy outside the Earth makes attractive the electricity production in space. Initial proposals have been based on the energy transportation by microwaves. However, recent progress in laser science has stimulated research into the feasibility of using laser-based orbit systems in which laser radiation is utilyzed for transport of converted solar energy to the ground [1, 2]. Note that for even for diffraction limited beams one must have precision focusing optics in the space and a large ground-based receiver to facilitate efficient power transport and collection. Here we propose to exploit a self-focusing effect in the atmosphere to assist delivering powerful laser beams. Usually, for beams with power exceeding the critical power for self -focusing, uncontrolled beam filamentation and beam break up takes place. In this work we demonstrate that when the self-focusing length is comparable with the atmosphere height, the catastrophic self-focusing can be greatly suppressed and a smooth compression of the whole beam is possible. To illustrate the idea, without loss of generality we consider a laser beam propagating vertically through the earth's atmosphere from space to ground. The nonlinear refractive index change is proportional to density variation. Introducing the density at sea level as ρ0, the corresponding nonlinear refractive index at an arbitrary height is, n2(z)=n2(0)ρ/ρ0;ρ=ρ0e−z/h. Here the refractive index on the ground is n2(0)=5.6×−19 cm2/W. Self-focusing in homogeneous media starts when the beam power exceeds the critical value, Pcr=0.93λ02/2πn0n2. The two key parameters we vary in our simulations are the mirror radius and the beam power. Figure 1 shows the intensity distribution on the ground for a mirror radius R0 = 1m and for several beam powers. One can observe a strong (factor of five) beam compression in comparison with linear propagation without any indication of filamentation. Figure 2 presents results of modelling of the beam radius evolution in the atmosphere for different mirror radii.