太赫兹激光作为大角度发散的高斯波束不能简化为平面波或球面波.经典电磁理论和ABCD法则传输理论建模显示:正透镜实现太赫兹激光束的会聚,会聚后其像距明显小于透镜的焦距;焦距和太赫兹激光光束波前半径相匹配的负透镜可以实现太赫兹激光束的准直.实验证实厂=-188,的负透镜位于与太赫兹激光光束波前半径相匹配的位置时,即z=100 mm,太赫兹激光的发散角从6°提高到0.1°,20 m传输实验中,负透镜准直探测方案比正透镜准直探测方案更加简单有效.%Terahertz beams are a kind of Gaussian beam with a big divergence angle and its wave front cannot be simplified to a plane or a spherical wave.All the theoretical derivations from the classical electromagnetic theory and ABCD laws support the following conclusions:firstly,a positive lens converges the terahertz beam in front of the focal plane instead of on the focal plane;secondly,a negative lens which matches the radius of the Gaussian beam's wave front is more appropriate for terahertz beam collimation.Experiments show that a negative lens with f'=-188 mm at the matching position,z =100 mm,can improve the terahertz beam collimation from 6° to 0.1°.And 20-m terahertz space transmission was realized with a very simple optical scheme.
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