Experiments with an Underground Lens Waveguide

If a beam of coherent light is to be transmitted along the surface of the earth it will be necessary to redirect and focus it at intervals by means of lenses or mirrors to follow the terrain. 1,2 Furthermore, it must be shielded from atmospheric temperature fluctuations which result in variations of the index of refraction. A laboratory experiment which, in a 100-m metal pipe, folded a transmission path back upon itself gave the impression that the atmospheric effects could be overcome by choosing the proper beam enclosure. 3,4 Other experimenters, using a 1-km long pipe above ground, considered evacuation as the most reasonable means to avoid temperature effects. 5 In any case, the transmission path would most likely be installed underground where temperature variations are much smaller than in the open air. To gain information about the transmission characteristics in this case a £-mile underground iron pipe was used to build a lens waveguide with focusing lenses 400 feet apart. The temperature field and the beam displacement in the tube were measured simultaneously and compared with theoretical estimates. This gave valuable indications for the construction of underground lens waveguides. 721