Pulse Shuttling in a Half-Mile Optical Lens GuideI

Lens guides with widely separated glass lenses for long distance optical communication have been proposed in several varieties 1 - 2 and investigated in various laboratory and field experiments. 8-5 The use of dielectric coatings reduces the loss to one per cent per lens or less,3 an estimate which has been confirmed by the experiments reported here. This loss figure permits 1000 lenses to be placed in the beam path before the beam intensity is down by 40 to 50 dB and a repeater or amplifier is necessary. An average distance of 100 m between lenses would lead to a repeater spacing of 100 km. There is legitimate doubt that a light beam can be kept bundled and centered on the guide axis over this distance. Measurements in a 140 m underground air-filled pipe showed a slow beam drift of several mm per week correlated with the ground temperature field. 8 Thermal insulation of the pipe could reduce this effect, but eventually along the line there will have to be some beam position control to cope with this and with an expected slow ground drift. Careful investigation in the same pipe revealed that density fluctuations or any short time influence of the gas in a well-shielded 767