Pulse Shuttling in a Half-Mile Optical Lens Guide

To investigate the distortion of a gaussian light beam in a long lens guide a light pulse was shuttled back and forth up to 150 times in a straight underground conduit with lenses (quality λ/10) positioned confocally every 140 m. The pulse was formed by suddenly “dumping” the light stored in a He — Ne laser cavity into the guide. Part of the round-trip loss was replenished by a He — Ne amplifier. In scanning the intensity in the guide cross-section on various round trips of the pulse it was found that the gaussian profile was virtually undisturbed.