Losses Suffered by Coherent Light Redirected and Refocused Many Times in an Enclosed Medium

Abstract

If a beam of light is to be transmitted for any considerable distance along the surface of the earth, it will have to be redirected at intervals in order to follow the terrain and focused repeatedly to counteract diffraction. The directing and focusing elements, whether lenses or mirrors, will introduce some loss in addition to that produced by the transmission medium itself. The experiment described in this paper was performed to determine the magnitude of the total loss encountered with such transmission and to determine how much of this loss is due to each of the contributing factors. A beam of light, enclosed in a metal pipe, was redirected many times by confocally-spaced spherical mirrors, and the loss as a function of the distance over which the beam had been transmitted was determined. At the operating wavelength of 6328 Å these losses, which were found to be almost entirely due to mirror deficiencies, amounted to about 1 per cent per reflection. As a result of the loss being largely in the mirrors the loss per mile depends to a considerable extent upon the spacing between these optical elements. The expected loss for a number of assumed spacings is tabulated. The experimental results encourage the belief that beams of coherent light can be redirected and focused many times without excessive loss, and that the mechanical stability required can be obtained — in the laboratory at least.