On the Feasibility of Long Distance Laser Strain Meter Measurements through the Uncontrolled Atmosphere

Abstract

Summary This paper presents a theoretical investigation of the feasibility of long distance laser strain meter measurements through an uncontrolled atmosphere. The principle impediments to the realization of the so-called ' Free-Air Laser Strain Meter ' are: (a) the need to distinguish between the effects of the atmosphere and the effects of Earth strain on the changing optical path length, and (b) the degeneration of laser beam coherence due to propagation through a turbulent atmosphere. The method proposed here circumvents the first impediment by the use of atmospheric dispersion and lasers of three different wavelengths to separate the ' geometrical ' and ' refractive ' contributions to fluctuations in the optical path length. While the latter exhibit trends at least as long as a year and may even be changing secularly; the former can only exhibit a trend if the distance between the end mirrors of the device is changing. The operation of a free-air laser strain meter will consist of the detection of a trend in a running average of the ' geometrical ' fluctuations in optical path length. The second impediment is circumvented by the use of small receiving apertures for the return atmospheric beam. This restriction to ' coherence patch ' size restores spatial coherence and results in high fringe visibility. The loss of temporal coherence causes rapid fringe motion which simply requires wide bandwidth fringe counting systems. The analysis presented in this paper concludes that such devices are feasible with the current ' state of the art ' of electro-optical technology and appear to be capable, in principle, of making Earth strain measurements of parts in 10 with an accuracy of a few per cent over distances of several kilometres through the uncontrolled atmosphere. The realization of such a device would provide geophysicists with a research tool of unusual capabilities and perhaps find engineering applications in the remote sensing of the stability of large man-made structures such as buildings bridges and dams.