Fiber Optics and Strain Interferometry

By use of glass fibers as dielectric waveguides, interferometric measurements of light along the propagation path can be used as a new type of strain gauge. Two novel strain interferometers are proposed: one which uses the phenomenon of crosstalk between adjacent light pipes, and another which is a waveguide version of the Michelson interferometer. Both designs are distinguished by the possibility of digital readout and a basic simplicity. The theory of the instruments is described, and explicit estimates for the strain sensitivity are calculated. The numerical data show that, with digital readout, strains of the order of 10-8 can be resolved with a basic instrument ten meters long, and strains of the order of 10-9 can be resolved by a hundred-meter device. With the aid of various refinements, the sensitivity can be enhanced by at least another order of magnitude.