Optical strain gauge

Abstract

Optical strain gauge employing evanescent field coupling between optical fibers. Optical fibers are spaced apart for evanescent field coupling on a structure whose strain is to be measured. Light is introduced into an input fiber which couples into an output fiber depending on the separation between the fibers. Light coupled into the output fiber is detected to generate an output signal indicative of the separation between fibers and hence strain in the supporting structure. In one embodiment, apparatus is provided to measure strain at a plurality of locations with but a single input and a single output fiber. In this embodiment, the input fiber and the output fiber are separated for evanescent coupling at a plurality of locations. The lengths of the input and output fibers between the locations where strain is measured are unequal to produce light propagation time delays. A light pulse is injected into the input fiber and couples into the output fiber at the successive locations where strain is to be measured. Because of the introduced time delay, the output train of pulses represents strain at the plurality of locations. In another embodiment, the optical fibers are embedded in a composite material such as an epoxy graphite structure. At points of strain measurement, the fibers are brought close together and positioned so that the positioning element transduces strain displacement in the composite into relative motion between the fibers. The positioning element further acts to maintain a gap between the fibers which may contain a suitable optical medium such that evanescent coupling can occur through the medium.