A Temperature Insensitive Smart Optical Strain Sensor
Kimberly A. Thomas, Everett E. Crisman, Otto J. Gregory, and William B. Euler, Proceedings SPIE: Smart Structures and Materials 2000: Smart Systems for Bridges, Structures, and Highways, S. C. Liu, editor, SPIE Press, Belligham, WA, vol. 3988, 2000, 429 – 439
Abstract
An optical strain gage, employing a hollow polyimide-coated glass capillary tube, is currently under development. The capillary tube serves as a waveguide, in which a source signal is attenuated in an amount proportional to the applied strain. The source signal is supplied to the waveguide through an optical fiber and a second optical fiber delivers the attenuated signal from the waveguide to a photodiode detector. The inherent compatibility of this optical strain sensor with fiber optic telecommunication systems makes it amenable for incorporation into intelligent systems for the continuous monitoring and damage assessment of bridges, highways, piers, and buildings. The application of thin films to the interior and/or exterior surfaces of the waveguide can optimize the dynamic range and sensitivity of the smart optical waveguide. This optical strain sensor exhibits advantages in comparison to commercially available metal foil (resistance) strain gauges, including gage factors at least 100 times larger and temperature insensitivity. The determination of the sensor's gage factor will be presented, as will experimental results showing that this sensor out performs other types of sensors and its signal response remains constant when subjected to operating temperatures ranging from –20 °C to +47 °C.