High resolution optical time domain methods for measuring strain

Abstract

High-resolution optical time-domain methods applied to measuring strain in an optical fiber are discussed. The use of this optical time-domain fiber sensor for measuring quasi-distributed strain along a cantilevered beam is experimentally demonstrated. This is accomplished by segmenting the sensor with air-gap sites, allowing reflections to be monitored. Physically looping these fiber segments many times over their interaction regions is shown to improve the sensitivity of the sensor. Also discussed are techniques to improve sensitivity by using a special tap-off coupler to recirculate optical pulses many times through the sensing region. Important in modeling these sensors is determining the photoelastic coefficient, which accounts for the photoelastic and Poisson effects on a strained fiber. The photoelastic coefficient is theoretically modeled using two methods involving waveguide and ray-optics theory. The results of these analyses are compared to experimentally determined values.