Fully distributed fiber optic strain sensor based on the Kerr nonlinear optical effect, the photoelastic effect, and counterpropagating optical pulses

• Main Author: Kapteyn, Kelvin Lloyd
• Format: Dissertation
• Language: English
• Subjects: engineering, electronics and electrical; engineering, mechanical; physics, optics

Since the first fiber optic strain gage was described in 1978, a lot of research effort has been applied to the development of various fiber optic strain measurement schemes. Most of the approaches that have been studied so far measure the total strain from one end of the fiber to the other. Other approaches make "quasi-distributed" measurements based on measuring the change in length of several segments of the fiber. The eventual goal of these quasi-distributed systems is to reduce the segment length or the gage length until a continuous strain distribution could be measured.
In this dissertation, a fully distributed fiber optic strain and temperature sensor is developed. This sensor is based on a strong Kerr effect in combination with a greatly increased photoelastic response and short, counterpropagating optical pulses. In combination with much shorter optical pulses, this approach promises great improvements in strain sensitivity and resolution. In addition, this system is capable of separating out strain components in both transverse directions of a polarization maintaining fiber as well as the axial strain component and the temperature at every point along the fiber. The measurement of three dimensional strain data and temperature in a fully distributed sensor is a very significant improvement over any previous system.
The sensor should provide a very powerful tool for crack and flaw detection as well as other applications that need a strain distribution. Primary applications are expected to be nondestructive testing and health monitoring of composite structures by embedding the fiber sensor into the composite during manufacture. Such a sensor would be protected from environmental damage and would provide data on the internal integrity of the structure.