Self-Repairing Polymer Optical Fiber Strain Sensor

This research develops a self-repairing polymer optical fiber strain sensor for structural health monitoring applications where the sensor network must survive under extreme conditions. Inspired by recent research in self-healing material systems, this dissertation demonstrates a self-repairing strain sensor waveguide, created by self-writing in a photopolymerizable resin system.;In an initial configuration, the waveguide sensor was fabricated between two multi-mode (MM) optical fibers via ultraviolet (UV) lightwaves in the UV curable resin and operated as a strain sensor by interrogation of the infrared (IR) power transmission through the waveguide. After failure of the sensor occurred due to loading, the waveguide re-bridged the gap between the two optical fibers through the UV resin. The response of the waveguide sensors was sensitive to the applied strain and repeatable during multiple loading cycles with low observed hysteresis, however was not always monotonic. The strain response of the original sensor and the self-repaired sensor showed similar behaviors. Packaging the sensor in a polymer capillary improved the performance of the sensor by removing previous “no-response” zones. The resulting sensor output was monotonic throughout the measurement range. The hysteresis in the sensor behavior between multiple loading cycles was also significantly reduced. However, a jump in sensor output voltage was observed after the sensor self-repair process, which presents challenges for calibration of the sensor.;The sensor configuration was modified to a Fabry-Pérot interferometer to improve the sensor response. The measurable strain range was extended through multiple sensor self-repairs, and strain measurements were demonstrated up to 150% applied tensile strain. A hybrid sensor was fabricated by splicing a short segment of MM optical fiber to the input single-mode (SM) optical fiber. The hybrid sensor provided the high quality of waveguide fabrication previously demonstrated through self-writing between MM optical fibers with the high fringe visibility of SM propagation. The peak frequency shift of the reflected spectrum Fabry-Pérot sensor was extremely linear with applied strain for the hybrid sensor, with a sensitivity of 2.3 x 10 -3 per nm per % strain. The calibrated peak frequency shift with applied strain was the same for both the original sensor and the repaired sensor, therefore the fact that the sensor has self-repaired does not need to be known. Additionally, this calibration was the same between multiple sensor fabrications. In contrast to a conventional air gap Fabry-Pérot cavity sensor, no decrease in the fringe visibility was observed over the measurable strain range.