In-Line Long Period Grating and Brillouin Scattering Fiber Optic Sensors for Strain, Temperature, Chloride Concentration, and Steel Mass Loss Measurement in Bridge Applications

Corrosion is the main reason for costly maintenance of aging transportation infrastructure in the U.S. Since 2008, the PI’s group has developed long period fiber grating (LPFG) sensors for point strain and steel mass loss measurements. When attached on a steel bar, a LPFG sensor doped with nano iron/silica particles and polyurethane can monitor the corrosion process of steel. However, the coating of particles with polyurethane was not robust. In addition, chloride concentration is important for the prediction of early corrosion in practice. Compared to grating sensors, Brillouin scattering based sensors have lower spatial resolution but offer a cost-effective solution to the monitoring of large-scale civil infrastructure. Therefore, integrating LPFG sensors into a distributed sensing system for multiple parameter measurements is important in bridge applications. Unlike fiber Bragg grating (FBG) sensors that have been recently applied to civil infrastructure, LPFG sensors and distributed sensing systems are still tested in laboratory. Their packaging is critical in field applications. This project aims to: (1) Develop a physically and optically protected LPFG strain sensor that is hermetically packaged in a fused silica capillary tube, (2) Develop a Fe-C coated LPFG sensor for life-cycle corrosion monitoring (chloride ion and mass loss) of nearby steel members, (3) Understand how many LPFG sensors of different types and wavelengths can be multiplexed to measure multiple parameters for the monitoring of large-scale bridges, and (4) Understand potential interference between the LPFG sensor interrogation and the pulse pre-pump Brillouin optical time domain analysis (PPP-BOTDA) measurement