Optical Fiber Based Sensors for Harsh Environments

The primary objective of this study is to develop optical fiber-based sensors that are capable of operating in extreme conditions. Silica-based optical fibers are well known for their resilience to harsh environments. Whether they are integrated into distributed sensing schemes or as point sensors, optical fibers offer low-costs, highly accurate sensing platforms for various physical quantities. In this dissertation, state-of–the-art Al-doped radiation sensitive optical fibers for distributed ionizing radiation measurements are presented for the first time. This optical fiber sensor, coupled with a Rayleigh scattering-based optical frequency domain reflectometry (OFDR) scheme, was used to monitor and quantitate ionizing gamma radiation from a 60Co radioactive isotope. An alternative multi-core optical fiber was deployed to simultaneously monitor two different parameters. The dual-core fiber has been fabricated with two distinct optical cores to allow for differences between the cores’ temperature and strain coefficients. With such differences, temperature and strain changes were discriminated using a Brillouin scattering time domain analyzer (B-OTDA). Ultrafast lasers are commonly used to inscribe thermally stable nanostructures on optical fibers’ cores. IR laser-induced structures inscribed on low-loss, radiation-hard silica fibers were used to develop point and distributed sensors for in-pile nuclear reactor measurements. The sensors were subjected to, arguably the most challenging of artificial environments, with temperatures above 600°C, and high neutron fluxes at levels above 1.2×1014 n/s/cm2. The sensors were also used to monitor the temperature distribution inside of a solid oxide fuel cell (SOFC). The information obtained from the operational SOFC can be used to prolong its lifetime and increase its efficiency. Lastly, additive manufacturing embedding of optical fibers into metallic parts were attempted. A nickel-iron alloy, Invar-36, was investigated as a coating material for silica. The coefficient of thermal expansion (CTE) of Invar-36 can be carefully engineered to be close to that of silica. With a reduced CTE mismatch at the glass-metal interface, problems of adhesion and delamination can be deterred to extreme conditions. The proposed sensor designs and implementations would allow monitoring complex structures, and harsh environments like in SOFCs, gas turbines, robotics, or in high performance machinery, with minimal invasiveness