Shi, Guannan2024-10-112024-10-112024-10-09vt_gsexam:41500https://hdl.handle.net/10919/121326Optical fiber sensors are widely applied in modern sensing systems. Taking advantage of the high sensitivity of optical interference, optical coherence fiber sensors, such as fiber Bragg gratings (FBGs) and Fabry-Perot interferometers (FPIs), have been investigated intensively and utilized broadly in optical sensing systems. Multimode fibers (MMFs) offer low coupling loss, high compatibility with various light sources, and insensitivity to ambient fluctuations, which are preferred for reliable and low-cost sensing systems. Therefore, the combination of optical coherence sensors and MMFs have facilitated and will continue to contribute to various optical sensor designs with desirable performances. This dissertation addresses the design and construction of optical coherence sensors in multimode fibers and presents several fully multimode fiber sensing systems with low coherence light source. Moreover, a theoretical analysis of fiber mode excitations, model coupling, and multimodal interference is conducted, and a numerical model is constructed to study the behaviors of optical coherence sensors in MMFs. With the femtosecond laser point-by-point inscription method, parallel fiber Bragg gratings (pFBGs), scattering array interferometers (SAIs), and densely multiplexable scattering array interferometers (DMSAIs) in sapphire fibers are proposed, fabricated and characterized, achieving excellent performance in multiplexed high temperature sensing. The study on SAI signals also revealed the unique coherence properties of MMFs. This work points out that the coherence properties in MMFs play a significant role in affecting the performances of optical coherence sensors, and such properties are closely related to both geometrical and optical properties of the fibers. This work also presents both theoretical and experimental tools to explore such properties and predict and test the performance of optical coherence sensors in MMFs, which is of great significance in the applications of such sensors in the real world.ETDenIn CopyrightOptical fiber sensorsoptical coherencemultimode fiberssapphire fibersDissertation