Study of Optical Fiber Grating Sensors Fabricated by Femtosecond Laser Pulses

The thesis has investigated some fiber grating sensors fabricated by femtosecond laser pulses.;FBGs are written in H₂-free and H₂-loaded SMFs by femtosecond laser radiation with a phase mask for a high temperature sensor use. Thermal stability of the FBGs are tested by short-term and long-term annealing experiment. Then, temporal thermal response is investigated theoretically and experimentally. A fast testing system is developed by means of periodic CO₂ laser radiation.;FBGs are successfully fabricated in all solid photonic bandgap fiber by femtosecond laser radiation and two reflection peaks are obtained due to dual core-mode guidance. LPGs are also fabricated in such fiber by fs laser with line-scanning method. The coupling from LP₀₁ core mode to LP₁₁ core mode is obtained and the LPG is highly sensitive to tensile strain but nearly insensitive or only slightly sensitive to temperature, curvature, and external refractive index.;FBGs are successfully fabricated in microfiber by femtosecond laser radiation. It demonstrates many propagation modes in the transmission spectrum and the higher-order mode shows a higher refractive index sensitivity. The other refractive index sensor is developed by twisting a pair of microfibers to form a coupler. Transmission spectrum of the microfiber coupler is highly sensitive to surrounding refractive index.;A multi-parameter (refractive index and temperature) fiber sensor is developed by embedding a Mach-Zehnder interferometer in an FBG. The fiber in-line Mach-Zehnder interferometer is fabricated in SMF by femtosecond laser micromachining. The refractive index sensitivity of 9148nm/RIU is obtained between 1.30 and 1.325, and the temperature sensitivity of 12pm/°C is obtained ranging from 30°C to 90°C.