| A study of developing harsh environment multi-parameter fiber Bragg grating sensors using femtosecond IR laser irradiation was conducted. The overall technical objective is to develop an intelligent multi-parameter optical fiber sensor system for real-time temperature, pressure, vibration or chemical concentration monitoring in high temperature (above 1000 ° C) harsh environments.;Femtosecond laser pulses have opened up a new phase in the exploration and applications of optical breakdown, revealing new physical processes and scenarios behind the laser damage of materials on the femtosecond timescale. The fiber gratings inscribed by femtosecond laser illumination have annealing characteristics similar to type II fiber gratings and demonstrate stable operation at much higher temperature than conventional UV laser induced gratings on photosensitive fibers.;In this dissertation, six types of fiber Bragg grating sensors were fabricated via femtosecond laser inscription and their applications were investigated -- single mode silica fiber Bragg grating sensor (SMFBG), multi-mode silica fiber Bragg grating sensor (MMFBG), ultra thin multi-mode silica fiber Bragg grating sensor (ThMMFBG), polarization maintaining single mode silica fiber Bragg grating sensor (PMFBG), ultra thin polarization maintaining silica fiber Bragg grating sensor (ThPMFBG) and multi-mode sapphire single-crystal fiber Bragg grating sensor (SaFBG).;The SMFBG sensor shows good distributed sensing capability and excellent long term thermal stability in high temperature environment (up to 1000 ° C). The novel multi-parameter (temperature, strain, vector bending) sensor based on asymmetric Bragg gratings fabricated in MM silica fiber and PM fiber have more than one resonant dip in the transmission spectrum. Bending and ambient temperature fluctuations affect the shapes of multiple transmission dips in different ways. In bending, different dips have different sensitivities. Temperature fluctuations tended to influence the dips uniformly across different dips. By analyzing the changing spectra of the dips, one can distinguish the changes induced by bending or temperature fluctuations.;THMM and THPM grating sensors are also investigated for liquid pressure sensing applications. After etching away most of the cladding, both MM and PM fiber grating spectra become sensitive to ambient refractive index changes, and their sensitivity to bending and strain is also enhanced. This enables the application for multi-parameter chemical/bio and pressure sensing. Furthermore, an ultrahigh temperature harsh environment sensor with operation temperature higher than 1500 ° C is developed by fabricating higher-order mode rejected fiber Bragg gratings in sapphire crystal fiber. This unique in-fiber grating in single crystal sapphire fiber is very promising in extreme high temperature environmental sensing applications.;The fabrication method and grating writing mechanism is described in Chapter 2, then in Chapters 3 and 4, a detailed discussion about their applications in harsh environments multi-parameter sensing such as temperature, bending, pressure, bio and chemical sensing etc. is conducted. Finally, extremely high temperature sensing using higher-order-mode rejected sapphire single-crystal fiber is demonstrated in Chapter 5. The grating was tested in a high temperature furnace up to 1600 ° C. |
