|Title:||Study of optical fiber grating sensors fabricated by femtosecond laser pulses|
|Subject:||Optical fiber detectors.|
Laser pulses, Ultrashort.
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Electrical Engineering|
|Pages:||xiv, 132 leaves : ill. ; 30 cm.|
|Abstract:||This thesis investigates the optical fiber grating sensors fabricated by use of femtosecond (fs) laser pulse irradiation. Three types of optical fibers have been used in the investigation, i.e. single mode fiber (SMF), photonic crystal fiber (PCF) and micro and nano optical fiber (MNF). For a high temperature sensor use, fiber Bragg gratings (FBGs) written in H2-free and H2-loaded SMF by use of IR fs radiation with a phase mask have been presented. Type II-IR FBGs fabricated in H2-free and H2-loaded SMFs can both sustain long-term annealing at 1000°C while their high reflectivity is maintained. Temporal thermal response of the FBGs has been investigated theoretically and experimentally. A fast testing system is developed to measure the thermal response time by means of periodic CO₂ laser irradiation. Temporal thermal response is independent of the heating power and the heating direction. The measured value of the temporal thermal response is ~240ms for heating and ~280ms for cooling. The simulation result based on lumped system equation is in good agreement with the experimental data. All solid photonic bandgap fiber (AS-PBGF) is an optical fiber that confines the light to the fiber core by the photonic bandgap of the cladding material. FBGs have been successfully inscribed in such fiber by IR fs radiation and two reflection peaks are obtained due to dual core-mode guidance. Long period fiber gratings (LPGs) have also been inscribed in this fiber by use of fs laser with line-scanning method. The coupling from fundamental core mode to LP₁₁ core mode is readily obtained and the resonant wavelength is highly sensitive to tensile strain but nearly insensitive or only slightly sensitive to temperature, curvature, and external RI. FBG has been successfully fabricated in microfiber (MF) by IR fs radiation. The FBG can be directly exposed to the surrounding medium without etching treatment and shows a high RI sensitivity. It demonstrates a number of propagation modes in the transmission spectrum and higher-order mode shows a higher sensitivity. Another RI sensor has been developed by twisting a pair of MFs to form a coupler. Transmission spectrum of the device is highly sensitive to surrounding RI. The highest sensitivity obtained is 2377nm/RIU at 1.3680 for 4.2μm-diameter MFs and sensitivity is increased up to 2735nm/RIU for a smaller fiber diameter of ~2.8μm. A multi-parameter (RI and temperature) fiber sensor has been developed by embedding a Mach-Zehnder interferometer (MZI) in an FBG. The compact fiber in-line MZI is fabricated in SMF by IR fs laser ablation. The RI sensitivity of 9148nm/RIU is obtained between 1.30 and 1.325, and temperature sensitivity achieved is 12pm/°C ranging from 30 to 90°C.|
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