Fiber Grating Based Devices And Their Sensing Applications

Optical fiber grating sensor has become one of promising sensing technologies. Fiber grating sensors are based on an absolute wavelength measurement, which has many advantages, e.g. immunity to electromagnetic interference, high sensitivity, compact size, wavelength multiplexing capability for sensor networking. They are widely employed in numerous areas, such as seismology, civil structures, aerospace systems, ocean survey, oil and electricity industry, chemical and biological industry, and medical instrument.In this thesis, several novel fiber grating-based devices for sensing applications have been developed and experimentally demonstrated. The main content and some innovative achievements are listed as follows:Firstly, the physical model and analysis approach for fiber gratings are introduced. The spectral properties of fiber Bragg gratings and long period gratings are theoretically calculated and analyzed with the coupled mode theory and transfer matrix method. Then, the photosensitivity mechanism of optical fiber and several photosensitization techniques are depicted in details. Fiber grating fabrication approaches and the fabrication system developed by ourselves are finally introduced.Short-cavity fiber grating laser, which operated in single longitudinal mode, is a promising candidate for high-performance sensing applications. Based on the theoretical analysis of fiber grating laser, we figure out the condition of the grating laser for single mode operation. The short-cavity grating lasers are fabricated with our grating fabrication platform. Then, we use the fiber grating laser to measure temperature, strain, and transverse load. Experimental results show that the fiber laser sensor offers higher sensitivity than normal FBG sensors. The fiber laser can also be utilized as hydrophone to detect high-frequency ultrasound for medical applications. We develop a wet-etching technique to partially etch fiber cladding. The relationship of the cladding diameter and frequency response of the fiber laser hydrophone is experimentally investigated.There are two key issues for fiber grating refractive-index (RI) sensors, i.e. the sensitivity of sensor and the cross sensitivity with temperature. The principle of LPG based refractive index sensor is discussed. A fiber taper seeded LPG interferometer is developed for high sensitively RI measurement. To discriminate RI and temperature cross sensitivity, we demonstrate three new schemes, i.e. the sandwiched LPG structure, the dual-LPG scheme, and the multimode fiber taper interposed dual-FBG structure. The experiments on the simultaneous measurement of RI and temperature are carried out and analyzed. Finally, we introduced and analyzed techniques on signal demodulation of fiber grating sensors for different applications. Two novel signal demodulation schemes are developed. First scheme is based on the fiber grating microwave photonic filter. It is temperature independent and very suitable for strain sensing applications. The second scheme is based on the chirped and apodized fiber grating triangular filter. It is demonstrated to measure dynamic strain monitored by FBG sensors. The experimental results show that the proposed schemes offer good performance.