Study On Novel Fiber Grating Refractive Index Sensors Based On Optical Heating

Optical fiber grating is a kind of important passive optical fiber device, it is of much importance in the optical fiber communication and sensing applications. Considering its advantages, such as compact size, high sensitivity, resistance to electromagnetic interference and capability of multiplexing and remote monitoring, and existing deficiencies of optical fiber grating sensing technology, this thesis focuses on the study of the research of novel refractive index sensors based on fiber gratings, proposing a temperature-insensitive refractive index sensor and two reflective refractive index sensors with high sensitivities.Firstly, the thesis briefly introduces the background and significance of optical fiber grating research, and then introduces the development of optical fiber grating and its applications in optical fiber communication and sensing systems in detail. Then the main content and innovation of this thesis are presented. After that index modulation distributions and spectral characteristics of various types of fiber gratings are summarized, and according to the grating diffraction and coupled-mode theories combined with the structural model, fiber gratings are basically analyzed.The thesis has proposed a novel temperature-insensitive fiber-optic refractive index sensor based on optically heating. According to the loss characteristics and mechanism of the cobalt-doped fiber, we have developed a temperature-insensitive refractive index sensor based on combining the cobalt-doped fiber with the long period grating and fiber Bragg grating. Through the generated heat by the non-radiative transition of the cobalt ion in the cobalt-doped fiber under laser pumping, the sensing unit can be thermally compensated to achieve the temperature-insensitive refractive index sensing. Experimental investigation of the refractive index sensing characteristic, thermally compensation characteristic and wavelength adjustment characteristic of the refractive index sensor, has been conducted. The experimental results demonstrate that the sensor can simultaneously achieve the refractive index sensing and temperature compensation.Furthermore, two desired reflective refractive index sensors base on the optical fiber gratings have been introduced. One of them is the wavelength-coded refractive index sensor based on the cobalt-doped optical fiber and fiber gratings. The other is the power-coded refractive index sensor based on the combination of thin-core fiber and the fiber grating. The former one utilizes the shift of the LPG’s resonant wavelength caused by the change of external refractive index to modify the power of pump laser for heating, which results in the Bragg wavelength shift of the fiber Bragg grating inscribed in the cobalt-doped fiber. One can deduce the surrounding refractive index change by monitoring the reflective Bragg wavelength shift. The latter one achieves the external refractive index sensing through monitoring the power of the cladding mode, which is induced by the core diameter mismatch between the single-mode fiber and thin-core fiber and reflected back by the fiber Bragg grating in the cobalt-doped fiber. Meanwhile, the core mode, which is not sensitive to external refractive index, is also reflected by the fiber Bragg grating. Thus the core mode reflection can be used as the reference to eliminate the sensing deviation caused by light source fluctuation. The theoretical analysis and experimental results are detailedly introduced in the thesis. And the experimental results have demonstrated such two sensors have many advantages, such as high sensitivity, compact structure, low cost and high practicability.