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New Fabrication of Long Period Fiber Gratings Based on Femtosecond Laser and Its Sensing Application

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Tutor: JiaLei; SuiQingMei
School: Shandong University
Course: Detection Technology and Automation
Keywords: Long period fiber grating,femtosecond laser,fiber grating fabricationsensing cha
CLC: TN253
Type: PhD thesis
Year:  2012
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With the optical fiber and photon devices fabrication technology improving continually, fiber grating has become one of the most representative and promising fiber-optic passive components. And it has been widely used in optical communication, optical sensing and other fields.The long-period fiber grating (LPFG) as a new kind of fiber grating has been studied by many researchers all around the world recently, and has got great progresses in a few years. It attracts wildly attention of home and abroad scholars for its advantages such as low insert-loss, broad-band, low back-reflection, sensitive to the changes of environments, low-cost and easy to be fabricated. In addition, designing the new structure, new features, and multi-function photonic devices has become an inevitable trend as the requirements of the fiber Bragg grating functional standard improves. It has shown a very broad application prospects within recent years development.In this dissertation, based on the LPFG inscription technology, coupled-mode theroy and different fiber grating write systems were analysised, an LPFG write system using an800nm laser and point-by-point technology was designed. Using this inscribe system, we inscribed many different LPFGs with different paraments. And, some sensing characteristics, including temperature, axial strain, bending and refractive index characteristics, were analysis and experiments were performed. Then, using chemical corrosion and electrostatic self-assembly method, the long period fiber grating refractive index sensitizing experimental study. Finally, the design of the LPFG and high-birefringence fiber Sagnac interferometer based multi-parameter measurement method was proposed.The main contents of this article include:1. According to three-layer cylindrical fiber model, the fiber mode distribution is analyzed. And the coupling of fiber core-mode and low first-order cladding mode is also further studied. Then using coupling constant describes the strength of mode coupling. Theoretical analysis and numerical simulation of transmission spectral characteristics is a good reference to the fabrication and sensor application of LPFG.2. Main fiber-gratings-inscribe methods were introduced and compared, including interfermeter method, phase mask plate method and point-by-point method, and so on. And the common types of lasers and an800nm femtosecond laser system was introduced in detail. Then a long period fiber gratings inscribed system based on point-by point technology using the800nm femtosecond laser was designed. The functions of the major components were introduced. Using this system, the fiber position could be precisely controlled, and laser pulses could focus precisely on the fiber core using two CCDs and two monitors. Then, many different characteristics LPFGs with different resonance peaks were inscribed in non-hydrogen-load bare fiber. The LPFG is with low road loss and the resonant peak changes in smooth, line width narrow. According to the experiment of monitoring the process of inscription, trend of the resonance peak transmission line is clear, and this system is verified that the alignment of the laser pulse focus is accurate and stable.3. Based of deap study of long period fiber grating sensing characteristics theory, high/low temperature experiments, axial strain experiment and bending experiment was perfomed on the LPFG inscribed with femtosecond laser. It was result from the experiments that in the two temperature range30¡«250¡æand300¡«800¡æ, the linearity of the temperature response curve were maintained above0.99, the temperature characteristics is stable and smooth. The resonance peak of the LPFG changes linearly to longer wavelengths at range of0¡«800¦Ì¦Å with pull, and the response sensitivity of two resonant peak axial strain were0.472¡Á10-3nm/¦Ì¦Å and0.501¡Á10-3nm/¦Ì¦Å, respectively. And the linearity is no less than0.993. The resonance peak of the LPFG decreases with the decrease of the radius of curvature (curvature increases), and the resonance peak drifts to the shortwave direction. But the resonance peak loss peak trends will appear different due to the different direction of bending.4. The refractive index characterstics of LPFG was analyzed, and refractive index sensitivity experiments were performed using concentration of10to60%sucrose solution and a series of organic reagents. It is result from the experiments that the refractive index sensitivity of the LPFG, responses to the more sensitive when the refractive index of the external environment close to the fiber cladding refractive index. Then, refractive index increased sensitivity experiments were performed using of hydrofluoric acid LPFG corrosion and electrostatic self-assembly film of two polymers(PDDA and PSS) on the corrosion of the LPFG. Refractive index characteristics experiment of the sensitized LPFG was performed. It was result that the refractive index sensitivity of LPFG has been improved after sensitizing, and the refractive index of the sensitivity increased about three times. The result showed that after sensitizing, the LPFG measurable range and accuracy of the solution are both increased.5. Based on analysis of high-birefringence fiber Sagnac interferometer, a simultaneous measurement system using LPFG and high birefringence fiber Sagnac interferometer was proposed and experiments were executed with the simultaneous changes of temperature and strain measurement. The result showed that in the system, the system temperature resolution of0.03¡æ, the strain resolution of1.7¦Ì¦Å, and temperature and strain measurement accuracy can reach¡À0.26¡æand¡À17.4¦Ì¦Å, respectively. Compared with other methods, this system maintains a higher measurement accuracy, and has lower the optical path.
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