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The Spectral Characteristics and Sensing Application of Long-period Fiber Gratings Fabricated by Fem

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Tutor: SuiQingMei
School: Shandong University
Course: Detection Technology and Automation
Keywords: long-period fiber grating,femtosecond laser,grating inscription,mechanism of ref
CLC: TN253
Type: PhD thesis
Year:  2012
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The long-period fiber grating (LPFG), as one of the important new fiber grating filter, has been widely used in the fiber sensing and communication aspects for its unique characteristics, such as low insertion loss, wide stopband, low backward reflection, high sensitivity, resistant to corrosion and small size. However, the application of LPFGs is restricted by their performances which are mainly related to the fabrication methods. The LPFG fabricated by traditional ultraviolet(UV) exposure method, which utilizes the photosensitivity to induce refractive-index change, has a low temperature stability. The gratings fabricated by CO2laser pulses,which based on releasing of residual stress caused by thermal effect, usually have bigger insertion loss and it is easy to cause fiber deformation when fabricating ultra-long period gratings.The refractive-index change induced by femtosecond (fs) laser is based on multi-photon absorption process. Compared with the traditional fabrication methods, fs laser irradiation can remove the need for fiber photosensitivity at the wavelength of the writing laser, and gratings inscribed by fs laser pulses have superior temperature performances, narrow bandwidth and large refractive index modulation, so they are potentially useful for high temperature sensor. And the point-by-point (PbP) irradiation of an optical fiber by a focused fs laser with suitable intensity is the most popular one used for recording a long-period fiber grating (LPFG).Based on thoroughly studying the fabrication technology of new fiber optic sensors using fs laser pulses, the PbP writing LPFG technology, the spectral features and sensing properties of LPFG fabricated by fs laser were mainly studied in this dissertation. The detailed research contents are as following:1. The research background and significance of this subject were introduced. The fs laser micromachining technology was also introduced. The grating fabrication methods using fs laser pulses were elucidated. The research developments in fabrication of LPFG with PbP method and fabrication of miniature fiber-optic interferometer sensors by fs laser were reviewed and discussed. The sensing characteristics of fiber sensors fabricated by fs laser were summarized at last.2. The interaction mechanism of fs laser and transparent bulk dielectrics were discussed, including the photoionization, avalanche ionization, plasma formation and self-focusing effect. The explain model of refractive index change induced by fs laser pulses were also summarized, including thermal model, color center model, structural change model and microexplosion model.3. The fiber grating non-uniform refractive index modulation and temperature distribution model were established. The spectral changes under different exposure time were simulated, and the effects of fs laser with low pulse energy on fiber Bragg grating were experimentally studied. The experimental results fit well with the theoretically simulated ones.The single point and scan exposure experiments on ultraviolet laser inscribed type ¢ñ fiber Bragg grating by focused infrared fs laser is performed. Experimental results show that the grating spectrum during irradiation will move to the long wavelength direction, and the spectral structure is no longer a single transmission peak but more irregular transmission peaks. However after stopping exposure, the resonant wavelength will show blue shift phenomemon and the transmissivity will increase, which tend to be saturated with the time increasing. Although the laser pulse energy is low, the Gaussian energy distribution of the focused spot lead to the grating exposure region in a non-uniform temperature field. Let the focused laser pulse as a series of heat source, analyzed the effects of the heat source diffusion process on the grating refractive index, given the non-uniform temperature distribution model and the modulation profile model of the refractive index after irradiation in the single-point laser pulse irradiation raster process, theoretically simulated the spectral changes in different exposure time and exposure process, it is in good agreement with experimental results. The results show that in the case of low pulse energy, the grating refractive index change is mainly caused by the focus thermal field.4. One LPFG fabrication system with fs laser pulses was designed, which is based on vertical and horizontal dual-CCD alignments. The system improved the accuracy and efficiency of the laser and fiber alignment. LPFGs were successfully inscribed in single-mode fiber with the absence any sensitizing treatment. These LPFGs have high spectra quality such as low backward reflection loss, strong resonance strength, narrow line width and smooth spectra. The system design principles and inscription process were explained in detail. The effect of grating period, grating length and laser pulse energy on LPFG transmission spectra studied.5. Based on the comprehensive analysis of the refractive-index change profile induced by fs laser, the low amplitude resonant peak phenomenon of grating spectrum was theoretically studied, and the spectral analysis method of LPFG inscribed by fs laser were given. First, according to the three layers fiber structure model, and on the basis of the coupled mode equations, the spectral characteristics of LPFG with cosine refractive index modulation were numerically analyzed. Second, according to the cross-sectional profile and the axial rectangular distribution of refractive-index change induced by fs laser, the spectral characteristics and analysis methods of the LPFG were analyzed for three cases:with uniform refractive index changes in the core, with non-uniform refractive index changes in the transverse, and with mean inscription position deviated from the fiber center.6. The temperature and refractive-index (RI) sensing characteristics of LPFGs fabricated by fs laser are researched. First, the temperature sensitivities of loss-peak wavelength and amplitude with over-coupled and under-coupled gratings were analyzed and measured, including the high temperature properties. The temperature sensitivitis of LPFGs fabricated by fs laser, CO2laser pulses and traditional UV-light exposure were measured. Second, the influencing factors of LPFG RI sensitivity were theoretically discussed. Further solution concentration sensing experiments of sodium chloride and sucrose were performed, The results show that it is feasible to measure the solution concentration with fast response using LPFGs fabricated by fs laser. Finally, three methods to improve refractive sensitivity are proposed.Finally, the main contents and conclusions in this thesis were summarized and the contents for next study were pointed out.
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