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Study On Characteristics Of Cascaded Long Period Fiber Gratings With Rotary Refractive Index Modulation

Posted on:2012-06-14Degree:MasterType:Thesis
Country:ChinaCandidate:Y E FanFull Text:PDF
GTID:2178330338497372Subject:Optical Engineering
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In the past several years, the mechanism and the characteristics of the uniform long period fiber gratings (LPFGs) have been studied extensively. And, their applications have gained prominent achievements in the field of fiber sensing and communication. However, the simple transmission spectrum is gradually becoming the bottleneck for the further development for this kind of gratings. Novel LPFGs with unique transmission spectrum should be proposed to satisfy further the application requirements. Recently, some non-uniform fiber gratings with different transmission spectrum were developed, such as the phase-shifted grating, the chirp gratings, the Gaussian gratings, the apodized gratings, the super-structure gratings, the titled gratings, the beating gratings and so on. In this thesis, two special grating structures combined the beating and phase-shift and cascaded mechanisms are proposed and analyzed both experimentally and theoretically. Such kind of gratings with unique optical characteristics will find potential applications in the field of fiber sensing and communication. The main work is as follows:1. At first, the coupled-mode theory and the transmission matrix method are introduced briefly. Then, the similar theoretical methods based on coupled-mode theory and the transmission matrix method are used to analyze long period moirégrating, the super-structure grating and the beating grating. At last, the uniform LPFGs, the phase-shifted gratings, and the cascaded LPFGs are modeled and calculated. The optimized parameters are also gotten for the reference of the later experiments.2. Phase-shifted beating grating fabricated by writing phase-shifted LPFG (PS-LPFG) in twisted SMF by high frequency CO2 laser pulse is investigated, which is also called PS-LPFG(R-PS-LPFG). (1) R-PS-LPFGs with different phase shiftφand twisting periodΛT are fabricated. (2) For R-PS-LPFGs with the phase shiftφ=πand the twisting periodΛT≤24mm, both beating effect and phase-shift effect act on the grating when the fiber is released, producing four loss peaks. (3)The characteristics of the temperature, the axial strain and the polarization dependent loss (PDL) for R-PS-LPFG withφ=π, the twisting periodΛT=24mm are tested experimentally and analyzed theoretically.3. Two LPFGs with rotary refractive index modulation (RLPFG) are cascaded to form Mach-Zehnder interferometer (RLPFG-MZI). (1) The optical characteristics and the sensing principle for ambient refractive index (RI) measurement of RLPFG-MZI are analyzed theoretically. (2) Different fabrication parameters are analyzed and tested experimentally, which provides reference for further experiments. (3) The RI experimental results show that, the wavelength shift of such simple structure sensor exhibits good linear response in the RI range from 1.3344 to 1.3637. Especially, for RLPFG-MZI with rotary angle CG=130°, the RI sensitivity can be as high as 14.7pm/mm (0.01RI, with interaction length of 40mm), which is nearly 3.5 times higher than that of MZI formed by LPFG written by high frequency CO2 laser pulse in SMF (4.2pm/mm,0.01RI, with interaction length of 40mm).In conclusion, the characteristics of cascaded LPFGs with rotary refractive index modulation are investigated. It has further branched out the structure of fiber grating, and provided several alternative devices to the fiber sensing and communication field, such as CWDM, the wavelength controllable coupler, and the tunable fiber ring laser, etc. It has great academic significance and social value.
Keywords/Search Tags:Fiber Optics, Optical Fiber Gratings, High-frequency CO2 Laser, Environmental Response Characteristics, Refractive Index sensing
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