Writing Mechanism And Theoretical Analysis Of Novel Long-Period Fiber Gratings

In this thesis, some theories of novel long-period fiber gratings (LPFGs) inducedby focused high-frequency CO₂ laser pulses are proposed. The main work is provided asfollows:1. The mechanism of LPFG's formation written by high-frequency CO₂ laserpulses was studied. When fiber was formed after pulling, there will be residual thermalstress remained due to different material in different layers, elastic and inelastic strainwill also cause mechanical stress remained in fiber. Because the material of fiber isalmost SiO₂,when exposure with high-frequency CO₂ laser, the vibration of SiO₂ willcause CO₂ laser's absorption, then in the exposure region, the temperature will rise andcause residual mechanical stress's release. So when stress change in this region, therefractive index will change because of optielastic effect, so at last periodical refractiveindex will be formed along fiber, then the LPFG will be formed.2. The modes coupling theory of LPFG written by CO₂ laser pulses was studied.Because the novel LPFG was written by CO₂ laser on one side, so the change ofrefractive index will be asymmetric on the transverse section of LPFG, i.e. the changeon the upper side will be larger than the change in the lower side. In this thesis, we willdeduce the modes coupling theory from the primary Maxwell equation. At differentpoison on the fiber's transverse section, a Fourier transformation will be used to deducethe refractive index's average change, the first order resonant index change, the higherorder resonant index change, we find the refractive index's average change will causedifferent resonant wavelength at different polarization state, while the introduce ofhigher order resonant index change will cause a resonant wavelength's slightly changefrom that of sinusoidal index modulation, so the new theory will be more accurate.3. Use the new coupling theory, we can explain some new characteristics of LPFG,such as the bending orientation relativity; When bending LPFG written by highfrequency CO₂ laser, we can think there is a pre-bending loaded on an ideal LPFG, thepre-bending and the bending out world put on it together cause the happen of bendingorientation relativity. The theory can also be used to analyze the bending orientationrelativity of LPFG written by bending insensitive fiber. Also, we can use a pre-load onthe transverse section of LPFG to explain then transverse loading orientation relativity.