Theoretical Research On Fabricating Long Period Fiber Gratings Using Electron Irradiation

Electron irradiation is widely used in the material modifying field. Present, it has also been attempted in manufacturing planar waveguide, optical wavelength division multiplexer with low crosstalk, and some other optical devices. Popular techniques in the long period fiber gratings manufacturing are not adapted to mass producing. Based on the present conditions, the theory of manufacturing the long period fiber gratings by electron irradiation SiO₂ fiber is referred firstly in this dissertation. Considering the theory that electron irradiation could modify the SiO₂ fiber, the technique for mass producing quality fiber gratings is also researched.The dissertation is started with the induction of theories and lab experiences about electron irradiation SiO₂. The function of electron irradiation in SiO₂ material modified is introduced, the threshold of atomic dislocation and the calculation method of electron irradiation depth in samples are both researched, the reason for the refractive index changed by electron irradiation is analyzed, and the calculation and measurement method for the SiO₂ refractive index changed by electron irradiation is provided in this dissertation. On this foundation, considering one SiO₂ molecular as an atomic, the physical model that SiO₂ fiber irradiated by electrons is set up, and the calculation method of electron irradiation depth in fiber is introduced in this essay. Based on the Monte Carlo method, the energy deposition for SiO₂ irradiated by the electrons with energy of 1MeV is calculated and simulated, which includes the two parts respectively promoted by crash and Bremsstrahlung. Some functions for energy deposition are referred, that are the important indexes for describe the physical procedure of SiO₂ irradiated by electrons. It is calculated that the electrons with energy of 0.4474MeV will generate the largest refractive index modulation in the single mode fiber core. To provide the theoretical support for grating mask design, the influence of duty cycle to the grating spectrum is analyzed. The effective refractive indexes of the first twenty first-order cladding mode in fiber are calculated, and its coupling law is analyzed. The law of the harmonic wavelength of harmonic gratings drifted with the increase of refractive index modulation is researched in the following, and the grating duty cycle design demands for manufacturing no addition resonance gratings or sensor gratings is provided in this dissertation. In addition, it is referred that the duty cycle of grating mask cannot be 0.5 for manufacturing long period fiber gratings when using the technique of SiO₂ fiber modified by electron irradiation.The achievements of this dissertation could provide the theoretical supports and consultations for the depth research on the manufacturing long period fiber gratings utilizing the technique of fiber modified by electron irradiation.