Apodization Phase Shift Of The Design And Production Of Long Period Fiber Grating Devices

Fiber grating has developed into a critical component for many applications in optical communication and sensor systems. They have the advantages of all-fiber geometry, low insertion loss, high return loss or extinction, and potentially low cost. But the most distinguishing feature is the flexibility for achieving sophisticated spectral characteristics through the control of the grating parameters such as induced index change, length, period chirp, phase shift and so on. However, duo to the limitations of present writing methods, long-period fiber grating with special grating profiles, which is crucial in fiber communication and sensing, has not been studied extensively both in theory and in experiment.This dissertation first analyzes the transmission spectrum of apodized phase-shifted long-period fiber gratings by solving a coupled mode equation system using transfer matrix method. The simulations undertaken by MATLAB demonstrate that only a small number of uniform sections are sufficient to approximate an ideal apodization function for suppressing the ripples in a uniform grating and in a phase-shifted grating. The effect of index apodization is also compared to that of length apodization. The simulations pave a way for the next-step experimental research.The computer-controlled CO2-laser writing method allows the position of the writing beam to be computer-programmed to produce the required grating profile without using any mask. Besides, CO2-laser written gratings have a much higher thermal stability than UV-written gratings. However, because of the strong absorption of the CO2-laser radiation in the fiber cladding, exposure to intense CO2-laser radiation from one side of the conventional single-mode fiber can induce an additional, asymmetric index change across the cladding area, which allows light coupling to both axially and non-axially symmetric cladding modes and thus leads to a more complex transmission spectrum.The use of a heavily doped fiber allows the grating to be formed in the fiber core with one-side exposure at a low CO2-laser energy density. Such a grating couples light only to axially symmetric cladding modes and thus generates a clean transmission spectrum with negligible polarization dependence. We also insert a beam splitter to tap 10% of the output radiation from the laser, so that the actual power used keeps small and stable. Our writing method facilitates the fabrication of practical long-period fiber grating based devices.Based on the simulations, apodized phase-shifted long-period fiber gratings are fabricated for the first time by applying the computer-controlled CO2-laser writing method. The results presented in this dissertation provide some first experiment verifications of the theoretical predictions and demonstrate the flexibility and the controllability of the method by using a germanium-boron-codoped fiber, which surely has laid a solid foundation for further research on long-period fiber gratings with complicate grating profiles.