Research on mode coupling properties based on gratings in GRIN MMF
Date of Issue2014
School of Electrical and Electronic Engineering
This dissertation mainly studies the fiber gratings in GRIN MMF. Fabrications, mode coupling analysis and applications are presented and concluded among the titled fiber Bragg Grating and long period grating. This article focuses on the LPG in GRIN MMF using C02 laser fabrication method and TFBG using UV laser method. The method using C02 laser is conducted by deforming the fiber with laser beams. Strong mode coupling will appear when the period of grating is near to the critical spatial period. Modes distributions and properties in the GRIN MMF are analyzed, and the mode coupling theory is also presented in order to investigate the modes coupling theoretically. Besides, TFBG in GRIN MMF is fabricated by titling the phase mask of normal FBG. Theoretical analysis and spectrum properties are also simulated in this dissertation. Experiments are also conducted to support the theoretical conclusions. In this dissertation, high power laser and long exposing time are used to make long period grating. Due to the conclusions, there will not be an obvious band gap in the spectrum of the output, even though in other reports power changes have been reported to support the obvious mode coupling. By contrast, experiment on TFBG in GRIN MMF has significant spectrum responses which match the simulation conclusions. A new idea is proposed to increase the performance of LPG fabrication, and a band gap emerges but the resonant wavelength shifts. Further analysis about the new fabrication should be studied, and the new idea is significant for manufacturing long period gating in GRIN MMF in the future.