| Summary: | 碩士 === 國立雲林科技大學 === 電子工程系 === 105 === In this research, we investigate the thermal effect occurring in optical fibers which are used for high-power fiber lasers. We use the heat transfer equation and the finite element method (FEM) to simulate the behaviors of the index-guided (IG) fiber and index-antiguided (IAG) fiber having step refractive-index-distributions. Characteristics of the fundamental mode (FM) and the higher order modes (HOMs), including the effective index and the effective mode-field diameter (MFD), are investigated under different heat loads.
From numerical simulation, the FM intensity distribution of the IG fiber increases into the core region when the heat load increase, and we analyzed by using thermal lens parameters. It is found that the FM of the IAG fiber will transfer from stage to stage when heat load of the fiber increases. There are three stages of the FM, i.e., the antiguided stage, the quasi-guided stage, and the guided stage. When the IAG fiber is operated in the guided stage, the thermally guided effect resulted from the high heat load will dominate over the antiguided effect arose from the refractive-index-profile of the IAG fiber, which makes the optical power confining in the fiber core. The single core-confined mode region should be in the thermal load range from the FM into the guided stage to the HOMs into the core guided stage .In addition, for fiber lasers, the concentration of the rare earth element, doped in the fiber core region to provide optical gain, will affect the refractive index of the core. In this research, we also investigate the refractive index difference becomes larger, the heat load into the guided stage and the single core-confine region will increase.
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