有限包层半径光纤Bragg光栅的理论研究

采用光纤波导三层模型,对有限包层半径光纤Bragg光栅导模有效折射率的改变进行了理论分析,结果表明:当包层直径小于16μm时,单模光纤Bragg光栅(纤芯直径为8.3μm)的导模有效折射率才开始发生明显变化.在包层外添加外包层,通过改变外包层的折射率可以实现对光栅Bragg反射波长的调谐,同时对不同芯子直径的光栅Bragg波长移动进行了数值计算.在保证光纤归一化频率不变的前提下,芯径越小Bragg波长调谐范围越大,当包层厚度为1μm时,芯径为a＝2.2μm的光栅Bragg波长调谐范围约为3.9μm.

Analysis of Shift in Bragg Wavelength of Fiber Bragg Gratings With Finite Cladding Radius

The effective refractive index of the fundamental mode is calculated using three-layer model for single mode fiber Bragg grating (FBG). The changes of effective refractive index for FBG with finite cladding radius are demonstrated. The result shows that appreciable change of effective refractive index for FBG (4.15 μm-core radius) happen below cladding diameter of ～16 μm. The Bragg reflection wavelength for single-mode FBG with thin cladding radius can be tuned by changing the refractive index of the surrounding material called as outer cladding. The numerical analysis of Bragg wavelength shift of FBG with different core radius a=4.15 μm,3.1 μm and 2.2 μm are also presented respectively. On condition that normalized frequency keeps constant, the tunable range of Bragg reflection wavelength increases as the core radius decreases. For the 2.2 μm-core radius FBG with absolute cladding radius of 1 μm, a tunable range of about 1.7 nm can be obtained for the FBG surrounded by outer cladding, of which refractive index changes from 1.33 to n 2. When the refractive index of the outer cladding changes from n 2 to the cut-off point, a tunable range of about 2.2 nm can be also obtained. The whole tunable range is near to 3.9 nm.