结构性改变长周期光子晶体光纤光栅成栅机理研究

Research on Transmission Mechanism of the Structurally Changed Long Period Photonic Crystal Fiber Grating

详细分析了光子晶体光纤包层气孔塌缩对光纤传输特性的影响,建立了包层气孔塌缩结构模型,利用有限元法和局域耦合模理论对环形结构改变下形变区域的有效折射率分布和模式耦合系数进行了计算,得到了调制区域各模式的有效折射率和耦合系数分布。研究了基模和包层模的耦合规律,得到了纤芯基模(LP01)和包层模(LP11,LP02)耦合下的传输谱,探讨了此种结构性改变成栅方法的可能性。在此基础上进一步分析了结构性参数如周期长度、周期个数和塌缩深度对传输特性的影响,得到了谐振峰随这些参数变化的规律。研究结果表明,光子晶体光纤包层空气孔塌缩可以形成光栅,调整塌缩参数可以改变所成光栅传输特性。研究结论为结构性改变光子晶体光纤光栅的刻写和应用提供了理论依据。

The influence of holes collapse in photonic crystal fiber cladding on transmission characteristics is analyzed in detail. The structure model of photonic crystal fiber with collapse holes is built. Using finite element method and coupled local-mode theory, the distribution of effective refractive indexes and mode coupling coefficients are calculated in the structural change region, where periodic ringlike collapse is induced. Then the mode effective refractive indexes and the distribution of coupling coefficients in modulation region can be obtained. The coupling rules between the core-mode and the cladding-modes are studied, and the transmission spectrum is obtained, which consists of two peaks formed by the couplings of LP01-LP11 and LP01-LP02 modes. The feasibility of this grating fabrication technique based on structural change is also discussed. On this basis, the influence of the fiber structural parameters on transmission characteristics are further studied involving in the grating period, the number of periods, the depth of cladding collapse. Thus the laws of resonance peaks varying with these parameters can be got. The research results indicate that the periodic holes collapse in photonic crystal fiber cladding can form grating. The transmission characteristics can be tuned by changing the collapse parameters. This result can provide theoretical basis for design and fabrication of structural photonic crystal fiber grating.