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讨论了麦克斯韦方程在各向同性均匀电介质中的电磁对偶变换不变性 ,定义了电磁混合比 ,求出了对偶场的充要条件 .把上述不变性和充要条件应用于阶跃光纤 ,分析其导波模式 ,证明在非轴对称情况下 ,阶跃光纤中不存在具有色散的受导简正模 .存在的解只有临界折射模 (n =n2 时的特解 ) .求出了它的分立频谱 .受导简正模只存在于轴对称情况中 .以上结论与实验事实不矛盾 ,并由此解释了基模的特性
The electromagnetic duality invariance of Maxwell equation in isotropic homogeneous dielectric is discussed, the electromagnetic mixing ratio is defined, and the necessary and sufficient conditions of dual field are obtained. The invariance and necessary and sufficient conditions are applied to the step fiber, Guided wave mode, it is proved that there is no guided simple normal mode with dispersion in non-axisymmetric case, and there exists only the critical refraction mode (special solution for n = n2). Its discrete spectrum is obtained. The guided normal mode only exists in the axisymmetric case.The above conclusion is not contradictory to the experimental fact, and thus explains the characteristics of the fundamental mode