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基于耦合模理论,提出一种判别圆双折射光纤的简易方法。首先指出在单模工作条件下的线双折射光纤中,一对互相正交的线偏振模间耦合系数为虚数,而在圆双折射光纤中,耦合系数为实数。从上述事实出发并根据单模光纤横截面上折射率分布的对称性,确定了光纤轴向倒置后几何形状不变是圆双折射光纤的三个必要条件之一。利用这种判别方法,可以确定对互易光纤而言,只有扭转圆光纤和熔融状态下制作的螺旋光纤才是真的圆双折射光纤。
Based on the coupled-mode theory, a simple method of discriminating circular birefringent fibers is proposed. First of all, it is pointed out that in a birefringent fiber under single-mode operation, a pair of linearly polarized modes with mutually orthogonal coupling coefficients are imaginary numbers, whereas in a circular birefringent fiber, the coupling coefficients are real numbers. Based on the above facts and according to the symmetry of the refractive index distribution in the cross section of the single mode fiber, one of the three necessary conditions for a circular birefringent fiber is determined that the geometric shape of the fiber is inverted after axial inversion. Using this method of discrimination, it can be determined that for a reciprocal fiber, only the circular fiber and the spiral fiber made in a molten state are true circular birefringent fibers.