多叶应力区扭转光纤的耦合模理论

Coupled-Mode Theory for Spun Multi-Lobe Stress Region Fibers

从线性极化模的耦合模理论出发,分析了双叶应力区扭转光纤,即扭转保偏光纤,和四叶应力区扭转光纤。推导应力引起的在光纤芯区各向异性的介质张量表达式;利用该表达式得到线性极化模在有转角时的多叶应力区光纤中传输的耦合模方程;从而推得多叶应力区扭转光纤中的超本地模,由超本地模的相位常数来确定该光纤的双折射。在双叶、四叶应力区理想对称的条件下,前者可以得到较高的椭圆双折射,而后者则无任何双折射。同样的结论可推广到8叶或16叶的扭转光纤,都无任何双折射。

Based on coupled-mode theory for linearly polarized （LP） modes, spun dual-lobe stress region fibers （i. e. spun linear birefringence fibers） and spun 4-lobe stress region fibers are theoretically analyzed. The expression of the anisotropic dielectric tensor of the additional stress-induced dielectric constant at the core region of the fiber is presented. By the expression, the coupled-mode equations for the linearly polarized modes propagating in multi-lobe stress region fibers with rotation angle are derived. Then, super-local modes in multi-lobe stress region spun fibers can be obtained by using mode transform. Finally, the fiber birefringence is decided from the phase constants of super-local modes. It is found that high elliptical birefringence can be obtained in spun dual-lobe stress region fibers with ideal symmetry, and there is no birefringence in spun 4-lobe stress region fibers if four lobes are perfectly symmetrical. The same result is valid for spun fibers with 8-lobe and 16-lobe stress region, both of which have zero birefringence.