Edavalath N N, Günendi M C, Beravat R, Wong G K L, Frosz M H, Ménard J-M, St J Russell P
Opt Lett. 2017 Jun 1;42(11):2074-2077. doi: 10.1364/OL.42.002074.
A hollow-core single-ring photonic crystal fiber (SR-PCF) consists of a ring of capillaries arranged around a central hollow core. Spinning the preform during drawing introduces a continuous helical twist, offering a novel means of controlling the modal properties of hollow-core SR-PCF. For example, twisting geometrically increases the effective axial propagation constant of the LP-like modes of the capillaries, providing a means of optimizing the suppression of HOMs, which occurs when the LP-like core mode phase-matches to the LP-like modes of the surrounding capillaries. (In a straight fiber, optimum suppression occurs for a capillary-to-core diameter ratio d/D=0.682.) Twisting also introduces circular birefringence (to be studied in a future Letter) and has a remarkable effect on the transverse intensity profiles of the higher-order core modes, forcing the two-lobed LP-like mode in the untwisted fiber to become three-fold symmetric in the twisted case. These phenomena are explored by means of extensive numerical modeling, an analytical model, and a series of experiments. Prism-assisted side-coupling is used to measure the losses, refractive indices, and near-field patterns of individual fiber modes in both the straight and twisted cases.
空心单环光子晶体光纤（SR-PCF）由围绕中心空心芯排列的一圈毛细管组成。拉丝过程中对预制棒进行旋转会引入连续的螺旋扭曲，这提供了一种控制空心SR-PCF模式特性的新方法。例如，扭曲在几何上增加了毛细管类LP模式的有效轴向传播常数，提供了一种优化高阶模（HOMs）抑制的方法，当类LP芯模与周围毛细管的类LP模式相位匹配时就会发生高阶模抑制。（在直光纤中，当毛细管与芯直径比d/D = 0.682时会出现最佳抑制。）扭曲还会引入圆双折射（将在未来的一篇快报中进行研究），并且对高阶芯模的横向强度分布有显著影响，使得未扭曲光纤中的双叶类LP模式在扭曲情况下变为三重对称。通过广泛的数值建模、解析模型和一系列实验对这些现象进行了研究。在直光纤和扭曲光纤两种情况下，都使用棱镜辅助侧面耦合来测量各个光纤模式的损耗、折射率和近场图案。
