We report the first, to the best of our knowledge, observation of cross-phase modulational instability (XPMI) of circularly polarized helical Bloch modes carrying optical vortices in a twisted photonic crystal fiber with a three-fold symmetric core, formed by spinning the fiber preform during the draw. When the fiber is pumped by a superposition of left-circular polarization (LCP) and right-circular polarization (RCP) modes, a pair of orthogonal circularly polarized sidebands of opposite topological charge is generated. When, on the other hand, a pure LCP (or RCP) mode is launched, the XPMI gain is zero, and no sidebands are seen. This observation has not been seen before in any system and is unique to chiral structures with $ N $-fold rotational symmetry. The polarization state and topological charge of the generated sidebands are measured. By decomposing the helical Bloch modes into their azimuthal harmonics, we are able to deduce the selection rules for the appearance of modulational instability sidebands. We showed that the four waves in the nonlinear mixing process must exhibit the same set of azimuthal harmonic orders.

中文翻译：

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手性三芯光子晶体光纤中带有光学涡旋的圆偏振螺旋Bloch模的交叉相位调制不稳定性

据我们所知，我们首先报道了在具有三重对称纤芯的扭曲光子晶体光纤中通过光学涡旋在圆偏振螺旋布洛赫模中携带光学涡旋的交叉相位调制不稳定性（XPMI）的观察结果在抽奖期间进行预成型。当通过左圆偏振（LCP）和右圆偏振（RCP）模式的叠加泵浦光纤时，会生成一对具有相反拓扑电荷的正交圆偏振边带。另一方面，当启动纯LCP（或RCP）模式时，XPMI增益为零，并且看不到边带。这种观察以前从未在任何系统中见过，并且对于具有$ N $的手性结构是唯一的倍旋转对称性。测量所产生的边带的极化状态和拓扑电荷。通过将螺旋Bloch模式分解为它们的方位谐波，我们可以得出调制不稳定边带出现的选择规则。我们表明，非线性混合过程中的四个波必须表现出相同的方位谐波阶数集。