Opt Express. 2023 Feb 27;31(5):7813-7824. doi: 10.1364/OE.482361.
A new type of spatially structured light field carrying orbital angular momentum (OAM) mode with any non-integer topological order, referred to as the spiral fractional vortex beam, is demonstrated using the spiral transformation. Such beams have a spiral intensity distribution and a phase discontinuity in the radial direction, which is completely different from an opening ring of the intensity pattern and an azimuthal phase jump, common features that all previously reported non-integer OAM modes (referred to as the conventional fractional vortex beams) shared. The intriguing properties of a spiral fractional vortex beam are studied both in simulations and experiments in this work. The results show that the spiral intensity distribution will evolve into a focusing annular pattern during its propagation in free space. Furthermore, we propose a novel scheme by superimposing a spiral phase piecewise function on spiral transformation to convert the radial phase jump to the azimuthal phase jump, revealing the connection between the spiral fractional vortex beam and its conventional counterpart, of which OAM modes both share the same non-integer order. Thus this work is expected to inspire opening more paths for leading fractional vortex beams to potential applications in optical information processing and particle manipulation.
一种携带任意非整数拓扑荷轨道角动量（OAM）模式的新型空间结构光场，即螺旋分数涡旋光束，通过螺旋变换得到。这种光束具有螺旋强度分布和径向的相位不连续性，这与之前报道的所有非整数 OAM 模式（称为传统分数涡旋光束）的强度模式的开口环和方位角相位跃变的共同特征完全不同。本文在模拟和实验中研究了螺旋分数涡旋光束的有趣特性。结果表明，在自由空间中传播时，螺旋强度分布将演变成聚焦的环形图案。此外，我们提出了一种通过在螺旋变换上叠加螺旋相位分段函数将径向相位跃变转换为方位角相位跃变的新方案，揭示了螺旋分数涡旋光束与其传统对应物之间的联系，它们的 OAM 模式都具有相同的非整数阶。因此，这项工作有望为分数涡旋光束在光学信息处理和粒子操纵等领域的潜在应用开辟更多的途径。
