Multimode optical fibers are essential in bridging the gap between nonlinear optics in bulk media and single-mode fibers. The understanding of the transition between the two fields remains complex due to intermodal nonlinear processes and spatiotemporal couplings, e.g., some striking phenomena observed in bulk media with ultrashort pulses have not yet been unveiled in such waveguides. Here we generalize the concept of conical waves described in bulk media towards structured media, such as multimode optical fibers, in which only a discrete and finite number of modes can propagate. Such propagation-invariant optical wave packets can be linearly generated, in the limit of superposed monochromatic fields, by shaping their spatiotemporal spectrum, whatever the dispersion regime and waveguide geometry. Moreover, they can also spontaneously emerge when a rather intense short pulse propagates nonlinearly in a multimode waveguide, their finite energy is also associated with temporal dispersion. The modal distribution of optical fibers then provides a discretization of conical emission (e.g., discretized $X$ waves). Future experiments in multimode fibers could reveal different forms of dispersion-engineered conical emission and supercontinuum light bullets.

中文翻译：

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多模光纤中的离散圆锥波

多模光纤对于弥合散装介质中的非线性光学器件和单模光纤之间的差距至关重要。由于模态非线性过程和时空耦合，对两个场之间跃迁的理解仍然很复杂，例如，在超短脉冲的体相介质中观察到的一些撞击现象尚未在此类波导中揭示出来。在这里，我们将散装介质中描述的圆锥波的概念推广到结构化介质（例如多模光纤），其中仅离散模式和有限数量的模式可以传播。无论色散范围和波导几何形状如何，都可以通过对它们的时空频谱进行整形，在叠加的单色场的范围内线性生成此类传播不变的光波包。此外，当相当强的短脉冲在多模波导中非线性传播时，它们也可以自发出现，它们的有限能量也与时间色散有关。光纤的模态分布然后提供圆锥形发射的离散化（例如，离散化）。$X$波浪）。未来在多模光纤中进行的实验可能会揭示色散工程设计的锥形发射和超连续谱子弹的不同形式。