We experimentally investigate the generation of high-order modes (HOMs) with flexible switchable output. Cascaded acoustically induced fiber gratings (AIFGs) act as mode converters. A periodic micro-bending in the AIFG is produced by travelling flexural acoustic wave, and two modes with adjacent azimuthal numbers can be coupled to each other. Different HOMs (LP21a/b and LP11a/b) could be obtained just by tuning the frequency of the radio frequency (RF) driving signal. The dependence of RF frequencies on the two acousto-optic (AO) coupling stages is theoretically analyzed and experimentally verified. Based on cascaded AIFGs mode converter, a multiple-level intensity modulator and an all-fiber Yb-doped laser with flexible mode switching output of HOMs (LP21a/b and LP11a/b) are demonstrated. Moreover, by optimizing the outer diameter of the second part of the AO coupling regions, the mode conversion between LP01 and LP21 can be realized by applying a single frequency on the piezoelectric transducer (PZT).

中文翻译：

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基于声致光纤光栅的多正交高阶模式转换器

我们通过实验研究了具有灵活可切换输出的高阶模式 (HOM) 的生成。级联声诱导光纤光栅 (AIFG) 充当模式转换器。AIFG 中的周期性微弯曲由行进的弯曲声波产生，并且具有相邻方位角数的两种模式可以相互耦合。只需调整射频 (RF) 驱动信号的频率，即可获得不同的 HOM（LP21a/b 和 LP11a/b）。RF 频率对两个声光 (AO) 耦合级的依赖性进行了理论分析和实验验证。展示了基于级联 AIFG 模式转换器的多级强度调制器和具有灵活模式切换输出的 HOM（LP21a/b 和 LP11a/b）的全光纤掺镱激光器。而且，