When an optical pulse is spatially localized in a highly multimoded waveguide, its energy is typically distributed among a multiplicity of modes, thus giving rise to a speckled transverse spatial profile that undergoes erratic changes with propagation. It has been suggested theoretically that pulsed multimode fields in which each wavelength is locked to an individual mode at a prescribed axial wavenumber will propagate invariantly along the waveguide at a tunable group velocity. In this conception, an initially localized field remains localized along the waveguide. Here, we provide proof-of-principle experimental confirmation for the existence of this class of pulsed guided fields, which we denote “space-time supermodes,” and verify their propagation invariance in a planar waveguide. By superposing up to 21 modes, each assigned to a prescribed wavelength, we construct space-time (ST) supermodes in a 170-µm-thick planar glass waveguide with group indices extending from \approx\! 1$\approx\! 1$ to \approx\! 2$\approx\! 2$. The initial transverse width of the field is 6 µm, and the waveguide length is 9.1 mm, which is \approx\! 257 \times$\approx\! 257 \times$ the associated Rayleigh range. A variety of axially invariant transverse spatial profiles are produced by judicious selection of the modes contributing to the ST supermode, including single-peak and multi-peak fields, dark fields (containing a spatial dip), and even flat uniform intensity profiles.

中文翻译：

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多模波导中的传播不变时空超模

当光脉冲空间定位在高度多模波导中时，其能量通常分布在多种模式中，因此会产生斑点横向空间分布，该分布会随着传播而发生不稳定的变化。理论上已经提出，脉冲多模场，其中每个波长被锁定到指定轴向波数的单个模式，将以可调谐的群速度沿波导不变地传播。在这个概念中，初始局部场沿波导保持局部。在这里，我们为这类脉冲引导场的存在提供了原理验证实验确认，我们将其称为“时空超模”，并验证它们在平面波导中的传播不变性。通过叠加多达 21 种模式，\大约\！1$\approx\! 1$到\约\！2$\approx\! 2$ . 场的初始横向宽度为 6 µm，波导长度为 9.1 mm， \约\！257 倍$\approx\! 257 \times$相关的瑞利范围。通过明智地选择有助于 ST 超模的模式产生各种轴向不变的横向空间分布，包括单峰和多峰场、暗场（包含空间倾角），甚至是平坦的均匀强度分布。