Synthesizing high-quality beams via principal modes in a multimode fiber

Multimode fibers offer exceptional potential for photonic applications, yet their output is typically scrambled into speckle patterns due to modal interference. While transmission-matrix-based wavefront shaping can recover desired fields at the fiber facet, its sensitivity to wavelength variations and the rapid diffraction of output beams in free space remain key limitations. To overcome these challenges, we extend the concept of principal modes of multimode fiber to tailor not only the field at the output plane but also its subsequent evolution in free space. By engineering the input wavefront to excite dispersion-robust modes with specific propagation characteristics, we experimentally demonstrate output beams that retain a well-confined spatial profile over an extended axial range. These modes exhibit significantly improved directional stability compared to conventional principal modes, effectively bridging the gap between complex multimode waveguides and the need for stable, high-quality free-space beams. This work establishes a framework for using multimode fibers as controllable sources of stable, well-confined free-space beams, with direct implications for beam delivery, free-space optical links, and far-field imaging, while also providing a foundation for multidimensional wavefront control in complex photonic systems.