Shaping the light amplified in a multimode fiber

Propagation of light in multimode optical fibers usually gives a spatial and temporal randomization of the transmitted field similar to the propagation through scattering media. Randomization still applies when scattering or multimode propagation occurs in gain media. We demonstrate that appropriate structuration of the input beam wavefront can shape the light amplified by a rare-earth-doped multimode fiber. Profiling of the wavefront was achieved by a deformable mirror in combination with an iterative optimization process. We present experimental results and simulations showing the shaping of a single sharp spot at different places in the output cross-section of an ytterbium-doped fiber amplifier. Cleaning and narrowing of the amplifier far-field pattern was realized as well. Tailoring the wavefront to shape the amplified light can also serve to improve the effective gain. The shaping approach still works under gain saturation, showing the robustness of the method. Modeling and experiments attest that the shaping is effective even with a highly multimode fiber amplifier carrying up to 127 modes. A technique that controls light propagation in rare earth-doped fiber optic cables makes it easier to peek inside disordered materials. Multimode optical fibers have large diametric cores that enable parallel transmission of multiple communication channels. Alain Barthélémy and colleagues at the Université de Limoges-Centre National de la Recherche Scientifique in France have used wave front shaping to turn multimode fibers into amplified imaging devices. Typically, random speckle patterns are seen when laser light scatters through large-core fiber optic cables. The French team used deformable mirrors and an iterative optimization process to modulate the phases of incoming light wave fronts so that the beam focused on a small, single spot. This approach eliminates the need for reference beams and preserves the amplification characteristics of doped fiber optic cables—a useful combination for imaging inside opaque, light-scattering substances.