« Wave chaos » is concerned with the study of waves in closed systems whose geometrical limit presents a chaotic dynamics. The latter is revealed, among other features, by the extreme instability of particular trajectories, namely periodic orbits. How do waves behave in a system whose geometrical limit shows a chaotic dynamics ? Under specific conditions, they can freeze the onset of the chaotic dynamics, and concentrate their energy in the vicinity of the unstable periodic orbits, thus allowing the existence of « scar » modes, which exhibit localized maxima of intensity along these trajectories. The work presented here results from the will to actively control the odes of a chaotic system in order to experimentally enhance the « scar » modes. We introduce a localized gain medium in a multimode optical fiber with a chaotic cross section, previously known as an ideal system for studying wave chaos. The spatial localization of the gain allows the control of the enhancement of a family of «scar» modes, through a selective amplification. We present numerical simulations validating the physical process of amplifica- tion, followed by an experimental study that demonstrates the selective amplification of «scar» modes in a multimode optical fiber.
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