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Nonlinear Absorption in Waveguides
Armand Rosenberg and James S. Shirk

We model the transmission of light in a nonlinear multimode waveguide with a core size compatible with imaging optical limiters, where the nonlinearity is assumed to be associated with a two-photon absorption. We compare the propagation of both individual spatial modes and Gaussian beams, in both the linear (low-energy) and nonlinear (high energy) regimes. Due to the discrete nature of the set of allowed spatial modes in these waveguides, the propagating light does not uniformly fill the waveguide core under the conditions investigated. Gaussian beams incident from free space propagate as a superposition of allowed modes that maintain a local variation in intensity across the core area during propagation. The higher local intensities of higher-order waveguide modes lead to increased nonlinear absorption. When the incident beam deviates significantly from the fundamental waveguide mode, high-order modes are excited and the waveguide’s nonlinear transmission decreases. These results help to account for the enhanced nonlinear response previously observed in capillary waveguides filled with a nonlinear material in solution.

Keywords: Nonlinear waveguide, optical limiting.

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