We present a novel approach to the analysis of a full model of scalar modulation instability (MI) by means of a simple geometrical description in the power vs. frequency plane. This formulation allows to relate the shape of the MI gain to any arbitrary dispersion profile of the medium. As a result, we derive a straightforward explanation of the non-trivial dependence of the cutoff power on high-order dispersion and obtain explicitly the power that maximizes the gain. Our approach puts forth a powerful tool to synthesize a desired MI gain with the potential application to a vast number of parametric-amplification and supercontinuum-generation devices whose functioning relies upon modulation instability.
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