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Modal dispersion in plastic optical fibers with W-shaped refractive index profiles (W-shaped POFs) is investigated. The authors focus on the effect of the refractive index valley at the boundary of core and cladding on the group delay. In this paper, particularly, the relationship between the depth of the index valley and the modal dispersion is highlighted. In order to analyze the relationship precisely, the authors prepare the W-shaped POFs with a high numerical aperture (NA ∼ 0.25), which are expected to show the effect of W-shaped profile more clearly than the W-shaped POFs with an NA of 0.2. As a result, the modal dispersion is strongly dependent on the depth of the index valley, which means that the control of the index valley is required, in order for minimum modal dispersion. Furthermore, by evaluating the output pulsewidth from the W-shaped POFs, the authors design the optimum index valley for the maximum bandwidth performance.