For high-density and high-speed optical interconnections using parallel polymer optical waveguides (PPOW), we theoretically and experimentally demonstrate that W-shaped refractive index profiles in the cores are capable of decreasing the inter-channel crosstalk compared to step-index (SI) and even to graded index (GI) waveguides. In this paper, first we analyze the optical characteristics of polymer waveguides with different index profiles by means of the ray tracing model with scattering effect previously we developed. The calculation results confirm the index valley surrounding each core works properly for preventing the power coupling from the cladding modes to the propagation modes. Next, we show the fabrication process (preform method) for such a waveguide, where the cladding material is composed of a copolymer of methyl methacrylate (MMA) and benzyl methacrylate (BzMA). Compared to a GI-core PPOW we previously fabricated with the same core material, the 1-m long PPOW with W-shaped refractive index profile exhibits not only a low propagation loss (0.027 dB/cm), but an even lower inter-channel crosstalk value (-32 dB).