Electro-optic activity of a nonlinear optical polymer is stabilized by crosslinking of the hyperbranched polymer, as presented by Shiyoshi Yokoyama and colleagues on page 1254. Since a nonlinear optical chromophore attached to the polymer side-chain has large molecular hyperpolarizability, the resulting electric-poled polymer film shows large electro-optic activity. When polarized laser light is used to irradiate the polymer film, polarization is effectively modulated by applying an electric field with large electro-optic coefficient. Hyperbranching of the polymer enhances its glass transition temperature, so that the poled polymer film is extremely stable with little decay of electro-optic activity. Despite crosslinking of the as-synthesized hyperbranched polymer, it forms a high optical quality film from many common solvents. Optical propagation loss is small enough for application in optical waveguide devices.