To determine its feasibility as a polymer optical waveguide, the system polymethyl methacrylate-benzophenone-coumarin is studied. The refractive-index change increases linearly up to ∼ 1.8% with coumarin quantity as a result of photodimerization of coumarin, while the propagation loss is restrained. A loss of 0.7 dB/cm and an index change of 1.4% are obtained for slab waveguides having 0.3 and 0.1 weight ratios of coumarin and benzophenone to PMMA, respectively.