A New Method for Accurately Measuring Temperature Dependence of Refractive Index

A new high-accuracy method for measuring temperature dependence of refractive index is proposed and applied to some polymer waveguides. The method uses 3 m-lines from a grating coupler on the waveguide surface, and has an accuracy of better than 1 × 10^–5 (1/°C). The grating period dependent on the temperature does not affect the accuracy. The measurement thermo-optic (TO) coefficients of polycarbonate (PC) and polymethyl methacrylate (PMMA) are strongly dependent on the waveguide structure in contrast with the previously reported value. It is found that the TM-mode refractive index of UV-curing polymer (J-91) increases with the increase in temperature, contrary to common expectations for polymers. A highly efficient TO switch is expected using the J-91 polymer waveguide.