Recently, various types of silica-based planar lightwave circuits (PLCs) have been used for optical networks. However, silica exhibits large temperature dependence, so the temperature of silica-based PLCs needs to be controlled. We can reduce the cost and the energy budget of optical equipment by using athermal waveguides. We propose an athermal optical waveguide comprised of trenches filled with low-refractive index material. The propagation constant of the waveguide is determined by the refractive indices of the core and the low-refractive index material; their thermo-optic coefficients have opposite signs, and therefore the temperature dependence of the waveguide can be suppressed. A 1-bit delay circuit was designed for a 12.5Gbit/s differential phase shift keying receiver using the athermal waveguide, and it displayed a wide operating temperature range.