A wavelength splitter and a polarization splitter with high compactness and extremely simple structures are designed for optical communication wavelengths. Operation principle of the devices is based on directional coupling in two parallel periodic dielectric waveguides. The device performances have been evaluated by the finite-difference time-domain simulations. The wavelength splitter with a coupling region length of 5 mum can route 1.31 and 1.55 mum wavelengths to corresponding outputs with a transmittance of more than 93%, while the polarization splitter with a coupling region length of 4.6 mum can divide lightwaves in TM and TE polarizations with a degree of polarization higher than 90% at 1.55 mum.