A narrowband optical filter on Silicon-on-Insulator based on a directional coupler of continuous and periodic segmented waveguide is theoretically investigated. The study shows that the filtering action is better when a slot waveguide is considered as the continuous waveguide instead of a photonic nano-wire waveguide. A filter of extremely narrow bandwidth at any desired wavelength can be designed by optimizing the feature size of the different components of the device. Comparison of the footprint shows that the proposed filter is much more compact than other available filter. Typical value of proposed filter bandwidth can be as low as 2.5 pm with a footprint of 0.44 mm2 which is much less than many available filters of similar bandwidth. In this study, the transfer matrix method has been used to find the effective refractive index and hence the propagation constant. © 2017 Informa UK Limited, trading as Taylor & Francis Group.