A Silicon-on-Insulator (SoI) technology is poised to revolutionize the photonic world in terms of device operational speed and device compactness. An approach is adopted using SoI technology to study coupling length (Lc) variation for numerous photonic applications where coupler/splitter is a part of the device eg, Racetrack resonator. However, coupler/splitter are itself applicable for communication purpose. The two and three rectangular straight waveguides are used to figure out the difference in Lc at the operational wavelength of 1550 nm. The propagational length used in this coupler simulation is 20 um and the gap between each waveguide considered is 55 nm. The Lc resulted for two and three straight waveguide coupler is 7.5 um and 5.5 um. All the simulation is performed on comsol multiphysics software following Finite Element Method (FEM) technique.