The optimum design of a photonic integrated circuit (PIC) based on optical semiconductor waveguide is presented. The proposed photonic devices circuit consists of a laser coupled through a diluted waveguide to a microdisk etched into silicon (Si) strip on a silicon dioxide layer (SiO2). We demonstrated that an efficient coupling between the laser and the integrated photonic components is possible by incorporating a uniform diluted optical waveguide. Using the Finite Difference Time Domain (FDTD) method, the optical and geometrical parameters are investigated. Varying the gap width between the diluted waveguide and the microdisk from 0 μm to 2 μm the coupling coefficient is calculated. It is shown that an efficient laser-to-microdisk coupling is achieved by incorporating the diluted waveguide as a spot size converter. A spot size converter combined with the microdisk is numerically demonstrated.