In this paper a typical waveguide in 2D photonic crystal of air holes in dielectric slab structure has been simulated to explore the possible transmission spectrum as shown later in various figures. The waveguide width is variable and correspondingly its transmission spectrum changes. This may be improved upon and used to design optical communication devices and photonic sensors. The algorithms used for simulation are finite difference time domain (FDTD), and plane wave expansion method (PWEM). © Springer Nature Singapore Pte Ltd. 2019.