finite-difference time-domain method

A digital comparator is a logic circuit used to compare two binary numbers. So far, various designs have been proposed using logic gates, which are mainly based on electrical signals and lack the desired high speed. This paper presents a new design of an ultra-fast all-optical single-bit comparator based on photonic crystal ring resonators consisting of highly nonlinear glass. The structure of this comparator consists of two inputs, four ring resonators with a number of waveguides for comparison and three outputs for displaying the result, all created in a photon crystal bed. Using the plane wave expansion method, its band structure is calculated and the results show that the fundamental photonic crystal has a photonic band gap in the polarized TM mode in S, C and L bands that is a suitable tool for telecommunication applications. To solve the Maxwell's equations, the  finite-difference time-domain method is used, which aims to investigate the light propagation inside the final structure. The results of numerical studies show that the designed structure has a very short response time of 3 ps, making it faster than all the comparators designed so far, including electronic, electro-optical and all-optical comparators. Also, its relatively small area of 826 μm2 makes it applicable in the design of photonic integrated circuits.

We investigate the transmission behavior of photonic crystals power splitter by using improved 2D FDTD and MMP methods. The Comparison between experimental data and simulation results are discussed in TE polarization. In this study, the 60° photonic crystal power splitter is optimized, which is composed of InPGaAs/InP material and air holes. The photonic crystal waveguide bend transmission data was measured by End-Fire technique. This computational method can be applied to investigate photonic crystals with irregularly shape and different materials. Comparison between experimental data and two dimensional simulations is evident that the improved 2D FDTD simulation shows a very good agreement with the experimental data of the transmission properties.

In this paper, the effects of wall structures on the parameters of realistic indoor wireless channel using two dimensional finite difference time domains (FDTD) method are investigated. Two types of wall are considered for the environment. In the first case, the walls are a type of concrete block with air gaps as inhomogeneous walls and, in the second case, the walls are a type of homogeneous oneswith effective constitutive parameters. For two cases, field coverage maps, average path loss, power delay profile and RMS delay spread are extracted and compared. Power delay profiles show that inhomogeneous walls create stronger multipath effects than homogeneous walls and that these effects are severe in none line of sight conditions. On average, the RMS delay spread for environment with inhomogeneous walls, in LOS conditions 5 ns and in NLOS conditions 8 ns is more than the environment with homogeneous walls.

The accuracy of improved two dimensional finite difference time domain method (2D FDTD) is verified by comparing with 3D FDTD method and experimental data. The optimized 60° photonic crystal waveguide bend، used in this study as a test case، is composed of InPGaAs/InP material and air holes and TE polarization is considered. The photonic crystal waveguide bend transmission data was measured by End-Fire technique. This computational method can be applied to investigate photonic crystals with irregularly shaped and different materials. Comparison of experimental data with two and three dimensional simulations، it is evident that the improved 2D FDTD simulation shows a very good agreement with 3D FDTD simulation and also with the experimental data of the transmission properties. The efficiency and simplicity of 2D FDTD makes this algorithm promising for simulation of photonic crystals on personal computers of reasonable speed. Since the improved 2D FDTD simulation has a high performance compared to its 3D counterpart and also could easily be run on PCs، the improved 2D FDTD simulation can be considered as a useful tool in studying، designing and improving photonic crystal devices.