CST MICROWAVE STUDIO® (CST MWS) lends itself well to the simulation of optical devices since it easily handles electrically large problems. In this case, an optical ring coupler with a substrate dimension of 20 by 25 wavelengths is simulated. The FIT-based time-domain Method allows the simulation of such electrically large structures on a modest PC configuration - in this case, just 512 MB system RAM was necessary for the simulation.
The geometry of the ring coupler is shown in figure 1. Waveguide ports have been defined at the waveguide feeds. The ring and feed lines are defined with a dielectric constant of 9. The frequency range simulated was between 170 THz and 250 THz.
CST MWS uses a 2D modal port solver which can accurately determine all possible modes for arbitrary geometries. The electric field of the fundamental mode at 210 THz and 250 THz are shown in figure 2. The frequency dependent change of mode patterns in the broadband time domain simulation can be considered by applying a "Full De-embedding" method.
The S-parameters derived from the Fast Fourier Transform of the Transient Solver results provide the broadband S-Parameters. These can be seen in figure 3.
In Figures 4 and 5 the frequency dependent behavior of the field can be seen. The electrical size of the problem can be appreciated by the number of visible waveforms in the field plots. Ports 3 and 4 are isolated at 211.6 THz whereas ports 1 and 2 are isolated at 250 THz. Since the Transient Solver has been applied to this problem, any arbitrary number of frequency monitors can be defined enabling the user to investigate the frequency dependence of the device in just one single run.