Comparison of Multimode Interference Couplers: Gaussian-mode and Sinusoidal-mode Cases

Analysis of the multimode-interference couplers can be done by using the sinusoidal eigen-modes and also the Gaussian eigen-modes for the multimode region. Study of the multimode interference couplers using the sinusoidal eigen-modes is based on the self-imaging concept of the multimode waveguides. These modes can be determined by using the planar waveguide theory. In Gaussian model, eigen-modes of the multimode region are obtained by using the Gaussian mode theory. In this paper, direct comparison of the multimode interference couplers with Gaussian eigen-modes and couplers with sinusoidal eigen-modes is done. The coupling coefficients of the field for these models are compared. In addition, a comparison is made between wave propagation analysis of Gaussian model and wave propagation simulation by FEMLAB software and FDTD method for MMI couplers with ultrashort lengths. It is shown that the Gaussian model results, unlike the sinusoidal model, are in good agreement with the simulation results. In other words, the Gaussian model is necessary for analysis of the ultrashort MMI couplers and also the sinusoidal model is not suitable for small lengths. In fact, building-up of the sinusoidal modes is the result of multiple interference of the reflected waves that appears in larger lengths. In addition, for long MMI couplers, the Gaussian modes are gradually transformed into the sinusoidal modes and the results of the sinusoidal model with self-imaging principle is true to handle these types of devices

Keywords:

MMI coupler , multimode interference , sinusoidal mode , Gaussian mode , self-imaging