Study on the switching characteristics in the multimode interference waveguide

Abstract

This thesis presents a novel $2\times2$ switching characteristics obtained from the multimode interference (MMI) patterns in a thick waveguide structure. The interference patterns vary according to the length as well as the width of the thick waveguide structure that may be in turn also controlled by change of refractive index. In other words, the self-imaging points formed in the constructive interference pattern of modes might be moved by the refractive index change. Finally, the signal path in the $2\times2$ optical switch could be the bar- or the cross-state according to the movement of the self-imaging points The mode propagation theory based beam propagation method (BPM) is used in the analysis of the MMI waveguide structure. Each mode in the MMI waveguide has not only different propagation constants but also different effective widths caused by Gooth - H?nchen shifts. The effective width differences among the modes become larger as the refractive index difference between core and cladding becomes smaller. Since the greater change in the interference pattern occurs in the lower index difference, the switching operation is available only in the very low-contrast MMI. In addition, it should be noted that these properties can not be analyzed by mode propagation analysis (MPA) but BPM because the same effective widths for all modes are assumed in MPA. In the simulation of the MMI waveguide structure with the width of 17.5$\mum$ and the length of 2045$\mum$, the initial bar-state gives the insertion loss of -0.542 dB and the crosstalk of -14.939 dB, while the cross-state gives the insertion loss of -1.118 dB and the crosstalk of -12.281 dB. The extinction ratio for the cross-state shows the value higher than 11.74dB, but the ratio becomes slightly better for the bar-state. Even though these outcomes do not show good performance, the MMI structure has big potential to overcome these results which might be further enhanced by modifying the structure. As a pr...