To address the increase in communication traffic, the development of high-performance photonic devices is required. Numerical simulation is now indispensable for developing photonic devices. However, three-dimensional full vector simulation of photonic devices with long device length is computationally expensive, then analytical approaches like coupled mode theory (CMT) are effective for analyzing some typical devices. In the conventional CMT, CMT parameters, such as coupling coefficients and phase mismatches, are calculated using the eigenmodes of isolated systems. However, when the coupling between modes is strong or when analyzing isolated modes is difficult, applying CMT becomes challenging. In this paper, we propose a supermode-based coupled mode theory (SB-CMT) that uses only the supermodes of coupled system to compute coupling coefficients and phase mismatches. To show the validity of the proposed approach in the two-mode coupling cases, numerical examples of directional coupler and mode converter are demonstrated.
CHEN et al. (2026) studied this question.