Effect of the first-order mode in access waveguides on the performance of unbalanced MMI couplers

Unbalanced multimode interference (MMI) couplers are optical integrated components widely used in photonic integrated circuits. Couplers with two fixed coupling ratios: 85/15 and 15/85 are studied in this paper. Low-loss MMI couplers are designed and fabricated. Excitation of the first-order mode in access waveguides of MMI couplers can influence the excess losses and the coupling ratio of these components. The effect of the first-order mode excitation was simulated and confirmed by the experimental results, which are presented in this paper.