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.