Design of multimode interference couplers and optical-path architecture for three-Axis integrated Fiber optic gyroscopes

To address the key requirements of integrated optical paths in highly precise and stable fiber optic gyroscope (FOG) systems, the performance of passive couplers was systematically analyzed, with a focus on multimode interference (MMI) couplers. Simulation results show that MMIs offer low insertion loss, broad bandwidth, and good polarization-independent characteristics, making them well suited for integrated FOG design. The effects of key structural parameters, including the length and width of the multimode interference region and the taper-port structure, were investigated to provide guidance for MMI optimization. On this basis, a notched MMI was proposed for low-loss nonuniform power splitting. In a 1 × 3 MMI, the proposed structure achieved a 2:1:2 splitting ratio with an insertion loss below 0.5 dB. Based on this nonuniform splitting capability, a 2 × 3 MMI and a corresponding novel optical-path architecture were designed for an integrated three-axis FOG system, enabling a crossover-free layout, same-side detector outputs, and balanced input power among the three fiber coils. These results demonstrate that MMIs are suitable not only as fundamental passive couplers in integrated FOGs, but also as enabling components for optical-path design and optimization.