Compact general interference hybrid-plasmonic multimode interferometers used for optical hybrid.
We propose three general interference multimode interferometers (MMIs) based on hybrid plasmonic waveguides (HPWs). Among them, the general 2×2 and 4×4 MMIs are designed for a 90° optical hybrid, while the 3×3 MMI is for a 120° optical hybrid. First, by considering the mode interference characteristics inside the multimode HPWs, a compromise between the number of guided modes and the device length is obtained at a determined height of the SiO2 interlayer of the HPW. Also, by analyzing the characteristics of multimode propagation in the HPW-MMI, it is found that the optimal positions of self-images would shift from their theoretical ones. In addition, tapered HPW sections are implemented to improve the coupling efficiencies for lights coupled into/out of the multimode section. Therefore, by optimizing the width and length of the multimode section, and especially the position of the input and output single-mode waveguides, the appropriate structure parameters of three HPW-MMIs are obtained, where the footprints of the 2×2, 3×3, and 4×4 HPW-MMIs are only 1.96×5.4  μm2, 2.18×12.0  μm2, and 2.52×11.5  μm2, respectively. The simulation results show that, at the wavelength of 1550 nm, the 2×2 HPW-MMI exhibits a transmission of 75.6%, a maximum transmissions imbalance of 0.55 dB, and a phase error of 3.68°; the 3×3 HPW-MMI exhibits a transmission of 69.2%, a maximum transmissions imbalance of 0.43 dB, and a phase error of 4.66°; and the 4×4 HPW-MMI exhibits a transmission of 68.5%, a maximum transmissions imbalance of 0.91 dB, and a phase error of 4.81°. All these performances meet the standard industry requirements.