The development of multi-core processors has greatly relieved the pressure of data processing. However, the capacity requirement for data transmission and exchange is still a challenge. Multi-dimensional multiplexing technologies are explored to meet the growing bandwidth requirements and provide a promising solution to address such a challenge. Among these technologies, mode division multiplexing in which each guided mode acts as an independent data channel has attracted much attention. Silicon-based optical mode switches are indispensable for reconfigurable on-chip mode division multiplexing. Previously, these switches have been demonstrated by a Y-junction combined with a multimode interference coupler and phase shifters, Y-junction couplers combined with 2×2 multimode interference couplers, and microrings. Although these devices can have good performance, sustained power consumption is required to maintain the switch states, which means they are volatile. In addition, due to the high refractive index contrast of the silicon-on-insulator platform, a strong polarization dependence would be formed. Thus, non-volatile polarization-insensitive silicon-based optical mode switches are highly desired. Owing to their outstanding properties, phase change materials are considered attractive candidate materials for realizing non-volatile integrated optical devices. To the best of our knowledge, non-volatile polarization-insensitive silicon-based optical mode switches employing phase change materials are never discussed before. Therefore, we wish to propose, design, and analyze a non-volatile polarization-insensitive silicon-based 1×2 optical mode switch using phase-change materials.

The proposed non-volatile polarization-insensitive silicon-based 1×2 optical mode switch using phase-change materials is composed of a polarization beam splitter, a polarization beam combiner, two directional couplers with phase-change materials operating in the TE₀ and TM₀ modes, a polarization-insensitive silicon waveguide crossing, and two-mode converter operating in the TE₀ and TM₀ modes. The proposed polarization beam splitter/combiner is based on a triple-waveguide coupler comprising two silicon waveguides on both sides and a Si-Si₃N₄ hybrid waveguide in the middle. By optimizing the structural parameters in the coupling region, a low-insertion-loss, and high-polarization-extinction-ratio polarization beam splitter/combiner can be obtained. For the two directional couplers with phase-change materials, a tapered silicon waveguide and a Si-Ge₂Sb₂Te₅-ITO hybrid waveguide are employed to form the coupling region. The finite difference time domain method and particle swarm optimization algorithm are adopted to optimize the coupling region to obtain low insertion loss and excellent crosstalk. Similarly, the two-mode converters are based on counter-tapered couplers. With an aim at achieving high conversion efficiency in a wide wavelength range, the corresponding coupling regions are optimized through the finite difference time domain method and particle swarm optimization algorithm. The proposed polarization-insensitive silicon waveguide crossing consists of two orthogonal multimode waveguides in which the input and output tapers are mirror-symmetrical. The finite difference time domain method and particle swarm optimization algorithm are employed to optimize these tapers and finally realize low insertion loss and excellent crosstalk. As a result, a non-volatile polarization-insensitive silicon-based 1×2 optical mode switch with good performance can be achieved by tuning the phase state of phase-change materials.

The functionality of the designed non-volatile polarization-insensitive silicon-based 1×2 optical mode switch is executed well (Fig. 13). When Ge₂Sb₂Te₅ is in the amorphous state, the input TE₀ and TM₀ modes are transformed into TE₁ and TM₁ modes and emerge from the port output₂. The input TE₀ and TM₀ modes will propagate forward and come out from port output₁, if Ge₂Sb₂Te₅ is in the crystalline state. When the designed device is operating in TE polarization, the crosstalk is less than -13.12 dB and the insertion loss is smaller than 1.37 dB within a bandwidth from 1535 nm to 1569 nm (Fig. 14). For TM polarization, within a bandwidth from 1535 nm to 1569 nm, the designed device exhibits a crosstalk of lower than -17.39 dB and an insertion loss of smaller than 1.61 dB. The corresponding fabrication tolerance is also discussed (Fig. 16). The waveguide width variation ΔW and the variation of Ge₂Sb₂Te₅ thickness Δh exert great influence on the crosstalk and insertion loss of the designed device. Thus, the waveguide width and Ge₂Sb₂Te₅ thickness should be precisely controlled. Ge₂Sb₂Se₄Te₁ can be employed to improve the crosstalk and reduce insertion loss further (Table 5).

We propose a non-volatile polarization-insensitive silicon-based 1×2 optical mode switch using phase-change materials. The proposed optical mode switch consists of a polarization beam splitter, a polarization beam combiner, two directional couplers with phase-change materials operating in the TE₀ and TM₀ modes, a polarization-insensitive silicon waveguide crossing, and two-mode converter operating in the TE₀ and TM₀ modes. The polarization-insensitive mode switching behavior is realized by adjusting the phase state of phase-change materials. The finite difference time domain method and particle swarm optimization algorithm are employed to design and analyze the presented device in detail. For the designed non-volatile polarization-insensitive silicon-based 1×2 optical mode switch using Ge₂Sb₂Te₅, when the TE₀ mode is input, the insertion loss is lower than 1.37 dB and the crosstalk is less than -13.12 dB within a bandwidth from 1535 nm to 1569 nm. As the TM₀ mode is input, the insertion loss is smaller than 1.61 dB and the crosstalk is lower than -17.39 dB within a bandwidth from 1535 nm to 1569 nm. The results can provide references for optimizing the design of non-volatile polarization-insensitive optical mode switches.