With the rapid development of internet businesses and information technology, optical fiber communication has become one of the cornerstones of our rapidly developing information society. With the rise of broadband services, such as augmented reality, 5G wireless networks, and cloud computing, global data traffic is growing exponentially. However, the capacity of standard single-mode fibers is gradually approaching the Shannon limit. Multiplexing techniques, including polarization division, wavelength division multiplexing (WDM) and mode division multiplexing (MDM), are often employed to improve the capacity and transmission rate of communication systems, with extensive studies conducted on MDM technologies. The multimode switch makes MDM more flexible, which is of great importance. However, most multimode switches are mode sensitive and cannot regulate the fundamental and higher-order modes simultaneously. This paper presents a mode-insensitive thermo-optic switch based on silicon waveguides, realizing mode insensitivity by optimizing the multimode interference coupler and phase shifter, whereby the first three transverse electric (TE) modes are simultaneously regulated through the thermo-optic effect. In addition, the influence of air trenches on power consumption is discussed.

The design of the proposed mode-insensitive thermo-optic switch uses a silicon-on-insulator (SOI) chip with an optical waveguide thickness of 220 nm. The heating resistance is generated by titanium nitride (TiN). First, we simulated the relationship between different modes and waveguide widths to determine the waveguide width that can transmit the first three TE modes. The Mach-Zehnder interference (MZI) structure, composed of multimode interference couplers and insensitive phase shifters, realizes the switching of optical paths. The parameters of the multimode interference coupler (the input position of the waveguide, maximum width and length of the gradient coupler, and length of the multimode waveguide) were optimized to achieve the 3 dB splitting ratio of TE₀, TE₁, and TE₂ modes. Subsequently, the relationship between the thermal optical coefficients of the different modes and waveguide width was simulated to determine a suitable width, to ensure that the phase change of each mode is the same at the same temperature. The waveguide temperature was varied by applying different heating powers to the heating resistor, to achieve light modulation. Finally, the influence of adding air trenches on power consumption is discussed, which has reference significance for the optimization of optical switches.

In calculating the effective refractive indices of three modes at different widths, the TE₂ mode was observed to transmit at a width of 1.2 μm, and the waveguide width was determined to be 1.45 μm. The optimized multimode interference coupler achieved a 1∶1 splitting ratio for the TE_0, TE₁, and TE₂ modes (Fig. 2). Simulation results show that as the waveguide width increases, the thermo-optic coefficients of the first three TE modes become increasingly closer, and the difference at 4 μm is already less than 2%. Therefore, the width of the phase shifter was determined to be 4 μm [Fig. 3(c)]. Simulations of the optical switch indicated that the TE₀ mode has a loss of less than 0.45 dB within the C-band, demonstrating good wavelength independence. The losses for the TE₁ and TE₂ modes were less than 1 dB in the range of 1550?1560 nm, with both achieving the minimum transmission loss at 1555 nm [Fig. 4(a)]. Without increasing the air trench thickness, switching of the optical signal output paths for the three modes was achieved at 22 mW [Figs. 4(b) and 4(c)]. By etching an air trench, power consumption was reduced. As the depth and width of the air trench increased, power consumption decreased to 9 mW. The extinction ratios of the three modes were greater than 20 dB (Fig. 7).

In this paper, a 2×2 mode optical switch based on silicon is proposed. The switch was cascaded by a mode-insensitive multimode interference coupler, a mode-insensitive phase shifter, and an S-bend waveguide. The modulation of the input optical signals of the TE₀, TE₁, and TE₂ modes was realized at the same heating power. The effects of adding an air trench on the power consumption of the optical switch and transmission of optical signals were discussed. The results indicate that the power consumption of the optical switch was significantly reduced after adding the air trench. The extinction ratio of the three modes at 1550 nm was not less than 29 dB, and the extinction ratio of the TE₀ mode in the C-band was more than 30 dB; the extinction ratio of the TE₁ and TE₂ modes in the C-band was more than 20 dB. The above results can provide a reference for the design of Mach?Zehnder interferometer-multimode interferometer thermo-optic switches based on SOI.