The mode splitter is employed in mode division multiplexing systems to separate and guide different modes in a bus waveguide. Traditional mode splitter methods are based on mode conversion processes, where the input higher-order modes are converted into the fundamental mode to separate them from the bus waveguide. This method is suitable for most mode division multiplexing systems, but in scenarios such as signal routing or signal selection, when different modes are separated, they are often switched or selected using multi-mode switches. If the mode splitter output is all in the fundamental mode, additional components should be added after the mode splitter to convert the separated fundamental mode back to higher-order modes before inputting them into multi-mode switches, which undoubtedly increases the system complexity. In the case of optical isolation, it is often necessary to remove the higher-order modes and just transmit the fundamental mode. If the mode splitter output is all in the fundamental mode, it cannot meet the requirements of practical applications. Therefore, if the original modes can be preserved with mode separation achieved, this will greatly simplify the system structure and reduce the system size.

The multi-mode interference (MMI) coupler is a basic silicon-based device, that features a compact structure, low loss, and easy fabrication. However, traditional MMI-based mode splitters are difficult to extend the mode separation function to higher-order modes due to the highly symmetric structure and mode self-imaging principle. This scheme is based on the silicon-on-insulator (SOI) platform. According to the modal analysis method, it is derived that the refractive index distribution in the multi-mode region can not only change the imaging position and shorten the self-imaging length but also separate different input modes. Thermal tuning is achieved by fabricating heaters on top of the waveguide, and the waveguide material refractive index changes with the temperature at a rate of 1.84×10^-4 K^-1 due to the thermo-optic effect. Therefore, by adding micro heaters to the multi-mode region of the MMI, designing the positions of the heaters, and accurately controlling the heating temperature, the separation of three modes, TE0, TE1, and TE2 can be achieved.

The designed structure is a 1×3 MMI, with the input port of the MMI located at the upper left of the multi-mode region, and four heaters are uniformly distributed in the multi-mode region. The optimized size parameters are w=w₃=1.9 μm, w₁=0.9 μm, w₂=0.6 μm, w_taper=w_taper1=w_taper3=4.4 μm, w_taper2=6 μm, w_m=15 μm, and l_m=227 μm (Fig. 6). The heating temperatures of the four heaters for the TE0 mode are 32 ℃, 26.14 ℃, 0 ℃, and 43.42 ℃ respectively, temperatures for the TE1 mode are 7.61 ℃, 0 ℃, 50 ℃, and 17.89 ℃, and those for the TE2 mode are 50 ℃, 50 ℃, 4.89 ℃, and 50 ℃. Under the wavelength of 1550 nm, the insertion losses are 1.03 dB, 1.04 dB, and 1.06 dB for the three modes, the crosstalks are -15.38 dB, -17.5 dB, and -20.4 dB respectively, and all have a 3 dB bandwidth greater than 97 nm (Fig. 8). It is proven that compared with traditional mode splitters, the proposed scheme has a larger number of separated modes, smaller insertion losses, and smaller mode crosstalks. The most outstanding feature is that it preserves the original modes during mode separation, which makes it have a wider range of applications in mode division multiplexing systems. Additionally, by increasing the number of micro heaters and output waveguides, the proposed scheme can achieve the separation of higher-order and more modes.

An MMI-based on-chip mode splitter scheme is proposed to separate the first three TE modes (TE0, TE1, and TE2) and preserve the modes. When the wavelength is 1550 nm, the insertion losses are 1.03 dB, 1.04 dB, and 1.06 dB, and the crosstalks are -15.38 dB, -17.5 dB, and -20.4 dB, with a 3 dB bandwidth greater than 97 nm. Meanwhile, when the temperature deviation is within 2 ℃, the insertion loss of each mode is less than 1.45 dB, and the crosstalkis less than -13.22 dB. Compared with traditional mode splitters based on mode demultiplexers, the proposed scheme can achieve separation without converting high-order modes into fundamental modes, which can greatly reduce system complexity in scenarios such as signal routing or signal selection. As more heaters are added, the proposed scheme can be expanded to separate higher-order modes. At the same time, this mode splitter also has high integration and low process difficulty and can be widely employed in optical communication systems.