Optical communications have advantages of low-loss long-distance transmission, large bandwidth and capacity. However, due to the existence of nonlinearity and dispersion, the Single-mode Fiber (SMF) communication networks is closing to the theoretical upper limit (~100 Tbit/s). Some multiplexing technologies, including Wavelength Division Multiplexing (WDM), Polarization Division Multiplexing (PDM), and Space Division Multiplexing (SDM) are adopted to increase the capacity. Due to the bandwidth restriction of optical amplifier and nonlinear effect of fiber, the WDM technique is approaching its capability ceiling. The limited polarization states in the PDM technique cast barriers to further enlarge the capacity, too. Recently, the SDM technology has been considered as a growing solution to expand the volume of fiber communications. Among SDM technologies, the Mode Division Multiplexing (MDM) can provide favorable data channels in one Few-mode Fiber (FMF) or multicore fiber. Different from the fiber transmission mechanism, the on-chip MDM technique has merits of compact size and functions integration.As a key device of on-chip MDM application, the mode switches play the role of mode conversion and routing. Due to the merits of compact size, large manufacturing tolerance and high mode Extinction Ratio (ER), the Multimode Interference (MMI)-based mode switches have been investigated, in which data exchange between different spatial channels can be implemented. These are considered as the Mode Selector Switch (MSS). Compared to reported mode switches that are implemented on the silicon or polymer platforms, silica technology has advantages of long-time stability, wideband transparence, compatibility to CMOS fabrication, and low-loss coupling with fiber. Therefore, silica waveguide is adopted in this work.According to the effective refractive index theory of the waveguide and the principle of MMI self-imaging, a silica waveguide mode selector switch constructed by cascading a 1×3 Multimode Interference (MMI) coupler and a 3×3 MMI coupler is proposed and fabricated. MMI-based mode switches have attracted wide attention due to their small size, large process tolerance, high Extinction Ratio (ER) and large bandwidth. The phase of the optical signal input to the 3×3 MMI coupler is changed by thermo-optic modulation, which can achieve the conversion from the input of the first-order mode (E₁₀) to the output of the fundamental mode (E₀₀) and selectively output from the target port. When the input is fundamental mode (E₀₀), E₀₀ can be output directly from the selected port. In addition, in order to enhance the efficiency of thermo-optical modulation and reduce thermal crosstalk, an air trench is used to concentrate the thermal field and increase the phase transformation amplitude, so that the heat can be concentrated in the waveguide core layer and reduce the additional power consumption caused by thermal diffusion.The switch is designed and optimized using the Beam Propagation Method (BPM) and fabricated using standard CMOS techniques including ultraviolet lithography, plasma etching and Plasma Enhanced Chemical Vapor Deposition (PECVD). The test results show that the first-order mode (E₁₀) switches between the two output ports when the drive power is 170 mW and 395.3 mW, respectively. In the wavelength range of 1 523 nm to 1 556 nm, the insertion losses, crosstalks, and extinction ratios of the three output ports are≤10 dB, ≤-15 dB, and ≥15 dB, respectively. The switch has a rise time of 0.941 8 ms and a fall time of 1.042 ms. The proposed switch has a higher extinction ratio and lower crosstalk, which is mainly attributed to the cascaded MMI design and the precise waveguide fabrication process, so that the actual waveguide size matches the theoretical design value. In addition, the biggest advantage of silica waveguide switch is its long-term stable operation reliability, which is of great significance for the practical application and large-scale production of devices. The proposed mode selector switch has good potential in on-chip signal routing applications.