In recent years, the research on silicon-based thermo-optic (TO) devices has focused on in-depth direction and become more complex, and the realization of higher-performance silicon-based TO devices is the main purpose of our research. There are many kinds of silicon-based optical switches developed so far. We design Mach-Zehnder interferometer (MZI)-type silicon-based TO switches with large bandwidth, simple structure, and high robustness, and the N×N TO-integrated switches and the electro-optic (EO)-integrated switches have been widely studied. The EO switch has a fast switching speed (nanosecond level), but its crosstalk and insertion loss are high due to the free carrier absorption effect. In contrast, TO switches excel in maintaining low loss and low crosstalk, but their switching response time is intrinsically limited, typically on the microsecond scale. Optical switches in hybrid network systems are typically used to handle high-capacity and high-bandwidth optical communication services, making TO switches the preferred choice due to their low loss and low crosstalk characteristics.

The MZI-type 1×8 silicon-based TO switch proposed and prepared in this paper is composed of one 2×2 MZI and six 1×2 MZI switching units connected by a binary tree structure, which has two input ports and eight output ports in the optical switch, with the first stage comprising a 2×2 MZI switching unit, the second stage comprising two 1×2 MZI switching units, and the third stage comprising four 1×2 MZI switching units. Effective control of the optical signals is achieved by the phase shift of the TO-tuned phase shifter, which directs the light to the destination branch waveguide, thus realizing the optical switching function. The coupler and phase shifter in the optical switching unit are optimized by using the finite-difference time-domain method and the particle swarm optimization algorithm to improve the switching performance and reduce the chip size. The long connecting waveguide is designed as a wide waveguide of 2 to reduce the waveguide transmission loss. The package connects the optical switch chip to a 14-channel optical fiber array by using an end-face coupling, curing it with an ultraviolet curing adhesive. In addition, a multi-channel voltage source is designed, which mainly consists of a CPU, op-amp LM324, analogue switches, and four DAC modules. This multi-channel voltage source has 32 selective ports, and the synchronous switching of the optical switching ports is achieved by simultaneously regulating the voltages of all levels of thermal phase shifters through the host computer. The results show that the optical switch achieves low on-chip insertion loss, lower crosstalk, and a reduced response time of the optical switch.

The experiments demonstrate that the designed and prepared 1×8 TO switch performs well in all aspects; its average on-chip insertion loss is about 1.1 dB (Fig. 8); the fiber-to-fiber loss fluctuates and varies in different paths because the difference in connecting waveguide lengths of the different paths and different ports with different coupling efficiencies to the fiber may not be the same. The crosstalk of its eight output ports is less than -23.6 dB (Fig. 9), and the crosstalk is the leading cause of switching signal degradation. The 2×2 MMI coupler can reduce crosstalk. The response time of the switch is less than 60 μs (Fig. 11) because the thickness of the silica cladding layer at the bottom is more significant than that of the buffer layer. The thermal conductivity of silica is about 1/100 of that of silicon; after heating, the heat diffusion from the core layer to the substrate is slower, and the falling-edge time is longer than that of the rising-edge time. When the input electrical signal's frequency is high, the TO phase shifter's response can no longer follow the electrical signal due to its limited response bandwidth, and the response is no longer square-wave in character. However, the rise and fall time becomes shorter. In addition, there is a difference in the response time of the switch due to manufacturing process errors in different phase shifters.

Our proposed 1×8 optical switch chip is based on a tree structure consisting of one 2×2 MZI and six 1×2 MZIs, with a TiN heater in each thermal phase shifter's upper/lower arm. The switch chip is constructed on a SOI platform by using a CMOS-compatible process with a size of 1.75 mm×3 mm. The chip exhibits an on-chip insertion loss of about 1.1 dB at the operating wavelength of 1550 nm, a crosstalk of less than -23.6 dB, a response time of the switch of better than 60 μs, and an average power dissipation of the switch of about 34.09 mW. The experimental results show that the 1×8 TO switch has the advantages of compactness, low loss, and low crosstalk.