With breakthroughs in fabrication techniques, integrated optical components have been developed on a lithium niobate on insulator (LNOI) platform with unprecedented performance. However, with the requirement for large-scale integration of devices on the LNOI platform, multimode crossing waveguides, which have low crosstalk and loss, are essential to enhance integration density and routing flexibility. Because of the anisotropic characteristics of lithium niobate, the design of crossing waveguides must consider the different refractive indices in different propagation directions. In this work, we propose and demonstrate a 2×2 crossing waveguide based on self-imaging theory under different waveguide sizes in the X-cutting Y-propagating and X-cutting Z-propagating directions to satisfy practical requirements.

This study employs self-imaging theory to design the crossing waveguide. First, the relationship between the lithium-niobate waveguide width and effective refractive index is obtained by simulation, and the optimal beat length is calculated using the self-imaging principle. Subsequently, the calculated parameters are introduced into the software as initial values. In this process, the structural parameters of the multimode waveguide in both directions are set to be the same, and the optimal values are found by parameter scanning. The structural parameters in one direction are fixed as previously described, and those in the other directions are scanned to find the best structural parameters. Finally, a simulated crossing waveguide is fabricated and measured. The LNOI wafers are cut into 25 mm×21 mm pieces. All pieces are cleaned with a piranha solution. Hydrogen silsesquioxane (HSQ) resist is then spin-coated onto the samples. The HSQ resist is exposed by electron beam lithography and subsequently developed. The patterns are then transferred to the LNOI device layer via reactive-ion etching, where the etching depth of the LNOI layer is 300 nm. Finally, a 500-nm-thick SiO₂ layer is formed on the lithium niobate waveguide through plasma-enhanced chemical vapor deposition (PECVD). The crossing waveguide is connected to the fiber using a grating coupler. The crosstalk and insertion losses are determined by subtracting the grating coupler spectrum from the measured spectrum. The insertion loss of the crossing waveguide is measured and averaged using cascade numbers. To understand the effects of process errors, the relationships among the device performance, waveguide width, and angle error are obtained by simulation.

The insertion loss and crosstalk of the device are measured in two directions (Fig.8). The minimum insertion losses of the crossing waveguides in the X-cutting Y-propagating and X-cutting Z-propagating directions are 0.094 dB and 0.356 dB, respectively. The corresponding crosstalk values are less than -33.31 dB and -30.81 dB in the C band, respectively. However, resonance dips appear in the transmission spectra in the X-cutting Y-propagating direction (Fig.8). This is because the cascaded crossing waveguides can be regarded as long-period waveguide gratings, which introduce resonance dips due to the coupling of the waveguide guiding mode to the radiative cladding modes. The insertion loss in the X-cutting Z-propagating direction is approximately 0.4 dB. The crosstalk values in both directions follow the same trend, which decrease in the C band. Differences in the experimental and simulation results can be seen, which mainly derive from fabrication imperfections, scattering losses, and waveguide absorption losses caused by the roughness of the sidewall edge of the lithium niobate waveguide. These process errors can be further optimized in subsequent processing to improve device performance. The relationships among the device performance, waveguide width, and angle error are obtained by simulation (Fig.9). Waveguide width errors have less effect on device performance than angle errors do, whereas the angle error has a greater effect in the X-cutting Z-propagating direction. Overall, the designed device has relatively high process tolerance. In addition, compared with the previously reported lithium niobate crossing waveguide, this work shows an improvement in the device size and crosstalk.

In this study, a 2×2 crossing waveguide with different structural parameters in the X-cutting Y-propagating and X-cutting Z-propagating directions is fabricated by single-step etching on X-cutting LNOI. The minimum insertion losses of the crossing waveguides in the X-cutting Y-propagating and X-cutting Z-propagating directions are 0.094 dB and 0.356 dB, respectively. The corresponding crosstalk values are less than -33.31 dB and -30.81 dB in the C band. The waveguide crossing has a footprint of 36.57 µm×31.90 µm. The performance can be further enhanced by optimizing the fabrication process.