  Waveguide crossings are elementary passive components for signal routing in
photonic integrated circuits. Here, we design and characterize two multimode
interferometer (MMI)-based waveguide crossings to serve the various routing
directions in the anisotropic x-cut thin-film lithium niobate (TFLN) platform.
To address the large measurement uncertainties in traditional cut-back
characterization methods, we propose and demonstrate a resonator-assisted
approach that dramatically reduces the uncertainty of insertion loss
measurement(< 0.021 dB) and the lower bound of crosstalk measurement (-60 dB)
using only two devices. Based on this approach, we demonstrate and verify TFLN
waveguide crossings with insertion losses of < 0.070 dB and crosstalk of < -50
dB along all three routing directions at 1550 nm. The low-loss and
low-crosstalk waveguide crossings in this work, together with the simple and
efficient characterization strategy, could provide important layout design
flexibility for future large-scale classical and quantum TFLN photonic
circuits.
