Design of LiNbO₃ polarization converters

In the literature, the design of polarization converters typically involves estimating the coupling length and polarization conversion efficiency based on the difference in effective refractive indices using full-vector mode solvers, and by simulating light propagation through the waveguide—for example, using the three-dimensional finite-difference time-domain method. In this work, we estimate the polarization conversion efficiency based on effective refractive indices using semi-vector beam propagation method, grounded in coupled mode theory. This calculation process is applied to design the L-shape LiNbO₃ polarization converters. The results are compared with those obtained using the three-dimensional finite-difference time-domain method. An ultrashort polarization converter with a device length of 8.2m is demonstrated. The proposed calculation process can significantly reduce design time for parameter scanning, facilitating the development of more compact and efficient polarization converters.