High voltage, bandwidth-limited, segmented CPW transmission lines for a thin-film lithium niobate electro-optic modulator using a conical-shaped high-permittivity material

Thin-film lithium niobate (TFLN) has emerged as a leading platform for integrated electro-optic modulators because of its large Pockels coefficient, low optical propagation loss, and wide optical transparency window. In electro-optic modulators, there is an inherent trade-off between the half-wave voltage V _π and the 3-dB electro-optic bandwidth. Consequently, the voltage-bandwidth limit (BW/V _π ) serves as a metric for comparing modulator performance among different device architectures. However, many previously reported TFLN modulators prioritize either low V _π  · L through strong field confinement or large bandwidth through transmission-line optimization, often without simultaneously addressing both constraints. In this work, a TFLN Mach-Zehnder modulator is proposed that employs a segmented coplanar waveguide (CPW) transmission line with a conical high-permittivity cladding to enhance electro-optic performance. This design improves the V _π  · L while maintaining favorable velocity matching and reduced microwave loss. As a result, low half-wave voltage and a high 3-dB electro-optic bandwidth are achieved simultaneously. To the best of the authors' knowledge, this work demonstrates the highest reported voltage-bandwidth limit for TFLN modulators employing CPW transmission lines, with a BW/V _π of 177.