0.33 V·cm thin-film lithium niobate Mach-Zehnder modulator with ridge-top transparent electrode

Thin-film lithium niobate (TFLN) electro-optic modulators are crucial for high-baud-rate digital communication and high-carrier-frequency analog links. Typically, TFLN modulators have half-wave voltage-length products (V_π⋅L) much larger than that of silicon modulators or InP modulators due to the weak overlap between the long device optical and RF fields long device roll. In this work, we present a high-efficiency TFLN traveling-wave (TW) Mach-Zehnder modulator (MZM) with ridge-top transparent electrodes on the TFLN waveguides. A low V_π⋅L of 0.33 V·cm is achieved, while the EO response of a 300-μm-long device rolls off 2.4 dB at 110 GHz, with a 3-dB EO bandwidth extrapolated to be 145 GHz. The on-chip insertion loss is 2.7 dB. A high-frequency model of TFLN TW-MZM with transparent electrodes is proposed. Three operation regions exist according to the resistivity of transparent electrodes, that is, the slow-wave region, the dispersion region, and the high residue in the weak overlap region. For high modulation efficiency and high-speed modulators, a slow wave region with resistivity less than 3 ×10^-3 Ω·cm is required.