Ultra-wideband (>1 THz) compact thin-film lithium niobate modulators using segmented coaxial transmission lines

This study presents a thin-film lithium niobate Mach-Zehnder modulator (TFLN MZM) based on a segmented coaxial transmission line (TL), designed to offer ultra-wide bandwidth and a significantly reduced width compared to conventional designs. The modulator operates in the optical C-band at a wavelength of 1550 nm. The coaxial TL structure ensures uniform current distribution across the electrodes, minimizing RF loss. Additionally, the segmented design enables impedance matching near 50 Ω for small electrode gaps, facilitating compact integration. All results in this work are obtained from full-wave electromagnetic simulations using Lumerical and HFSS. One design, with a gap of G = 5 μm, features a compact cross-section dimension of 21 × 21 μm², achieving a DC half-wave voltage-length product (V_π ·L) of 2.3 V·cm, a V_π of 4.7 V, and a 3-dB electro-optic bandwidth (EO BW) of 216 GHz. The radio-frequency (RF) electrode exhibits a loss of 0.82 dB/(cm·GHz^1/2), yielding a bandwidth-to-voltage ratio (BW/V_π) of 46. A second design, incorporating a larger gap of G = 10 μm, achieves a bandwidth of 1 THz-to the best of our knowledge, the highest bandwidth reported to date in thin-film lithium niobate (TFLN) modulators. This variant has a cross-section dimension of 36 × 36 μm², a V_π ·L of 3.6 V·cm, an RF electrode loss of 0.50 dB/(cm·GHz^1/2), and a half-wave voltage (V_π) of 9 V for a 4 mm device length, resulting in a BW/V_π of 111.