High-speed mid-infrared Mach–Zehnder electro-optical modulators in lithium niobate thin film on sapphire

In this study, high-speed mid-infrared Mach–Zehnder electro-optical modulators in x-cut lithium niobate (LN) thin film on sapphire were designed, simulated, and analyzed. The main optical parameters of three types of Mach–Zehnder modulators (MZMs) (residual LN with thickness of 0, 0.5, and 1 μm) were simulated and calculated, namely, the single-mode conditions, bending loss, separation distance between electrode edge and lithium niobate waveguide edge, optical field distribution, and half-wave voltage–length product. The main radio frequency (RF) parameters of these three types of MZMs, such as characteristic impedance, attenuation constant, RF effective index, and the –3 dB modulation bandwidth were calculated depending on the dimensions of the coplanar waveguide traveling-wave electrodes. The modulations with residual LN thickness of 0, 0.5, and 1 μm were calculated with bandwidths exceeding 140, 150, and 240 GHz, respectively, and the half-wave voltage–length product achieved was 22.4, 21.6, and 15.1 V cm, respectively. By optimizing RF and optical parameters, guidelines for device design are presented, and the achievable modulation bandwidth is significantly increased.