Thin‐Film Lithium Niobate Optical Modulators with an Extrapolated Bandwidth of 170 GHz

High‐speed modulators with low driving voltage, low loss, and compact size are essential for future optical communication systems. Thin‐film lithium niobate modulators have met each of these criteria separately, but simultaneous achievement of all of them has been challenging on this platform. Low driving voltage electro‐optic modulators necessitate either a narrow gap between the electrodes or an elongated Mach–Zehnder arms, both of which adversely affect the microwave loss, hence the bandwidth. Herein, this trade‐off is alleviated by placing the optical waveguides nonsymmetrically with respect to the electrodes and by including a dielectric buffer layer beneath the electrodes. Exploiting this novel design yields a modulator with a measured roll‐off of only 2 dB from low frequencies up to 100 GHz, and with an extrapolated 3 dB bandwidth of 170 GHz. The measured voltage–length product of this subterahertz device is 3.3 V cm. Another device, optimized for a lower voltage–length product of 2.2 V cm, exhibits a 3 dB electro‐optic bandwidth of 84 GHz. The devices are also tested for eight‐level pulse‐amplitude modulation (PAM‐8) and demonstrate data rates of up to 240 Gb s−1 at 80 Gbaud, validating that the modulators are a propitious candidate for next‐generation optical communication systems.