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. Here, this trade-off is alleviated by placing the optical waveguides non-symmetrically 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 sub-terahertz 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 phase-amplitude modulation (PAM-8) and demonstrate data rates of up to 240 Gbit/s at 80 Gbaud, validating that the modulators are a propitious candidate for next-generation optical communication systems.