Low-voltage modulators using thin-film lithium niobate

This paper presents an overview and application of thin-film-lithium-niobate (TFLN) modulators that have low drive voltages, i.e., V_π. Such modulators are critical components for realizing high-speed operation in a modern telecommunication networks, wireless communications, and RF-Photonic applications. Recent developments in crystalion- slice TFLN have enabled a new class of electro-optic modulators that have a tighter mode confinement, compact footprint, ultra-high bandwidth, and low modulating voltages. However, lithium-niobate suffers from difficult microstructuring in comparison to silicon-based materials, since it can have an etch resistance greater than many metal-based hard masks. To overcome this challenge, a hybrid material system combining the electro-optic properties of TFLN with the ultra-low propagation loss of silicon nitride has been developed. In this work, we demonstrate an integrated hybrid phase modulator, based on a silicon-nitride strip loaded waveguide on a TFLN material platform, which provides tight optical mode confinement, without the need to etch lithium-niobate. As a result, the drive electrodes can be placed closer to the optical waveguide thereby resulting in a strong RF and optical mode overlap. A 2.4 cm long phase modulator and Mach-Zehnder modulator with a demonstrated V_π of 1.5 V and 0.875 V, respectively in DC are presented along with other candidate and demonstrated devices, such as multimode interference coupler, micro-ring, and racetrack resonator.