The strong electro-optic properties of lithium niobate have long established optical modulators based on the material as the technological backbone for long-haul telecommunication networks. The recent advancements in thin-film lithium niobate (TFLN) permit the fabrication of compact waveguide structures, to produce low-voltage and high-bandwidth modulators, as well as provide potential integration with silicon photonics. Here we propose the use of cascaded apodized gratings to enhance the optical group index of the modulator. By cascading inward, and outward Bragg gratings etched directly into the TFLN waveguide, an optical passband is established between the two Bragg frequencies of the cascaded grating sections. The group index of the passband can be engineered to provide an enhancement to the modulation efficiency by effectively increasing the device interaction length between the optical field, and the RF field. We demonstrate the potential for a 5x reduction in drive voltage while maintaining greater than a 5 nm optical bandwidth.
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