Design and fabrication of broadband thin-film lithium niobate phase modulators
Date
2018
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Despite the present ubiquity of lithium niobate (LiNbO3) in fiber-optic telecommunications and its attractive nonlinear electro-optic (EO) properties, the evolution of LiNbO3 integrated photonics lags behind that of its silicon (Si) and III-V material platforms. Discrete devices fabricated in bulk single-crystal LiNbO3 generally rely on low-index-contrast optical waveguides with large bend radii [1] and specialized micromachining processes to sustain broadband operation [2], which inhibits dense integration. A number of existing thin-film LiNbO3 devices, however, demonstrate that there is potential for further development and suggest a way to surmount some of the disadvantages inherent to bulk LiNbO3. With the recent widespread availability of full 75 mm wafers of crystal-ion-sliced (CIS) thin-film LiNbO3 from a number of distributors, there has been a plethora of LiNbO3 device research and innovation since the early 2010s [3], [4]. ☐ Notable devices that leverage the high-index-contrast provided by a thin LiNbO3 device layer include tunable ring resonators [5], Mach-Zehnder interferometers [6], switches [7], and standalone phase modulators [6], [8]–[10]. Developed in parallel to these devices are various hybrid devices that rely on either Si [11]–[16] or silicon nitride (Si3N4) [15], [17], [18] for loading and guiding an optical mode. A common feature of all devices mentioned herein is reduced optical mode size. This reduced mode size leads to devices with vastly improved electro-optic activity over their bulk predecessors, resulting most notably in reduced modulator half-wave voltages. Coupling the reduced half-wave voltage length products with the ability to bend and fold the high-index-contrast optical waveguides results in a substantially decreased device footprint ideal for future integrated photonic systems. ☐ The focus of this work is on broadband phase modulators in thin-film LiNbO3 which until now have not been investigated outside of bulk LiNbO3. The first challenge is to obtain thin films of LiNbO3 with a consistent sub-10 µm thickness. Mechanical thinning is investigated and implemented; however, thickness variation on the order of 2 to 3 µm results in inconsistent velocity matching and therefore reduced bandwidths in fabricated devices. This variation necessitates the development of a tuning method that allows broadband mechanically-thinned devices to be repeatably produced. After investigation of mechanically-thinned substrates, this work focuses on CIS-based phase modulators. In CIS substrates, a high-conversion-efficiency device is developed which exhibits a measured EO response up to an unprecedented 500 GHz.