Optical Single-Sideband Modulator for On-chip Frequency Shifting in the Near-Infrared

Recent development of atomic sensors, such as atomic accelerometers and optical clocks, has led to a growing interest in producing photonic integrated circuits (PICs) to reduce the size, weight, and power of laser systems used in these applications. Optical frequency shifters in the form of integrated electro-optic modulators are essential components for these PICs that enable precise frequency tuning of laser inputs to the atomic transitions, but modulation can also produce spurious sidebands that cause parasitic transitions and light shifts in atomic sensors. To address this challenge, we propose a dual parallel Mach-Zehnder modulator (MZM) fabricated with lithium niobate (LN) waveguides for frequency shifting via single-sideband modulation. Each MZM branch will be a travelling-wave electro-optic modulator composed of a radio frequency (RF) transmission line with the RF phase velocity matched to the optical group velocity in the waveguide, which produces efficient electro-optic modulation without the capacitive bandwidth restriction of lumped-element electrodes. Similar devices have been demonstrated with greater than 20 dB sideband suppression and low electrical power draw, but the commercially available dual parallel MZMs are fiber-coupled devices typically optimized for 1550 nm light. Therefore, our approach will involve optimizing the RF transmission lines and LN waveguides of a dual parallel MZM for near-infrared (NIR) wavelengths, such as 780 nm used in rubidium-based sensors. However, dual parallel MZMs are limited to suppressing only three neighboring sidebands. Therefore, we also propose to analyze a higher-order MZM design for suppression of up to seven neighboring sidebands, and investigate the feasibility of integrating a tunable add-drop filter that tracks the output of the frequency shifter for further sideband suppression. To evaluate the merits of different designs, we will calculate how persistent high-order sidebands could degrade the performance of an atomic sensor, using the coherent population trapping clock as a model system. Furthermore, we plan to investigate the feasibility of integrating the frequency shifter into silicon nitride (SiN) PICs via heterogeneous integration of LN using transition tapers that impedance match optical modes between the SiN and LN waveguides.The Phase I Base efforts will be focused on analyzing different MZM-based designs by calculating the sideband spectrum, optimizing the RF transmission lines and LN waveguides using electromagnetic simulations, and fabricating MZMs to test the optimized components. The Phase I Option efforts will be directed towards simulating, fabricating, and testing the transition tapers for heterogeneous integration, as well as the add-drop filter. Overall, the results of Phase I will allow us to predict the sideband suppression, electrical power draw, optical power handling, and modulation bandwidth of prototypes to be developed and performance tested during Phase II.