Lithium Niobate In-Phase/Quadrature Modulator for High-Bandwidth Data Transmission at Cryogenic Temperatures

High-speed optical modulators are essential devices for datalinks using optical interconnects, which offer higher bandwidths and lower power consumption than conventional copper interconnects, and advancements in photonic integrated circuits (PICs) has led to more compact and efficient versions of these modulators. The lower heat dissipation and lower thermal conductivity of optical interconnects also make this technology particularly useful in cryogenic applications, such as data transmission from infrared sensors that need to be cooled to liquid nitrogen temperatures (~77 K) for optimal performance.áWe propose a lithium niobate (LN) in-phase/quadrature (IQ) modulator for high-speed modulation at cryogenic temperatures. Bulk LN has been a popular material for electro-optic modulators (EOMs) due to its high Pockels coefficient, and the recent availability of single-crystal thin film LN-on-insulator (LNOI) wafers has led to an emerging interest in miniaturizing these devices in PICs. The operation of LNOI-based Mach-Zehnder modulators (MZMs) has also recently been demonstrated down to 4 K with 30 Gbps data bandwidths. However, those devices used a simple on-off keying modulation scheme. For this reason, we propose an IQ architecture for more sophisticated modulation schemes like high-order quadrature amplitude modulation, which enables significantly higher data bandwidths while occupying a die area similar to that of an MZM. This architecture will be a dual parallel MZM composed of traveling-wave EOMs fabricated from LN ridge waveguides and coplanar radio frequency (RF) transmission lines, in which the optical group velocity matches the RF phase velocity for efficient electro-optic modulation without the capacitive bandwidth restriction of lumped-element electrodes.The objectives of this project will include characterizing the performance of the proposed IQ modulator at cryogenic temperatures using a liquid nitrogen cold plate under high-vacuum, investigating alternate methods of bias control that limit heat loading, designing high-bandwidth RF connections for electrical integration of the IQ modulator on semiconductor chips, and developing an architecture for combining IQ modulation with wavelength division multiplexing (WDM) for parallelization of data transmission from focal plane arrays. The alternate methods of bias control we propose include electro-optic phase shifting and phase delays in an unbalanced MZM configuration, which would eliminate Joule heating from the resistive microheaters typically used for thermo-optic phase shifting. For integrable RF connections, we propose compensated V-bonds or through silicon vias fabricated with the Bosch process for deep reactive ion etching through the silicon substrate of our LNOI chips. To combine our modulation approach with WDM, we propose to integrate photonic add-drop filters with our IQ modulators to pick off spectral components of the WDM signal via critical coupling to ring resonators.