Enhanced steering efficiency through substrate removal in 1D optical phased arrays

The thermo-optic effect allows for the modulation of phase with very low induced optical losses in otherwise passive materials used for integrated optical phased arrays. A key drawback of this effect is the required electrical power, typically requiring tens or even hundreds of milliwatts to induce a sufficient phase shift in traditional waveguide materials such as silicon. In this work we describe a foundry-implemented method to increase the efficiency of silicon waveguide-integrated thermo-optic phase shifters by more than an order of magnitude compared to traditional devices. By reducing paths of conductive heat loss through the use of a thermal isolation trench, we experimentally demonstrate efficiency increases of 19x in Si waveguide thermo-optic phase shifting efficiency. We extend this application to OPAs and observe a 15x increase in beam steering efficiency in an 8 channel integrated OPA as well as an effective field of view extension in a wide-pitch 32 channel OPA. We also measure the modulation bandwidth and find a correlation between steering efficiency gains and device speed. Detailed finite-element simulations show excellent agreement with measurements. These simulations provide unique insights into the thermal behavior of these devices and inform tradeoffs between phase shift efficiency and temporal response in future device design.