Broadband acousto-optic modulators on Silicon Nitride

Thin silicon nitride integrated photonics platforms rely on weakly confining waveguides and thick oxide cladding layers to enable ultra-low-optical losses on chip. Due to these favorable properties, this technology is seeing increased use in chip-scale systems tasked with manipulating and controlling classical and quantum optical systems. Phase-modulators play a crucial role in the low-noise control loops of such systems, but optically broadband phase modulators remain elusive as the same properties used to enable ultra-low-optical losses make interaction with the optical mode difficult. Here we show an unreleased and optically broadband acousto-optic modulator architecture on this platform enabled by long modulation lengths in a compact spiral structure. These devices achieve a V_π of 8.98 V at a modulation frequency of 704 MHz across an optical bandwidth exceeding 90 nm at telecom wavelengths. We demonstrate their use as part of the control loop in an optomechanical sensing system.