Dual-stage laser stabilization with a frequency-tunable integrated 118 million Q reference cavity disciplined to 780 nm rubidium spectroscopy

Integrated narrow-linewidth lasers in the visible and near-IR are a critical component of next-generation atomic systems for quantum sensing, time keeping, and navigation. Technologies such as low frequency noise lasers that are tunable and referenced to an absolute frequency set by atomic transitions are required for sensing applications such as cold atom interferometers. While bulk-optic reference cavities can be used for laser noise reduction and stabilization, longer-term frequency drifts are mitigated with a secondary lock to an atomic reference using power-consuming bulk optic technologies such as an acousto-optic frequency shifter. Photonic integrated cavities based on ultra-low-loss silicon nitride (SiN) waveguides enable laser stabilization in a wafer-scale integration platform. Incorporating a secondary lock of these integrated cavities to an atomic reference is an attractive solution for compact chip-scale long term stable references. In this paper we demonstrate the use of a thermo-optic tunable, 118 million Q, 0.44 dB/m loss reference cavity for simultaneous laser frequency noise reduction and absolute frequency referencing to 780 nm rubidium spectroscopy. By tuning the integrated cavity resonance by a range >200 MHz using a thermal tuner with 20 MHz/mW efficiency at over 1 kHz tuning rate, we demonstrate long-term locking of the stabilized laser to rubidium saturation absorption spectroscopy. We achieve up to 4 orders magnitude frequency noise reduction, integral linewidth (beta-separation) reduction from 5 MHz (free-running) to 326 kHz (dual lock) and simultaneously a fractional frequency drift of 8.5e-12 at 1 second, two orders of magnitude improvement compared to locking to the cavity only. These results represent a compact and robust laser for photonic integrated atomic systems that can be extended to probing and locking to narrower linewidth transitions such as the rubidium two-photon at 778 nm and for laser frequency control sequences in cold atom experiments.