Octave-spanning operation of a photonic integrated coil resonator as a reference cavity

Photonic integrated resonators can be used for laser stabilization and disciplining light to atomic transitions. Such integrated structures hold the potential to augment or replace table-top reference cavities, improve the robustness, lower the cost, and enable portability of precision applications such as atomic quantum sensing and computing, optical clocks, and ultra-low noise microwave generation. Realizing resonators that can span an octave of optical bandwidth will enable stabilization across a wide range of atomic species, quantum architectures, and precision metrology tools. Examples include second-harmonic generation, dual-comb line stabilization, and stability transfer of atomic-disciplined lasers to other lasers operating across large optical bandwidths for applications such as Rydberg sensing. Here, we present an octave-spanning photonic-integrated silicon nitride 4-meter-coil resonator that operates from 780 nm to 1560 nm. The 4-m coil resonator, based on a tunable bus-ring coupler design, provides a spectral resolution of 47 MHz over the octave span with intrinsic quality factors (Q_in) from 24 to 77 million over the octave bandwidth. With the same coil resonator, we demonstrate stabilization of lasers operating at 780 nm, 1320 nm, 1550 nm, and 1560 nm, achieving greater than 2 orders of magnitude frequency noise reduction within the 1-10 kHz offset range and thermorefractive noise-limited performance between 1 kHz and 10 kHz at 1320 nm, 1550 nm, and 1560 nm. Additionally, we demonstrate linewidth stabilization that lowers the 1560 nm, 1550 nm, 1320 nm, and 780 nm 1/π reverse integral linewidths (ILW) by an order of magnitude to 159 Hz, 250 Hz, 289 Hz, and 1 kHz, respectively. The resonator is fabricated in the CMOS-compatible silicon nitride platform that supports a wide range of active and passive components, unlocking the potential to integrate precision and quantum systems on-chip.