Scientists at the National Physical Laboratory (NPL) used an optical reference cavity — a well-established method to enhance laser stability — to improve optical storage time in laser frequency stabilization and to cancel spurious stabilization noise. Cavity-stabilized lasers are key tools in optical frequency metrology, with applications ranging from optical atomic clocks to high-precision spectroscopy and gravitational wave detection. The researchers enhanced optical storage time by optimizing the optical cavity design. To achieve a record optical storage time of 300 µs with ultrahigh frequency discrimination, they developed an optical reference cavity measuring 68 cm long. The light trapped between the high-reflectivity mirrors at both ends of the 68-cm-long cavity can travel approximately 100 km — the equivalent of twice the length of the Eurotunnel. This enabled the team to demonstrate exceptional stability and storage in a laser using a lengthy optical reference cavity. Experimental scheme for active RAM cancellation and characterization. Courtesy of Optics Letters (2025). DOI: 10.1364/OL.560815. To block spurious stabilization noise, the team developed a robust scheme to actively cancel residual amplitude modulation (RAM), a source of technical noise that stems from the phase modulation technique required for stabilization. The team focused on reducing the RAM-induced fractional frequency deviation by actively canceling the RAM at its source — an electro-optic modulator (EOM). The researchers mitigated the RAM effect at its source by reducing the RAM spatial inhomogeneity, by limiting etalon effects, and by compensating for the birefringence of the EOM crystal. Fiber coupling provided spatial homogenization of the RAM through propagation inside the fiber. The researchers actively canceled RAM at the 10-7 level in an annealed-proton-exchanged lithium-niobate waveguide EOM. By optimizing the cavity design for low RAM susceptibility and actively canceling the RAM, they reduced the RAM-induced frequency deviations to the 10-19 fractional level. “This has been an interesting challenge to work on, and I’m glad to have contributed towards this improvement in control of residual amplitude modulation, an effect that can seriously limit frequency stabilization if not properly managed,” researcher Adam Parke said. Beyond atomic clocks, which are based on optical transitions, the implications of the research extend across various fields including national timekeeping, positioning, navigation, telecommunications, characterization of laser sources, and fundamental science. Broadly, the work highlights the potential to improve methods of scientific measurement. Researcher Marco Schioppo said, “Since cavity-stabilized lasers are ubiquitous tools in high precision time and frequency measurements, our work will have a broad positive impact on a variety of technological applications and science.” The research was published in Optics Letters (www.doi.org/10.1364/OL.560815).