Scientists at NPL recently published findings on laser frequency stabilisation, demonstrating an unprecedented level of performance using an optical reference cavity. This advancement features a beyond state-of-the-art optical storage time and a novel approach to actively cancel spurious stabilisation noise.
Frequency stabilisation of lasers to optical reference cavities is a well-established method for achieving superior stability. The recent work significantly reduces technical stabilisation noise, enabling the realisation of lasers with enhanced stability performance. 
The team developed an optical reference cavity measuring an extraordinary 68 cm in length, achieving a record optical storage time of 300 microseconds. To put this achievement into perspective, the light trapped between the high reflectivity mirrors at either end of the 68 cm cavity can travel approximately 100 kilometres, equivalent to twice the length of the Eurotunnel.
In addition to the advancements in cavity design, the researchers also tackled the challenge of spurious stabilisation noise. They successfully implemented a technique to actively cancel a source of technical noise known as Residual Amplitude Modulation (RAM), which arises from the phase modulation technique required for stabilisation.
This innovative work paves the way for the development of more stable lasers, which will enhance the performance of optical clocks—the next generation of atomic clocks based on optical transitions. The implications of this research extend across various fields, including national timekeeping, positioning, navigation, telecommunications, characterisation of laser sources, and fundamental science.
The findings underscore the potential for improved measurement capabilities, which could lead to significant advancements in technology and scientific research.
Marco Schioppo, principal scientist, said: we are glad to share these results on improved laser frequency stabilisation to optical cavities to enable the development of better and better lasers.
Since cavity stabilised 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.
Adam L. Parke, assistant scientist, said: 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 stabilisation if not properly managed.
The publication has been selected as “Editors’ Pick” by the journal Optics Letters, recognition given to highlight articles with excellent scientific quality and is available here https://doi.org/10.1364/OL.560815.
Picture 1 shows the 68 cm long optical reference cavity developed at NPL and used for this publication. Photo by Andrew Brookes.
Picture 2 shows of Adam (front) and Marco (back) in their laboratory at NPL. Photo by David Severn.