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  • NPL, UKSA and ESA develop optical clocks for future space missions

NPL, the UK Space Agency (UKSA) and the European Space Agency (ESA) are developing ultra-stable lasers and optical clocks for future space missions

Optical cavity-stabilised lasers and optical clocks are unique tools that can improve satellite navigation systems, Earth Observation and large-scale tests of physics

Scientists at the National Physical Laboratory (NPL) are working in collaboration with the European Space Agency (ESA) and UK and European partners to develop optical clocks in space for future improved navigation and timing.

Laser frequencies referenced to ultra-stable cavities are key components of optical clocks, and together comprise unique tools that have the potential to improve satellite navigation systems and measurements of the changes in the Earth’s gravity field, as well as test fundamental theories of physics, including searches for dark matter and unifying gravity and quantum mechanics.

NPL is working on several parallel-running ESA contracted and UK Space Agency (UKSA) funded activities focussed on these different applications, all with the aim, in each case, of increasing the Technology Readiness Level (TRL), which is an assessment of the functional maturity of the NPL cubic cavity for operation in the harsh environment of space. The NPL cubic cavity can operate in any orientation and has recently demonstrated TRL6 in vibration, shock and irradiation tests that emulate the space environmental conditions associated with launch and deployment in orbit, which is a significant step towards instrument space qualification. These projects are mainly collaborative with European and UK partners, particularly involving large-scale space integrator companies.

The patented NPL cubic cavity design makes the optical cavity frequency stability highly insensitive to vibrations. It is uniquely robust and can reduce the spectral linewidth of commercial laser systems from several MHz to below 1 Hz. This provides for ultra-stable lasers that can be used either as stand-alone frequency references or as sub-components of optical atomic clocks.

There is significant interest in incorporating such optical atomic clock and ultra-stable laser technology in applications in future Science (fundamental physics and cosmology), Earth Observation (relativistic geodesy) and Navigation (future GNSS) Space Agency programmes. For the latter, there are possibilities for optical clocks and cavity-stabilised lasers in a few-satellite low-Earth-orbit (LEO) constellation to provide faster updates to MEO constellation clocks, with less dependence on atmospherically limited orbit data uploads, from multiple ground stations.

In the NASA / ESA Next Generation Gravity Mission, NPL’s cubic cavity can be used to measure the Earth’s gravity field as a function of position on the Earth’s surface. This mission will consist of two satellites orbiting the Earth in LEO and separated by ~ 200 km. Changes in the distance between the satellites, measured using laser interferometry, reflect changes in the geoid level (nominal elevation from sea-level) due to changing land mass as the first satellite passes. The ultra-stable laser needed for this can be stabilised using the NPL cubic cavity. In the polar regions, this technique could be used to monitor glacial changes more accurately than was possible with the previous GRACE and GOCE missions. This data also feeds into climate change predictions and provides information that enables policy makers to adopt appropriate mitigation and adaption strategies.

In the future NASA / ESA 2030s Laser Interferometer Space Antenna (LISA) mission, NPL cubic cavity-stabilised lasers can be used as references for space-based gravitational wave measurements. The cavity will provide a short-term frequency reference in both ground support equipment and potentially in the space deployment. The antenna will be the largest space-based instrument conceived and launched into space, with three space craft in triangular formation separated from each other by 2.5 million km, a million times longer than ground-based gravitational wave interferometers. Each craft will house ultra-stable lasers and detectors, with a centralised laser frequency stabilisation system such as the NPL cubic cavity within the spacecrafts.

Patrick Gill, Senior NPL Fellow, states, “With the unique vibration-insensitive cubic cavity technology reaching space flight-qualified technology readiness levels of TRL8, its incorporation into space-based ultra-stable lasers and optical clocks should give better insights into our understanding of fundamental physics, such as whether fundamental constants (eg the fine structure constant or the speed of light) are changing over time or space. Overall, there are great opportunities for cubic cavity-stabilised lasers in space covering a wide range of different applications over the next two decades, from fundamental physics and climate change to improved satellite navigation, which are all important in strengthening the UK’s position as a global space science and technology innovator."

Cyrus Larijani, Strategic Business Development Manager for Space and Nuclear at NPL, states, “The NPL cubic cavity is the result of years of successful research by the team at NPL and I am delighted that it is now forming the foundation of our collaboration with international partners to develop the next generation of optical clocks. These projects form part of our overall strategy to deliver atomic timekeeping that underpins the technologies that are part of our daily lives. They illustrate the potential applications of optical clocks in space and are a great example of their impact on real life challenges such as improving navigation and monitoring climate change.”

Tony Forsythe, Head of Space Technology at the UK Space Agency, states, “Britain is already a leader in using space technology to understand and tackle climate change and the development of these optical clocks will improve our capabilities in this crucial area. We are proud to support NPL on this project, alongside other important climate change missions. While climate change is at the forefront of our minds, particularly as the UK gets ready to host the major UN climate summit, COP26, these clocks also deliver other benefits for us here on Earth, including improved navigation services.”

Eamonn Murphy, Laser and Optoelectronics Systems Engineer, ESA, states, “There has been pivotal program support provided by the ESA General Support Technology Programme (GSTP) over the last years, for NPL and the subsequent OSRC industrial team. TEC-MME, as part of the Technical Directorate in ESA, has provided continuous technical guidance, support and leadership culminating in the achievement of TRL 6 at sub-system level by the OSRC industrial team. Ideally, this development will now evolve into an industrial product, which can then be successfully implemented into relevant future ESA missions."

01 Nov 2021