Precision Astronomical Spectrographs using Single-Mode Photonic Technologies

Photonics is an area of science concerned with the generation, manipulation and detection of light. Modern photonic technologies include lasers and optical fibres - technologies that have revolutionised our world. Without photonics, many technologies we take for granted would be impossible, including the internet, DVDs and the iphone. But not all photonic technologies are created equally, and some are better than others, even when they might appear the same. For example, some optical fibres are "multimode", meaning that the light they guide can propagate in variety of distributions known as "modes". Fibres can also be single-mode, meaning that the light can only propagate with a well-defined shape. These differences may seem trivial, but they are hugely important. In telecommunication systems, the use of either single- or multimode-fibre has a huge effect on the speed with which data that can be transmitted down the fibre. If a pulse of light is sent down a multimode fibre, it spreads out in time because different modes travel at different speeds - this means that the information becomes distorted and the data rate must be reduced. The solution is to use single-mode fibre - since there is only one "mode" then the pulses cannot be distorted in the same way.

Optical fibres are also used in astronomy, to transport the light from the telescope to an instrument for analysis by a spectrograph. Currently, almost all astronomical spectrographs use multimode fibres, which, because they have more modes, are able to collect more light from the telescope focal plane. The use of multimode fibres does not come without its issues. These issues are particularly problematic in very precise spectrographs that have to be exceptionally stable. These issues would be completely solved by using single-mode fibres, but this would come with an unacceptable reduction in collection efficiency. This clearly brings about the question - can we not just efficiently couple the multimode fibre to a single mode fibre? Normally, the answer to this would be no, but a new photonic technology, known as the "photonic-lantern" provides the solution. The photonic-lantern remarkably enables multimode light to be efficiently coupled to an array of single-modes, thus providing a best-of-both-worlds situation, where single-mode performance can be provided and combined with the collection efficiency of multimode fibre. But the potential benefits of using single-mode photonic-technologies for astronomical spectrographs don't stop there. By operating in the single-modes regime, the calibration of the spectrograph can be greatly enhanced, particularly if the light used to calibrate the spectrograph originates from a laser frequency comb, another photonic technology that can provide remarkable absolute calibration accuracy over an indefinite period.

This STFC Consortium Grant thus brings together experts from the fields of photonics and astronomical instrumentation, with one clear and ambitious overall objective - to establish whether laser frequency combs and photonic-lanterns can facilitate astronomical spectrographs with unprecedented performance. To achieve this goal, we will perform basic technology research in photonic-lanterns and laser frequency combs, in order to establish performance specifications. This information will be used by the instrumentation experts within the Consortium to perform design studies of instruments for a variety of high impact science cases. A spectrograph will also be built to demonstrate that these instruments can deliver the performance levels indicated by the simulations. If the performance is sufficiently exciting, the instrument will be tested "on-sky" at a world-class telescope. If successful, this project will open up an entirely new way to building ultra-precise astronomical spectrographs, for future applications in areas such as exoplanetary science and cosmology.

1. Academics
The astrophotonics community: This Consortium project would be one of the first in the world, and certainly the first UK funded project, to specifically bring instrumentation and photonics experts together with one overall primary goal - to conclusively test the assertion that photonic-lantern-fed, laser-frequency-comb calibrated, spectrographs, can enable spectrographs of unprecedented performance. The project would thus have a considerable impact on academics working in the field of astrophotonics, by raising confidence in these emerging technologies, and acting as an example of how the two communities can collaborate.

The instrumentation community: If the project achieves its primary goal, then it will result in a paradigm shift in the design and construction of high-resolution spectrographs. If Project 1 achieves its secondary goals (e.g. mass-producible photonic lanterns for OH-line suppression, waveguide adaptive optics), then the design and construction of many astronomical instruments would be transformed forever. If Project 2 achieves its secondary goals, then broadband calibration of spectrographs from the visible to the mid-IR (~2500 nm) would be a "solved" issue. Thus, it is clear that these goals will result in a significant benefit to the instrumentation community, by demonstrating new technologies that can enable better astronomical measurements. As a UK Consortium, we also foresee a particular benefit to the UK instrumentation community, who will have the leading edge in exploiting the technologies we develop.

Astronomers: In the longer term, our goal as a Consortium is to raise the confidence in the technologies we develop to a sufficient level that we can realistically bid to build a facility class instrument. Such an instrument would, we believe, provide spectroscopic measurements of unprecedented absolute precision and sensitivity, with a clear benefit to astronomers around the world.

The photonics community: The technologies we will demonstrate are themselves cutting-edge in the field of photonics. We therefore anticipate a benefit to researchers in the field of photonics, by demonstrating new capabilities and applications. We will engage with the photonics community through publications in appropriate journals and by giving talks at photonics conferences (e.g. CLEO).

2. Industry and UK PLC
We believe that this project will result in opportunities for knowledge transfer, technology exchange and economic impact. As detailed in the Pathways-to-Impact document, we will ensure that opportunities for knowledge transfer and exploitation are not missed by engaging with industry through an industry-focused workshop, talks and publications at industry focused conferences (e.g. Photonics West and OFC). Intellectual property protection is key to maximise the possibilities for commercial exploitation, and IP protection decisions will be made at project review meetings. A collaboration agreement will be put in place before the project starts.

3. The Public
We see the public as direct beneficiaries, who will be enthused about astronomy and about the applications of photonic technologies in astronomical instruments. Outreach will form an important part of our impact strategy, and we will seek to engage the impact through a wide variety of routes (See Pathways-to-Impact).

4. The Investigators
Thomson (PI) is currently an STFC-AF, who has been working over the last ~5 years to develop new astrophotonic technologies. As a result of his research, and that of Birks, the UK is now a world-leader in the field of astrophotonics, but the time is right for a UK platform that will enable Thomson, Birks and Reid to fully engage with instrumentation experts. This project will provide exactly that platform, enabling the UK to maintain its world-leading position in astrophotonics and the investigators to demonstrate the full potential of single-mode photonics for astronomical instrumentation.