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.