Precision Astronomical Spectrographs using Single-Mode Photonic Technologies

光子学是一个与光的产生、操纵和探测有关的科学领域。现代光子技术包括激光和光纤-这些技术已经彻底改变了我们的世界。如果没有光子学，许多我们认为理所当然的技术将是不可能的，包括互联网、DVD和iPhone。但并非所有的光子技术都是平等创造的，有些比其他的更好，即使它们看起来一样。例如，一些光纤是“多模”的，这意味着它们引导的光可以以称为“模式”的各种分布传播。光纤也可以是单模的，这意味着光只能以明确定义的形状传播。这些差异看起来微不足道，但它们非常重要。在电信系统中，单模光纤或多模光纤的使用对光纤传输数据的速度有很大的影响。如果光脉冲沿着多模光纤发送，它会及时传播，因为不同的模式以不同的速度传播-这意味着信息会失真，数据速率必须降低。解决方案是使用单模光纤--因为只有一种“模式”，所以脉冲不能以同样的方式失真。光纤也用于天文学，将光从望远镜传输到仪器，由光谱仪进行分析。目前，几乎所有的天文摄谱仪都使用多模光纤，因为它们有更多的模式，能够从望远镜焦平面收集更多的光。多模光纤的使用并非没有其问题。这些问题在必须非常稳定的非常精确的光谱仪中尤其成问题。这些问题将通过使用单模光纤完全解决，但这将带来不可接受的收集效率降低。这显然带来了一个问题-我们不能只将多模光纤有效地耦合到单模光纤吗？通常情况下，这个问题的答案是否定的，但一种新的光子技术，被称为“光子灯笼”提供了解决方案。光子灯显著地使多模光能够有效地耦合到单模阵列，从而提供两全其美的情况，其中可以提供单模性能并与多模光纤的收集效率相结合。但是，将单模光子技术用于天文摄谱仪的潜在好处并不止于此。通过在单模状态下工作，可以大大增强光谱仪的校准，特别是如果用于校准光谱仪的光来自激光频率梳，这是另一种光子技术，可以在无限期内提供显着的绝对校准精度。因此，STFC财团资助汇集了来自光子学和天文仪器领域的专家，有一个明确而雄心勃勃的总体目标-确定激光频率梳和光子灯是否可以促进天文摄谱仪具有前所未有的性能。为了实现这一目标，我们将进行光子灯和激光频率梳的基础技术研究，以建立性能规格。联合会内的仪器专家将利用这些信息为各种高影响科学案例进行仪器设计研究。还将建立一个光谱仪，以证明这些仪器可以提供模拟所指示的性能水平。如果性能足够令人兴奋，该仪器将在世界级望远镜上进行“天空”测试。如果成功，该项目将为建造超精密天文光谱仪开辟一条全新的道路，用于未来的系外行星科学和宇宙学等领域。

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.