Mass-Producible OH-Line Suppression Technologies

Light travels fast - very fast. So fast, in fact, that every second it travels a distance of 300 million metres, or eight times the circumference of the earth. As fast as the speed of light is, light can still take a VERY long time to travel astronomical distances. For example, light takes approximately 4 years to travel from the Sun to our NEAREST star - Proxima Centauri. The fact that even light can take such a long time to travel across the universe means that astronomers are effectively looking back in time when they observe distant objects - the starlight we see on earth may have been emitted from the star billions of years ago. This fact is becoming very useful to physicists, who are now asking themselves some very "Big questions" about how the universe formed and what roles dark matter and dark energy played in the evolution of the universe. To answer these questions, physicists require observations of the very early universe, when the universe was only a few hundred million years old. Luckily, we can obtain these observations - simply by looking at very distant objects.

There is, however, an interesting phenomenon that occurs when we look at very distant objects. The universe is expanding, and what's more, the rate of expansion is increasing. This remarkable fact, which was first observed by Edwin Hubble in the first half of the 20th century, means that light from more distant objects is increasingly "redshifted" through the Doppler effect - the same physical phenomena that makes a retreating siren sound lower pitched than it really is. In the future, astronomers wish to observe objects which are so distant that the light has taken 13 billion years to reach us. This light is extremely redshifted, and the key spectroscopic emission lines, which normally have wavelengths of a few hundered nm, are actually observed by us in the near-infrared at wavelengths > 1 micron. Such high redshifts pose a particular problem for ground based astronomy since the night sky is actually extremely bright in the near-infrared - due to the fluorescence from oxygen-hydrogen (OH) molecules that reside about 90 km high up in the Earth's atmosphere. This fluorescence is actually contained in hundreds of emission lines throughout the near-infrared, making it extremely difficult to detect the light which reaches us from the celestial objects of interest. Luckily, however, the fluorescence lines generated by the Earth's atmosphere are spectrally very narrow - meaning that if they can be reflected by a set of very precise and narrow filters, then we can gain access to the light of interest that lies between the lines.

Previous attempts to develop efficient OH-line filters using traditional optical techniques have proven unsuccessful. Recently, however, a new approach using optical fibre filters known as Fibre Bragg-gratings has been successfully demonstrated on-sky. Currently, however, these filters are prohibitively expensive - with each one costing many tens of £k. This is a particular drawback if these filters are ever to be mass-produced - a capability that would be required if these filters are to be used in large "multi-object" instruments which would provide spectral information for thousands of objects within the telescope image.

The UK is currently leading the development of the technologies which will facilitate the mass-production of OH-line suppression filters. These technologies are based on two routes, the first is Ultrafast Laser Inscription - a revolutionary laser fabrication technique which enables three-dimensional optical circuits to be laser written into glass substrates, the second is based on the use of highly multicore fibres which contain many individual glass channels, each of which can guide light along the fibre. During this project, we will take these technologies from their current proof-of-concept demonstrations, to the point at which they can be confidently designed into the operation of future instruments.

Observations of the early universe are a key requirement for answering many of the "Big questions" in physics. These observations will be facilitated using telescopes to capture the light, and advanced instruments to analyse the light. If successful, the photonic spectral filters we will develop during this project would be a key part of future instruments.

Our filters will be developed using two different technologies. The first is a three-dimensional laser fabrication technique known as Ultrafast Laser Inscription (ULI). The second is using multicore fibres (MCFs). Both of these technologies have numerous commercial applications outside astronomy. For example, MCFs are currently attracting considerable attention for future applications in ultra-high-bandwidth telecommunications, whereas ULI is the only technology which facilitates the fabrication of interconnects for coupling light to and from MCFs. We therefore anticipate that this project will generate significant amounts of industrially relevant Intellectual Property (IP) in both the MCF and ULI areas, with the potential to impact industries such as telecommunications, aerospace, defence and security.

This combination of cutting edge academic research, conducted using advanced technologies and techniques which are themselves cutting edge, but still highly relevant to industry, gives this project significant impact potential in academic, economic and societal areas. We foresee that this project will result in wide-spread impacts. Specifically:

-The project is already well supported by industry (Renishaw PLC and Fianium Ltd). These industry partners are eager for knowledge transfer and we will engage with them on a 6 monthly basis face-to-face to keep them updated on the progress of the project. This will ensure that they are well informed of technology developments, and ensure that opportunities for rapid knowledge exchange are explored and exploited. We are already collaborating with Renishaw PLC on a confidential cancer-diagnosis related application of the technologies we will fully develop during the project.

- It is clear that astronomy is driving the development of new photonic concepts, devices and techniques. These will certainly find commercial applications in the coming years. To ensure that UK industry is well positioned to exploit these opportunities, we will hold a mid-project workshop on "Astrophotonic Technologies". We will also aim to write articles in photonics industry trade journals - where industry is most likely to become aware of them.

- We will train a number of young scientists, both Post-Doctoral Research Associates and Ph.D students. This training will include hands-on laboratory skills in areas such as photonics and electronics, but also in theoretical simulation techniques. These scientists will go on to impact both academic and industry areas - with a potential impact for the wider UK economy.

- The public is naturally excited by astronomy, and this project will contribute to the development of a new class of astronomical instruments which will inform us about the true nature of our universe. We will seek to engage with the public about this research project through a variety of outreach events. These will include public lectures and school lectures, a research website and blog. We will also seek to display our project technologies at events such as the Royal Society Summer Science Exhibition.