Mass-Producible OH-Line Suppression Technologies

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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.