Laser-sculpted optical devices for future giant telescopes

Future telescopes, with mirrors half the size of a football field, will need special components to deal with the light they collect. Astronomers are turning to photonic devices that guide and manipulate light inside specially-designed materials. The greatest potential, which is described in the latest issue of Optics Express, the Optical Society's (OSA) open-access journal, may lie in a laser-based technique that carves out micron-sized light pathways in three dimensions.

Astrophotonics lies at the interface of astronomy and photonics, the optical equivalent of electronics. Photonics has already been validated in astronomy, providing access to some of today's significant astrophysics observations. The field has experienced 10 years of breakthroughs in photonics instrumentation to enable the extent of today's interferometry and measurements. Fingernail-sized chips have been manufactured to manage the beams coming from up to three telescopes of existing interferometric arrays and are being considered for up to a six telescope beam combination in the coming years.

This burgeoning field has emerged over the past decade in response to the increasing demands of astronomical instrumentation. In particular, it has enabled new developments to combine signals from widely spaced telescopes and ultra-high precision spectroscopy to detect planets around nearby stars, among many other things. In essence, astrophotonics will allow optical fibres and photonic devices to carry and process the light from the stars and galaxies that surround us.

According to Joss Bland-Hawthorn and Pierre Kern, the editors of Optics Express' Focus Issue on the subject, 'Modern astronomy is on the verge of another revolution, which is going to be made possible by astrophotonics. For example, much of today's research in astronomy is focused on the detection of faint light from extrasolar planets in orbit around nearby stars and, at the other extreme, the detection of the first star-forming systems in the early universe. You need larger, more robust telescopes - and advancements in astrophotonics - to work on either task. It's really the birth of a new field in the international year of astronomy.'

Indeed, a new paper published in the current issue of Optics Express demonstrates how astrophotonics could be of particular benefit to the next generation of ground telescopes - the extremely large telescopes (ELTs) that will have mirrors 20 metres or larger. These gigantic instruments, like the planned 42-metre European ELT, will have the sensitivity to see galaxies at the edge of the universe, just as they were beginning to form. The ELTs will also be able to extract the age and possible origin of whole populations of stars in our own and nearby galaxies. This 'galactic archaeology' requires collecting the light from many different objects and analysing each of them separately. For an ELT, the number of objects to be simultaneously analysed could be as high as 100,000.

'As things stand, building up-scaled versions of existing instruments would require impossibly stiff materials, impractically large optics and too much money,' says Jeremy Allington-Smith, an astronomer at Durham University in England who co-authored the paper with his colleagues Ajoy Kar and Robert Thomson of Heriot-Watt University in Edinburgh.

Photonics devices can be made small enough to handle the expected demands of an ELT. The U.K. team has studied the potential of ultrafast laser inscription (ULI) as a route to creating astrophotonic devices. This relatively new technique for fabricating compact photonic devices makes use of ultrashort laser pulses - 'the shortest events ever created by humanity,' explains Thomson - a photonics researcher at Heriot-Watt University.

In less than a picosecond, these pulses can deliver peak powers readily in excess of 10 GW. When focused on the interior of a transparent substance, the absorbed energy can alter the structure of the material over tiny, micron-scale regions. Typically, the pulses affect the index of refraction, which describes the speed of light through the material. By changing the index of refraction along a continuous line, researchers have created hair-thin optical waveguides inside a material. 'The majority of the past work in the ULI field has focused on creating relatively simple telecom-type devices such as lasers, amplifiers and splitters, but the time is right to really push the boundaries of what can be fabricated,' Thomson says.

More complex photonic devices could be made for an ELT application. In their paper, the authors describe two potential instruments - a highly dispersive waveguide array to measure the spectrum of the light emitted by celestial objects, and an integrated filter for removing unwanted atmospheric emissions. Traditional manufacturing techniques that are derived from the electronic chip industry are not capable of making such three-dimensional devices, but ULI is able to sculpt them directly out of a glass substrate. The authors admit, however, that the technique is not yet mature. The ULI fabricated light channels still lose quite a bit of light, which prohibits the waveguides from being longer than a few tens of centimetres. Moreover, the waveguides cannot be bent sharply, so devices have to be relatively large to allow for low-loss curves. They and other groups are currently working to solve both problems. It may be possible, for example, to use ULI techniques to etch out tiny mirror surfaces that can be substituted for sharp waveguide bends.

The team plans to continue their research by developing miniature spectrometers and exploring the best way to funnel incoming celestial light into a photonic device that filters out specific emission lines that interfere with astronomical work.