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  • Astrophotonics Shows Promise
Feb 2009
WASHINGTON, Feb. 9, 2009 – The emerging field of astrophotonics could prove particularly beneficial to next-generation extremely large telescopes (ELTs).

Because future ELTs will house mirrors half the size of a football field, atronomers 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 of America’s open-access journal, may lie in a laser-based technique that carves out micron-size light pathways in three dimensions.

Astrophotonics lies at the interface of astronomy and photonics, the optical equivalent of electronics. Photonics already has 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-size 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 ultrahigh-precision spectroscopy to detect planets around nearby stars, among many other things. In essence, astrophotonics will allow optical fibers 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, “Modern astronomy is on the verge of another revolution, which is going to be made possible by astrophotonics. For example, much of todays 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. Its really the birth of a new field in the international year of astronomy.”

Indeed astrophotonics could be of particular benefit to the next generation of ground telescopes – ELTs that will have mirrors 20 m or larger. These gigantic instruments, like the planned 42-m European ELT, will have the sensitivity to see galaxies at the edge of the universe, just as they were beginning to form. The ELTs also will 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 objects and analyzing each of them separately. For an ELT, the number of objects to be simultaneously analyzed 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,” said Jeremy Allington-Smith, an astronomer at Durham University in the UK who, together with colleagues Ajoy Kar and Robert Thomson of Heriot-Watt University in Edinburgh, wrote the paper.

Photonic devices can be made small enough to handle the expected demands of an ELT.

The researchers have 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, and according to Thomson, “the shortest events ever created by humanity.”

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 said.

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 3-D devices, but ULI can 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 centimeters. Moreover, the waveguides cannot be bent sharply, so devices have to be relatively large to allow for low-loss curves.

The researchers 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 its 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.

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The scientific observation of celestial radiation that has reached the vicinity of Earth, and the interpretation of these observations to determine the characteristics of the extraterrestrial bodies and phenomena that have emitted the radiation.
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
An afocal optical device made up of lenses or mirrors, usually with a magnification greater than unity, that renders distant objects more distinct, by enlarging their images on the retina.
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