Astrophotonics: measuring up to the universe

03 Jul 2009

Photonics opens up new era for astronomical instruments.

Astronomers must embrace new technology to develop the next generation of astronomical instruments, says Joss Bland-Hawthorn, Federation Fellow at the University of Sydney, Australia. That was the headline message of his keynote presentation, "Astrophotonics: the next wave in observational cosmology", at the LASER World of Photonics Congress in Munich, Germany, last month.

The merging of astronomy and photonics over the last decade has evolved into the new field of astrophotonics, yielding bigger and better astronomical instruments. Current plans include extremely large telescopes (ELTs) measuring up to 42 m in diameter, optical frequency combs to measure spectra with even greater precision, and the use of multimode fibres to collect more light (Optics Express 17 1880). Despite the many technological advances, there remains great scope to further progress measurements beyond the diffraction limit and achieve a major step-up in sensitivity.

"We know an awful lot about the universe," began Bland-Hawthorn, "but there is an awful lot that we don't know." Headline goals, including the detection of extrasolar planets around nearby stars and observation of the first star-forming systems in the early universe, are limited by present capabilities. He believes that incorporating other technologies will help to answer some of the bigger questions surrounding the universe.

ELTs were developed with the aim of maximizing on the so-called D⁴ gain – increasing both the collecting area and angular gain with aperture diameter. However, the performance of these billion-dollar telescopes, such as the planned Giant Magellan Telescope in Chile or the European ELT, is limited by low-frequency atmospheric turbulence. "The largest telescopes will always be here on Earth and so we need to solve for the atmospheric problem," explained Bland-Hawthorn. "The new paradigm is to achieve diffraction-limited performance with adaptive optics."

One example in the field of adaptive optics is the hydroxyl (OH) molecule filter. It is the "most complex filter ever conceived, manufactured and demonstrated", he continued. The filter acts to remove sky background dominated by the bright spectral lines from the OH in the upper atmosphere. It will allow a host of new ideas to be explored, including the observation of early objects in the universe. Demonstrated in 2008, the first instrument that will incorporate the filter as a user facility is set to go live in the next two years.

Fibre technology also plays a key role in this new era of ELTs. "1980s technology has been completely surpassed by fibres," claimed Bland-Hawthorn. Chile's Very Large Telescope Interferometer (VLTI) is "on the verge of spectacular results" by using fibre optics for beam transportation and combination in telescope networks.

Photonic fibre "switchyards" are also of great interest. Again, inspiration has been found outside astronomy. "The telecoms industry has done really well with microelectromechanical systems [which will] have enormous applications in the future," he concluded.