An ultrafast laser-based technique that carves out micron-sized light pathways in three dimensions may be the answer to the challenges posed by future telescopes. A paper published in the current issue of Optics Express demonstrates how "astrophotonics" (where astronomy and photonics meet, according to OSA, the Optical Society) could be of particular benefit to the next generation of ground telescopes--the extremely large telescopes (ELTs) that will have mirrors 20 meters or larger.
These gigantic instruments, like the planned 42-meter 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 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," 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 centimeters. 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 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.
For more information, see the paper, Ultrafast laser inscription: an enabling technology for astrophotonics, in Optics Express, part of the publication's special focus on astrophotonics.
