Microspectrographs enable deeper space exploration

As telescopes get bigger and bigger – and more complex – engineers and astronomers recognize that simply scaling up existing technology would result in instruments that are limited by size and complexity. Integrated devices for multiobject spectroscopy could help, according to researchers at the University of Sydney.

The university’s astrophotonics research group is working on a solution: an optical device with a variety of functions that can be produced on a chip as small as tens of millimeters. Several single-mode integrated optics systems have been shown to work with existing interferometers, the team reported, adding that these systems offer better interferometer resolution, increased flexibility, better instrument stability, easy installation, and reduced overall cost and complexity.

The group, led by professor Joss Bland-Hawthorn of the Sydney Institute of Astronomy in the School of Physics, also is part of the Institute for Photonics and Optical Science and the Consortium for Australian Astrophotonics, which includes the Anglo-Australian Observatory and Macquarie University.

Joss Bland-Hawthorn holds a microspectrograph developed by his research group. He leads the astrophotonics team at the Institute of Astronomy in the School of Physics at the University of Sydney in Australia. Courtesy of Alison Muir, University of Sydney.

Bland-Hawthorn’s team is investigating various devices, including ultrabroadband fiber Bragg gratings – 1050 to 1750 nm – that knock out approximately 200 narrow frequencies.

“These are by far the most complex filters ever demonstrated,” he noted. Most filters simply isolate a region of the spectrum, but these filters go much further, he added. “They knock out hundreds of emission lines produced by the Earth’s atmosphere so that the sky appears far darker.”

The team also is working on multi-mode to single-mode converters (photonic lanterns) that allow single-mode action in a multimode fiber developed with the University of Bath. This is a world first, according to Bland-Hawthorn, who is also an Australian Research Council federation fellow.

“We are building an astronomical instrument called Gnosis based on this technology and incorporating the special filters,” he said. “The instrument will allow Australian astronomers to peer much further into the universe and to detect the most distant galaxies in the early universe.

“We can feed these multimode converters into an array waveguide such that we can accurately disperse the input light from an incoherent multimoded source with a single-mode photonic grating; e.g., array waveguide grating. We call this device PIMMS#1 and are building a device to demo at the telescope. You end up with a microspectrograph which outperforms existing technologies.”

This is a schematic drawing of the PIMMS#1 instrument concept. Courtesy of Joss Bland-Hawthorn, University of Sydney.

The figure for one array waveguide illustrates this. “The telescope light is imaged onto the front face of the lantern, which splits the light into seven single-mode fibers (in practice, the output number can be much larger),” Bland-Hawthorn said. “The light from these single-mode fibers then passes through an array waveguide before being dispersed into little spectra at the detector.

“The truly remarkable feature of PIMMS#1 is that the instrument is diffraction-limited regardless of the illumination pattern falling onto the lantern.” This means that it can be made very compact regardless of the size of the telescope with which it will be used. “With a little help from industry, the cost of instruments on the new generation of extremely large telescopes could be greatly reduced,” he noted.

Bland-Hawthorn is excited about microspectrographs and their versatility. “One spectrograph fits all telescopes,” he said, “and it’s tiny compared to conventional instruments.”

The PIMMS paradigm could lead to an array of possible future devices for a range of uses. The team is working to develop devices that will find wide application in oceanographic, atmospheric and space sciences. “In the area of remote sensing, we foresee arrays of small devices that can be carried by robots, nano-/microsatellites, dirigibles, balloons, submersibles and so forth,” he said.

He believes that microspectrographs will allow smaller university departments and observatories to start designing and building their own instruments without relying as heavily on support from major observatories and associated funding streams, as they have in the past.

The researchers also are developing imaging fiber bundles with 90 percent fill fraction. “The best to date has been about 30 percent,” Bland-Hawthorn said. “These will be moved around at the telescope with robots so that we can make images of hundreds of sources at a time.”

Systems operating with multimode fibers are in development in various research groups, the team noted, and future integrated optics projects are expected to involve multipurpose devices that can handle spectroscopy and other applications.