Researchers at the Australian National University (ANU) have developed a new optical chip for a telescope that enables astronomers to have a clear view of alien planets.
With current astronomical instruments, seeing a planet that is close to its host sun, similar to Earth, is very difficult due to the sun’s brightness. The innovative telescope chip cancels light from the host sun, allowing astronomers for the first time to take a clear image of the planet.
A new optical chip for telescopes
“The planar waveguide chip cancels the light from a host star in a distant solar system such that the much weaker light from the planet is not swamped out by the host star light,” explains Dr Steve Madden, associate professor in the ANU Research School of Physics and Engineering. “The cancellation works in a similar way to noise cancelling headphones in that a negative version of the light is added back to the incoming star light and they sum to zero.”
The method was first proposed by Bracewell in 1978 — hence, it is referred to as Bracewell nulling. It is very different from coronagraphy, the method used for direct imaging of exoplanets. In a coronagraph, essentially a physical block is placed where the star appears in the image plane to block its light. Due to diffraction effects, blur, etc., Madden explains the blocking disc has to be somewhat bigger than the star, and light still leaks around at a low level, so planets are typically not visible until they are quite a long way from the star. “This means the reduction in the star light is less than is possible with our chip, and, additionally, our chip can see much closer into the host star, we believe into the so called habitable zone at about the distance earth sits to our sun,” Madden says.
What is fundamentally new about this chip
“Bracewell nulling has been demonstrated before and also in a chip based format,” Madden notes. “However, this was performed at near-infrared wavelengths, where the contrast of the planet to the star is weak.”
The expert explains that forming exoplanets are at temperatures around 1000 degrees Celsius and so emit a lot of heat themselves. Their heat output peaks in the mid-infrared at around a wavelength of 4 microns, so there is a maximum in the contrast to the host star. The same is true for mature, earth-like planets, according to Madden, except their peak emission and contrast are at about 10 microns wavelength. “So you need a Bracewell nuller that works in the mid-infrared to exploit this,” the professor points out. “Unfortunately, there are not very many viable waveguide materials with infrared transmission to 10 microns and beyond, and we have built on our long experience with chalcogenide glasses to do this.” Chalcogenides, known for their ability to transmit up to about 20 microns, are quite difficult to work with, however. “So our chip is, we believe, the first demonstration of a low-loss, high-performance nulling chip in the mid-infrared.”
Promises the new chip holds
Built by PhD student Harry-Dean Kenchington Goldsmith at the ANU Laser Physics Centre, Madden deems this first-generation of chips a key stepping stone: “They will enable initial testing of mid-infrared Bracewell nulling on a telescope, hopefully in the next 12–18 months, in the 4 micron region. This will allow the astronomers to gain experience with the technique and resolve the inevitable complications that will arise.”
The telescope chip, the professor reckons, will also significantly contribute to the scientific debate on exoplanetary formation processes. “Once the technology is established, more complex devices at a variety of wavelengths in the infrared regime will be made, and astronomers can consider taking spectra of exoplanets and trying to find exoplanets with telltale signs of life — specifically ozone, at 9.7 microns in the infrared regime.”
The chip’s potential impact on the development of future light-based technologies
“With advancement in photonics we are slowly replacing optical features that are sometimes heavy and bulky,” Madden says. “If we can replace these to the extent where photonics are as good or better, we can reduce the cost of future space missions with little to no reduction in the science we can achieve.”
The professor further notes that mid-infrared is a spectroscopically very important region here on Earth, too. He says there are many applications in sensing that his and other teams are trying to tap into. “Here, integration enables massive cost reductions, offering the promise of very widespread deployment in industrial, environmental, medical, etc. applications.”
Coming a long way
This telescope chip builds on more than a decade of research on specialized optical materials and devices. The Australian National University initially began using chalcogenide glasses for waveguide chips in applications of ultrafast nonlinear optical processing of telecommunications signal as part of the Australian Research Council funded CUDOS centre of excellence. “Chalcogenides have the largest nonlinearity of any glasses, and this made them attractive for that application,” says Madden, adding that at that time, there were no very-low-loss waveguides in chalcogenide materials, and the glasses were considered to be very unstable. “Almost three years of research effort went into the breakthrough realization of very-low-loss waveguides and then devices that exploited their properties.”
Madden and his colleagues invested much effort in researching the glasses themselves to find ways to stabilize them and to improve their mid-infrared transmission. “This resulted in new glass formulations that are the ones used in the current chip,” the scientist reveals. “We are also currently researching their properties as laser amplifiers, where they have some unique characteristics that could lead to a new range of mid-infrared laser sources.”
Madden and his team have yet to attempt to see any exoplanets. “We would like to begin testing the device interferometically to understand exactly how much starlight we can reduce — and compare that to our models,” the expert says. The team is currently building a test setup to try out the device on a telescope. Though their initial telescope experiments may not aim to discover exoplanets, Madden expects they will likely test the new technology on bright binary systems first.
Written by Sandra Henderson, Research Editor, Novus Light Technologies Today