Eyeglass for the exoplanet
(appeared in Dec 2016)

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Slivers of glass that work like microchips may help astronomers get past the glare of distant suns, says S.Ananthanarayanan.

There has been an explosion in the discovery of planets in solar systems other than our own during the last few decades. While a brace of methods have been devised to glimpse these specks of light hidden by the brightness of the parent star, there are challenges in viewing images in the mid infra red, where information about the atmosphere of the planet can be found. A team of researchers in Australia report a miniature optical arrangement that could help sight a planet in this region of the spectrum as if the parent star were not there!

H-D. Kenchington Goldsmith, N. Cvetojevic, M. Ireland, P. Ma, P. Tuthill, B. Eggleton, J.S. Lawrence, S. Debbarma, B. Luther-Davies and S. J. Madden, from the Australian National University, the Australian Astronomical Observatory, the University of Sidney and the Macquarie University have just presented at the Congress of the Australian Institute of Physics, Brisbane, their success in manipulating light from a distant planetary system with the help of a device made of a special glass that is transparent to relatively longer wavelength IR radiation.

As the glare of the parent star prevents the dim, reflected light from a planet being seen by observers, the first method of detecting exoplanets has been indirect and ‘in the blind’. The movement of the planet in orbit results in a definite, if lesser, movement of the mother star too. This movement, back and forth, of the luminous star can be detected at the earth because of shift, the Doppler shift, in the colour of the light that comes from the star. The extent of the shift, and also the frequency of its reversal, enables an estimate of the orbit and the size of the planet.

The next method devised was by watching for the slight dip in the intensity of the light from the star when the planet goes across between the star and the earth. As we now have very sensitive instruments that can measure a drop in intensity, this method reveals the time the planet takes to go around and also an estimate of its size.

A more fancy method of seeing exoplanets is by blanking out the light from the star. One proposal was to launch a large, circular screen into orbit around the earth and then positioning the screen so that it exactly covers the disk of the star, to block the glare. The surrounding planets could then be expected to burst into view, as the background falls dark. This plan, unfortunately, was still-born, as there is a property of light waves that prevents a circular disk from acting like a screen. The outer rim of the disk, which is lit, acts as a circular source of light and focuses the starlight along the line where the telescope lies, and the brightness is not abated. A work-around has been to provide the opaque disk with a scalloped rim, so that half the light waves arriving at the centre travels a greater or shorter distance and finds itself ‘out of phase’ and hence cancels the other half. The method, however, is yet to be tried.

In the infra red

While these are methods in respect of visible light, a great interest, in the case of distant heavenly bodies and exoplanets, is to view them in long wavelength radiation, like the infra red. This is of interest for two reasons. One is that it is often the longer waves alone that survive, because light of shorter wavelength gets scattered. The other, more to the point in the quest for more proximate exoplanets, is that the chemical signature of gases in the atmosphere of the planets lies in the infra red, particularly the mid infra red region of the spectrum of light. A serious interest is to look for signs of the gas, ozone, in the planets’ atmosphere, as the presence of ozone indicates the possibility that the planet harbours living organisms and even intelligent life.

A major issue while dealing with long wavelength light is that getting images of good resolution calls for huge light collection, in the form of a very large objective mirror of the telescope. For the longer IR waves, the only way is to use multiple telescopes separated by large distances and then combine the images, to mimic a large collection area. With these techniques, however, the phase, or stage of vibration of the light waves at different telescopes needs to be managed. Very sensitive devices that integrate light of shorter wavelengths, like visible light, with micro-electronics have been developed, but this has not been possible with infra red light, mainly because most materials are opaque to infra red light. In fact, a method, just reported by a group in Limoges, in France, of converting mid IR waves to near IR or visible radiation, without disturbing the phase, is seen as a welcome possibility of being able to use visible light technology to deal with mid IR data.

This is the context in which the researchers in Australia announce their work of fabricating Photonic Integrated Circuits, which are self-contained assemblies that take in, process, compare and combine infra red light signals, in the same way that large computing ability is built into electronic ICs. With the help of the Photonic IC, in fact, the group is able to implement yet another way of blocking out the glare of the mother star and hence see more clearly the image of the exoplanet in infra red light.

Neutralising glare

The method used is similar to the adaptive optics, which optical telescopes use to make out a dim star in visible light. The feeble light wave front from a dim source gets distorted by variations in the composition of the earth’s atmosphere, before the light gets down to the telescope. What is then done is that a bright, nearby star is also sighted and the distortion of its otherwise strong image is measured. A low intensity light signal that is the exact opposite of the distortion is then generated and added to the feeble signal. This cancels the distortion and the feeble image can be correctly formed. Team member S J Madden explained that this is similar to what is done in noise cancelling headphones.

In the case of sighting exoplanets, the IR signal of the mother star is the dominating signal that is received. The faint signal from the exoplanet is also there, and invisible, but this signal is generally out of phase with the main signal, both because of its position and the fact that the exoplanet is in motion. Now, if a signal which is the inverse of the signal received is generated, and this is from the image formed by neighbouring telescopes, this will blank out the light coming from the mother star, but not the faint light from the exoplanet, which would then become ‘visible’, in IR light. This is the process, which was proposed by Ronald N Bracewell of Stanford University in 1978, which has now been implemented by the new photonic Integrated Circuit.

While silica based photonic chips have been useful in integrating photonic optics, these are opaque beyond the near infra red. A material called Chalcogenide glass, glass which contain the elements, sulphur, selenium, tellurium, has been found to be transparent well down to the mid infra red region. The group of researchers thus constructed optical units of Chalcogenide glass and integrated the functions of phase-shifting the main, infra red image for superimposition and blanking out of the strong, stellar signal. During trials, the new ‘Multi Mode Interference’ coupler demonstrated effective ‘nulling’ of stellar glare and revealing the weaker, reflected signal, in the mid IR, which opens up a great, new research area of investigation of the atmosphere or surface of exoplanets, many of which have been found to be ‘earth-like’.


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