FEATURE ASTRONOMY
frontier
The European Southern Observatory’s 4 Laser Guided Star System focused on the sky
Rachel Berkowitz looks at the photonics technology being developed for probing the universe, which in turn is leading to better commercial products
I
n the fifth century BC, Sophocles condemned astronomy as ‘impossible to understand and madness to investigate’. But thanks to advances in optics and photonics,
investigating astronomy is no longer the madness it was thought to be in ancient times. Space exploration and instrumentation developments are helping raise the profile of optics and photonics technology, both in terms of how important it is and what it can do. Novel telescopes and detectors extend the limits to which we can investigate the universe. In turn, these instruments drive technologies that benefit the optics industry.
Adaptive optics
Image distortions plague ground-based telescopes, because incoming light is blurred by the atmosphere. To compensate, historical
18 ELECTRO OPTICS l JUNE 2016
astronomers determined how much aberration the atmosphere caused and how to correct for it, based on light from a reference star with well-known properties. But as telescopes looked further into space, finding a neighbouring bright star became impossible. The modern solution is to create a reference ‘star’ – or a laser guide star – at a known distance and brightness using a high-powered laser. At the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile, an adaptive optics facility, known as the 4 Laser Guide Star Facility (4LGSF) has been used to create a high-power guide star – the ‘SodiumStar’ – whereby lasers excite sodium atoms at the mesosphere’s edge, 80- 100km above Earth’s surface; these fluoresce in a point-like fashion to create the star. ‘It’s like when you have a headlamp; that’s a bright source that lets you sample the atmosphere,’ said physicist and systems engineer Domenico Bonaccini Calia. ‘Adaptive optics lets us look from the ground, as if we were in space.’ In the telescope, light from the guide star is separated from astronomical data and analysed with sensors by measuring aberrations of the wavefront. Because the light travelled the same path as did light from real stars, wavefront aberrations are a direct measurement for atmospheric blurring. A real-time computer
adjusts the deformable mirror in the telescope’s optical train to compensate for measured distortions. ‘Amazingly, the whole control loop allows running up to roughly one kilohertz,’ said Martin Enderlein of Germany-based Toptica Photonics, who along with MPB Communications (MPBC) developed the system. The required wavelength is very difficult to lase. ESO patented a new narrow-band amplification principle, based on inelastic ‘Raman’ scattering of photons, to generate a 40W, 1,178nm, single- mode fibre source, which is then converted into a free-space 589nm source at 22W. The system is turn-key, insensitive to telescope environmental and gravity changes, and able to function reliably without supervision in Chile’s Atacama desert. Toptica and MPCB spent four years developing the product, under contract with ESO and with partial funding from Keck and the European Union. MPBC produced the fibre pump and narrow- band, high-power Raman amplifiers, while Toptica integrated the components and has been the main ESO contractor. ‘Until now, running a laser for adaptive
optics required specialised personnel on site and on shift, attending the laser and adjusting it continuously – a very special operation. Now it’s not a problem anymore. Telescopes will be able
@electrooptics |
www.electrooptics.com
Final
TNO/Henrij Werij
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48