Laser Prizes

Clear sight for large telescopes – solid-state laser with tunable wavelength enables breakthrough in laser guide star technology

The Guide Star Alliance, development teams working under Dr. Domenico Bonaccini Calia, Dr. Wilhelm Kaenders and Dr. Wallace Clements at the European Southern Observatory (ESO) in Garching, TOPTICA Photonics AG in Gräfelfing and MPB Communications Inc. in Montreal, receive the third prize of the 2016 Berthold Leibinger Innovationspreis.

The view of the night sky from the Cerro Paranal is just as breathtaking as Chile’s Atacama Desert itself. Here the atmosphere is dry and the air currents only weak; the stars seem close enough to touch. Thus, on a plateau 2635 meters above sea level, the astronomers of the European Organization for Astronomical Research in the Southern Hemisphere ESO have set up the VLT, or Very Large Telescope. To be able to take sharper images comparable to those that, up until now, have only been possible from the Hubble telescope in outer space, the scientists devised a trick called guide star technology. It uses the light of bright stars to recognize turbulences caused by air movements and corrects these by means of so-called adaptive, i.e. dynamically adjustable, optics in real time.

One requirement for guide star technology is thus the presence of a bright star in the telescope’s field of view. However, the deeper one looks into space and thus into the history of the universe, the smaller are the celestial regions observed and the fewer the number of stars of sufficient brightness. But a solution for this was quickly found. So-called laser guide stars are artificial stars located at the edge of the atmosphere that have been generated with laser beams at any given spot in the sky. Strong laser beams excite sodium atoms 90 kilometers above the Earth, debris of meteorites that combusted as they entered the Earth’s atmosphere, thereby constantly adding to this layer.

In contrast, it was more difficult to produce the laser beam needed to excite sodium D lines at 589 nanometers. Dye lasers were used early on in the development process, but this beam source is not easy to work with since the dyes that produce the laser light have to be dissolved in a liquid. They have limited power, require a great deal of maintenance and cannot be integrated and moved together with the telescope.

It thus became clear to ESO that a new beam source would be needed for its new laser guide star system for the VLT. The team of the Italian scientist Dr. Domenico Bonaccini Calia developed and patented the fundamental elements for this new beam source technology at ESO headquarters in Garching. Their idea: producing the necessary wavelength with diode and fiber lasers by means of so-called Raman amplification. In simplified terms, the light of an infrared laser diode is amplified in glass fiber at a wavelength of about 1178 nanometers and then doubled in frequency. What may sound quite simple at first – at least for a physicist – is actually extremely difficult to achieve in technical terms. The output power needs to lie well above 20 watts, the linewidth must be small, its stability high, the exit beam as parallel as physically possible and the entire system largely maintenance-free. In all, its development took eight years as of the first trials at ESO. This was followed by significant patented developments, such as the integration of a repumper, under the commercial leadership of TOPTICA. Finally, the Laser Guide Star Alliance managed to develop a suitable beam source that met industrial standards. A dream came true for the astronomers.

The great moment took place on April 26, 2016: four of the beam sources had already been mounted on one of the telescopes of the VLT and cast their first laser beams into the sky at "first light". It was a fantastic event and an indescribable sight for all involved. These new beam sources will now be integrated into all adaptive-optics telescopes currently either under construction or in the planning stages.

In addition to their role in laser guide star technology, the beam sources can also be used for other applications. Depending upon the flexibility of the Raman amplification, they enable the precise and tunable emission of many other visible wavelengths. In addition to other space applications such as tracking and following the course of satellites and space debris using LIDAR, which is the light counterpart to the radar, Dr. Wilhelm Kaenders, CEO of TOPTICA, and Dr. Wallace Clements, Head of Development at MPB Communications, are currently considering applications for their beam source in atomic spectroscopy and photon imaging processes.


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