First Light at The Very Large Telescope

TOPTICA’s Laser Guide Star system - dubbed SodiumStar 20/2 - projects a powerful orange beam into the cool air in Chile’s Atacama desert. (Source: ESO, J.Girard)

Toptica’s Laser Guide Star system – dubbed SodiumStar 20/2 – projects a powerful orange beam into the cool air in Chile’s Atacama desert. (Source: ESO, J. Girard)

Recently, the first unit of the 4 Laser Guide Star Facility 4LGSF has been installed at Unit Telescope 4 of ESO’s Very Large Telescope (VLT) at the Paranal Observatory, Chile. A team of Toptica and ESO engineers were able to successfully complete the foreseen set of verification tests, preceding the first light and several on-sky measurements. This is a major step towards the completion of the full Adaptive Optics Facility.

The core of the installed SodiumStar laser system is based on a narrow-band seed diode laser, a fiber-based Raman fiber amplifier module and a high-efficient resonant frequency conversion unit. Raman fiber amplification is a nonlinear interaction between a seed laser and a spectrally shifted broadband pump laser within an optical fiber. The amplifier is fed with the light of a DL DFB infrared diode laser and the output is frequency-doubled to achieve a total average output power of more than 20 W at a wavelength of 589 nm. The laser light has a linewidth of less than 5 MHz and a 10% repumper frequency sideband at 1.7 GHz.

The SodiumStar beam can be followed by the eye along its path through the atmosphere due to Rayleigh scattering, but the visibility vanishes after a few ten kms of height. When it enters the Sodium layer at 100 km altitude, however, it generates the artificial starwanted for the adaptive optics correction. Pointing the guide star at Saturn not only helps to orientate on the night sky but leads to impressive snapshots. (Source: ESO, D. Bonaccini Calia)

The SodiumStar beam can be followed by the eye along its path through the atmosphere due to Rayleigh scattering, but the visibility vanishes after a few ten kms of height. (Source: ESO, D. Bonaccini Calia)

The exceptional parameters of the SodiumStar laser system are prerequisite to efficiently excite Sodium atoms in the mesosphere that is located 90 – 110 km above earth’s surface. The re-emitted fluorescence light of the atoms undergoes the same distortions in the atmosphere as the light emitted from real stars further out in the universe. Therefore, the fluorescence light of the Sodium atoms can be used to measure and compensate for the distortions with the adaptive optics of the telescope. In this way, diffraction-limited images of real stars can be obtained with the ground-based telescope.

Toptica’s SodiumStar laser is a compact and reliable turnkey system that has been developed in collaboration with the Canadian MPB Communications. It is a unique alternative to power-limited and maintenance-demanding dye lasers or sum-frequency mixing of solid state lasers that was used to create sodium laser guide stars in the past. The installation of three additional SodiumStar lasers is planned to complete the 4LGSF in the near future.

Dr. Steffan Lewis, the ESO responsible of the Toptica contract: “I would like to explicitly thank the team of Toptica and MPB for their major contribution to this milestone.” “We are proud and happy to contribute to this major achievement of the 4LGSF ESO program, and to see the technology now broadly adopted also by other major telescopes.” said Dr. Wilhelm Kaenders, President of Toptica.” (Source: Toptica)

Links: Toptica

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