MUNICH — The first of four laser guide star systems developed by Toptica Photonics AG of Germany and MPB Communications Inc. of Canada has been installed and brought online at the European Southern Observatory's Very Large Telescope in Chile.
Previous systems used to create sodium laser guide stars have relied on power-limited and high-maintenance dye lasers or sum-frequency mixing of solid-state lasers, according to Toptica.
The SodiumStar laser projects a powerful orange beam into the cool air in Chile's Atacama Desert. Courtesy of Julien Girard/ESO.
The new system, called SodiumStar, presents an alternative. The core of the system is based on a narrow-band seed diode laser, a fiber-based Raman fiber amplifier module and a high-efficiency 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 distributed feedback IR 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 percent repumper frequency sideband at 1.7 GHz.
The laser excites sodium atoms in the mesosphere, 90 to 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 farther out in the universe. The fluorescence signal can be used to measure and compensate for these distortions using the adaptive optics of the telescope. In this way, diffraction-limited images of real stars can be obtained with the ground-based telescope.
The system was developed under a €5.2 million (approximately $5.7 million) contract awarded in 2010. Three additional SodiumStar lasers are expected to be installed in the near future.
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