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20 W at 589 nm via frequency doubling of coherently beam combined 2-MHz 1178-nm CW signals amplified in Raman PM fiber amplifiers

We report the recent breakthroughs of our research activities carried out in the framework of the ESO R&D programs to develop 589-nm CW sources for laser guide stars[1] (LGS). Large telescopes equipped with adaptive optics require 20–25 W 589-nm CW light sources with an emission linewidth ≪ 50 MHz. Towards this goal, we have been working [2.3] on the development of high-power narrowband 1178-nm Raman fiber lasers with subsequent frequency doubling to 589 nm. Fiber lasers are an asset and probably the best laser type in remote and difficult operation sites like astronomical observatories. Typically, they are compact, maintenance-free, turn-key, ruggedized devices. Moreover their output beam quality is excellent, an important requirement for LGS. The lasers which we are aiming at are part of Laser Guide Star Facilities, in which the laser beam is projected at 90 km at the Earth's Mesosphere, producing LGS by excitation and optical pumping of mesospheric sodium atoms.