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Large telescopes equipped with adaptive optics require 20-25W CW 589-nm sources with emission linewidths of
~5 MHz. These Guide Star (GS) lasers should also be highly reliable and simple to operate and maintain for many years
at the top of a mountain facility. Under contract from ESO, industrial partners TOPTICA and MPBC are nearing
completion of the development of GS lasers for the ESO VLT, with delivery of the first of four units scheduled for
December 2012. We report on the design and performance of the fully-engineered Pre-Production Unit (PPU), including
system reliability/availability analysis, the successfully-concluded qualification testing, long-term component and system
level tests and long-term maintenance and support planning.
The chosen approach is based on ESO's patented narrow-band Raman Fiber Amplifier (EFRA) technology. A master
oscillator signal from a linearly-polarized TOPTICA 20-mW, 1178-nm CW diode laser, with stabilized emission
frequency and controllable linewidth up to a few MHz, is amplified in an MPBC polarization-maintaining (PM) RFA
pumped by a high-power 1120-nm PM fiber laser. With efficient stimulated Brillouin scattering suppression, an
unprecedented 40W of narrow-band RFA output has been obtained. This is then mode-matched into a resonant-cavity
doubler with a free-spectral-range matching the sodium D2a to D2b separation, allowing simultaneous generation of an
additional frequency component (D2b line) to re-pump the sodium atom electronic population. With this technique, the
return flux can be increased without having to resort to electro-optical modulators and without the risk of introducing
optical wave front distortions. The demonstrated output powers with doubling efficiencies >80% at 589 nm easily exceed
the 20W design goal and require less than 700 W of electrical power.
In summary, the fiber-based guide star lasers provide excellent beam quality and are modular, turn-key, maintenance-free,
reliable, efficient, and ruggedized devices whose compactness allows installation directly onto the launch telescope
structure.
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