RFA-based 589-nm guide star lasers for ESO VLT: a paradigm shift in performance, operational simplicity, reliability, and maintenance

摘要

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 yearsat the top of a mountain facility. Under contract from ESO, industrial partners TOPTICA and MPBC are nearingcompletion of the development of GS lasers for the ESO VLT, with delivery of the first of four units scheduled forDecember 2012. We report on the design and performance of the fully-engineered Pre-Production Unit (PPU), includingsystem reliability/availability analysis, the successfully-concluded qualification testing, long-term component and systemlevel 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 masteroscillator signal from a linearly-polarized TOPTICA 20-mW, 1178-nm CW diode laser, with stabilized emissionfrequency and controllable linewidth up to a few MHz, is amplified in an MPBC polarization-maintaining (PM) RFApumped by a high-power 1120-nm PM fiber laser. With efficient stimulated Brillouin scattering suppression, anunprecedented 40W of narrow-band RFA output has been obtained. This is then mode-matched into a resonant-cavitydoubler with a free-spectral-range matching the sodium Dsub2/suba to Dsub2/subb separation, allowing simultaneous generation of anadditional frequency component (Dsub2/subb line) to re-pump the sodium atom electronic population. With this technique, thereturn flux can be increased without having to resort to electro-optical modulators and without the risk of introducingoptical wave front distortions. The demonstrated output powers with doubling efficiencies &80% at 589 nm easily exceedthe 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 telescopestructure.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.