Design and Performance of Raman Fiber Amplifier Based 589-nm Guide Star Lasers for ESO VLT and Their Suitability for Future ELT AO Systems
V. Karpov (1), V. Protopopov (1), W. Clements (1), W. G. Kaenders (2), A .Friedenauer (2), B. Ernstberger (2), W. Hackenberg (3), D. Bonaccini Calia (3), St. A. Lewis (3)
(1) MPB Communications Inc., (2) TOPTICA Photonics AG, (3) Laser Systems Dept, European Southern Observatory (ESO)
Large telescopes equipped with adaptive optics require 20-25W CW 589-nm light sources with emission linewidths of 5MHz. Towards this goal, ESO has been working for a number of years on the development of laser sources based on high-power narrow-band 1178-nm Raman fiber amplifiers (RFA) with subsequent frequency doubling to 589nm, demonstrating field tested lasers and powers beyond 50W CW. We present the design and performance of the guide star lasers being developed by industrial partners Toptica and MPBC, under contract from ESO, for deployment at the ESO VLT. The laser is designed and robustly engineered specifically for deployment on telescope facilities. The laser design is based on ESO’s patented narrow-band RFA. The linearly-polarized, fiber-coupled emission of a Toptica CW diode laser, emitting 20mW at 1178nm, serves as master oscillator signal with stabilized emission frequency and controllable spectral linewidth up to a few MHz. The narrow-band seed signal is amplified in a polarization-maintaining (PM) Raman fiber amplifier developed by MPBC. The amplifier is pumped by a high-power 1120-nm PM fiber laser. With efficient suppression of stimulated Brillouin scattering, an unprecedented 40W of narrow-band RFA output has been obtained. The RFA output is then mode-matched into a resonant cavity doubler with a free spectral range exactly matching the sodium D2a to D2b separation. This allows simultaneous generation of an additional frequency component (D2b line) in the output beam to re-pump the electronic population of sodium atoms, thereby increasing the return flux. We have demonstrated doubling efficiencies >80%, resulting in CW output powers at 589nm easily exceeding the design goal of 20W. Fiber lasers provide excellent output beam quality and are turn-key, maintenance-free, reliable, ruggedized devices whose compactness allows installation directly on the launch telescope structure. They are therefore well suited for LGS applications, also considering the challenging environment of astronomical observatories.