50W CW visible laser source at 589nm obtained via frequency doubling of three coherently combined narrow-band Raman fibre amplifiers

We demonstrate the cascaded coherent collinear combination of a seed-split triplet of 1178nm high-power narrow-band (sub-1.5MHz) SBS-suppressed CW Raman fibre amplifiers via nested free-space constructive quasi-Mach-Zehnder interferometry, after analysing the combination of the first two amplifiers in detail. Near-unity combination and cascaded-combination efficiencies are obtained at all power levels up to a maximum P-1178 > 60W. Frequency doubling of this cascaded-combined output in an external resonant cavity yields P-589 > 50W with peak conversion efficiency eta(589) similar to 85%. We observe no significant differences between the SHG of a single, combined pair or triplet of amplifiers. Although the system represents a successful power scalability demonstrator for fibre-based Na-D-2a- tuned mesospheric laser-guide-star systems, we emphasise its inherent wavelength versatility and consider its spectroscopic and near-diffraction-limited qualities equally well suited to other applications. (C) 2010 Optical Society of America