The possibility of using low pump power for cw Raman continuum generation is demonstrated by optimization of the pump peak power and by accounting for the loss-related reduction of the effective length of Raman interaction in holey fibers. A 3.8-W, 324-nm-wide cw Raman continuum with a spectral power density higher than 10 mW/nm is generated in a completely fiber-integrated, single-mode format.

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The frequency quadrupling of up to 50W average, 1.5kW peak power Yb/Er single-mode, fibre oscillator/amplifier sources sequentially in periodically poled and bulk LBO crystals allows to achieve UV average powers of 3-lOW around 388nm.In the ultra violet range 355-390 nm there is considerable call for high power sources to meet the multifarious applications demands such as micromachining and drilling with suppressed plasma formation, electronic device inspection, stereolithography and diverse applications in the biosciences such as photolysis of caged compounds and fluorescence imaging studies. Traditionally, these demands have been met by gas lasers or conventional bulk solid state lasers incorporating intra-or extra-cavity frequency doubling or quadrupling. Consequently, sources have tended to be relatively large and inefficient, moderately complicated in operation and fairly restricted in operational power levels. With the dramatic break through in the development of high power fiber laser and amplifier sources [1], new approaches to high average and peak power uv generation have been realized, that will revolutionize the size, efficiency and ease of operation of sources that are set to impact on applications. The possibility of a dual stage, single-pass SHG has been demonstrated with quasi-CW Yb/Er fiber pump operating around 1550nm [2], although the average powers generated in UV were limited due to the relatively low fundamental peak powers associated with a single-mode format fibre pumping and because of the optical-damage susceptibility of periodically poled crystal used in second stage SHG. Despite the demonstrated possibility of growth of new materials potentially suitable for damage-free high power UV generation, like stoichiometric LiTaO3, the questions about s-PPLT's photorefraction and optical damage susceptibility in UY remain open. At the same time, a possibility of kW-scale peak powers in a large core fiber sources has been demonstrated with single mode quality [3] and means that efficient, and robust non-resonant SHG configurations for UV generation can be successfully realized in materials with relatively low effecfive nonlinearities but proven high optical damage thresholds, like LBO [4] Here we report on a fiber-pump based source with a capability of generating 3 to lOW UV power by using single-pass quasi-phase-matched SHG in KTP and second-stage UV generation in bulk LBO.A master-oscillator power fiber amplifier (MOPFA) configuration was chosen as it allows control over temporal and spectral characteristics of the fiber pump which are essential for achieving high conversion efficiency in SHG materials. The source operated around 1550nm and seed signal from a directly modulated DFB diode was pre-amplified to 2W in conventional single-mode EDFA before being launched into a final amplifier stage based on a special, large core, high efficiency Yb/Er fiber (14gm mode field diameter, 3m length). The MOPFA source was capable of producing 10 to 22W average power with peak powers up to 6kW and excel...

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