Recent advances in the field of highly brilliant continuous wave fibre laser sources lead to an increase of available intensity. These intensities were formerly reserved to short pulse laser sources with short and ultra-short pulse durations. As a result of this combination of wavelength, intensity and cw operation, plasma formation becomes critical. Especially in contrast to cw Nd:YAG-laser sources, as a weakly ionized metal vapor is known to be present in this case. Results of analytical modeling and experimental work represent the basis for discussion of evidence of plasma formation and its potential influence on laser materials processing with use of single-mode fibre laser sources.

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