A high power fiber laser system for processing a workpiece, comprising: - a plurality of laser modules each having an output feeding single mode fiber which supports and guides laser radiation towards the workpiece, - a fiber combiner having the output feeding single mode fibers of the respective modules coupled together into a multimode delivery fiber which guides combined laser radiation from the laser modules along a light path, each feeding single mode fiber has a reduced diameter in said fiber combiner obtaimed by heating and stretching the individual single mode fibers, wherein each single mode fiber has a minimum numerical aperture below which the single mode touches the cladding, so that the combined laser radiation of the optical combiner has a minimally possible beam quality factor (M 2); and characterized by: - a laser head receiving the combined laser radiation and operative to direct the combined laser radiation towards a surface of the workpiece so as cause a backreflected radiation to propagate backwards along the light path through the laser head, delivery fiber and combiner towards the laser modules; and - a monitoring assembly receiving the backreflected radiation and operative to detect a spectrum of the backreflected radiation, - a controller receiving the detected spectrum from the monitoring assembly and operative to match the detected spectrum to a reference value, the backreflected radiation including thermal emission and backreflected laser signals in UV, NIR and visible ranges from about 400 nm to about 2000 nm, - wherein a control signal (Ic) is output to the laser modules whose parametes are continuously adjusted to obtain the desired spectra of the backreflected radiation. - a plurality of laser modules each having an output feeding single mode fiber which supports and guides laser radiation towards the workpiece, - a fiber combiner having the output feeding single mode fibers of the respective modules coupled together into a multimode delivery fiber which guides combined laser radiation from the laser modules along a light path, each feeding single mode fiber has a reduced diameter in said fiber combiner obtaimed by heating and stretching the individual single mode fibers, wherein each single mode fiber has a minimum numerical aperture below which the single mode touches the cladding, so that the combined laser radiation of the optical combiner has a minimally possible beam quality factor (M 2); and - a laser head receiving the combined laser radiation and operative to direct the combined laser radiation towards a surface of the workpiece so as cause a backreflected radiation to propagate backwards along the light path through the laser head, delivery fiber and combiner towards the laser modules; and - a monitoring assembly receiving the backreflected radiation and operative to detect a spectrum of the backreflected radiation, - a controller receiving the detected spectrum from the monitoring assembly and operative to match the detected spectrum to a reference value, the backreflected radiation including thermal emission and backreflected laser signals in UV, NIR and visible ranges from about 400 nm to about 2000 nm, - wherein a control signal (Ic) is output to the laser modules whose parametes are continuously adjusted to obtain the desired spectra of the backreflected radiation.