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, wherein - the laser modules each are configured with an active fiber and input and output passive fibers (56, 54) butt-spliced to respective opposite ends of the active fiber, the active fiber having a multimode core doped with a gain medium and configured to support propagation of a single mode at a desired wavelength, the passive fibers (56, 54) each being single mode fibers and having a mode field diameter substantially equal to an MFD of the active fiber and - the laser modules each further have a pumping unit provided with one or multiple single mode fiber lasers; - 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, the output feeding single mode fibers each supporting a mode which has substantially a Gaussian profile with a beam quality factor (M 2) about 1, - the optical combiner being configured so that the combined laser radiation has a minimally possible beam quality factor (M 2) obtainable by - coupling a plurality of single mode output laser beams into the respective output feeding single mode fibers; - stretching the output feeding single mode fibers while monitoring numerical apertures (NAs) of each individual output feeding single mode fiber so as to prevent coupling of a Gaussian-profile mode into a cladding of the output feeding single mode fiber; - simultaneously with stretching coupling the output feeding single mode fibers together so as to provide for the fiber combiner with the output multimode delivery fiber); - 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 plurality of laser modules each having an output feeding single mode fiber which supports and guides laser radiation towards the workpiece, wherein - the laser modules each are configured with an active fiber and input and output passive fibers (56, 54) butt-spliced to respective opposite ends of the active fiber, the active fiber having a multimode core doped with a gain medium and configured to support propagation of a single mode at a desired wavelength, the passive fibers (56, 54) each being single mode fibers and having a mode field diameter substantially equal to an MFD of the active fiber and - the laser modules each further have a pumping unit provided with one or multiple single mode fiber lasers; - the laser modules each are configured with an active fiber and input and output passive fibers (56, 54) butt-spliced to respective opposite ends of the active fiber, the active fiber having a multimode core doped with a gain medium and configured to support propagation of a single mode at a desired wavelength, the passive fibers (56, 54) each being single mode fibers and having a mode field diameter substantially equal to an MFD of the active fiber and - the laser modules each further have a pumping unit provided with one or multiple single mode fiber lasers; - 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, the output feeding single mode fibers each supporting a mode which has substantially a Gaussian profile with a beam quality factor (M 2) about 1, - the optical combiner being configured so that the combined laser radiation has a minimally possible beam quality factor (M 2) obtainable by - coupling a plurality of single mode output laser beams into the respective output feeding single mode fibers; - stretching the output feeding single mode fibers while monitoring numerical apertures (NAs) of each individual output feeding single mode fiber so as to prevent coupling of a Gaussian-profile mode into a cladding of the output feeding single mode fiber; - simultaneously with stretching coupling the output feeding single mode fibers together so as to provide for the fiber combiner with the output multimode delivery fiber); - coupling a plurality of single mode output laser beams into the respective output feeding single mode fibers; - stretching the output feeding single mode fibers while monitoring numerical apertures (NAs) of each individual output feeding single mode fiber so as to prevent coupling of a Gaussian-profile mode into a cladding of the output feeding single mode fiber; - simultaneously with stretching coupling the output feeding single mode fibers together so as to provide for the fiber combiner with the output multimode delivery fiber); - 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.