the invention describes techniques for the control of the spatial as well as spectral beam quality of multi - mode fiber amplification of high peak power pulses as well as using such a configuration to replace the present diode - pumped , neodynium based sources . perfect spatial beam - quality can be ensured by exciting the fundamental mode in the multi - mode fibers with appropriate mode - matching optics and techniques . the loss of spatial beam - quality in the multi - mode fibers along the fiber length can be minimized by using multi - mode fibers with large cladding diameters . near diffraction - limited coherent multi - mode amplifiers can be conveniently cladding pumped , allowing for the generation of high average power . moreover , the polarization state in the multi - mode fiber amplifiers can be preserved by implementing multi - mode fibers with stress producing regions or elliptical fiber cores these lasers find application as a general replacement of nd : based lasers , especially nd : yag lasers . particularly utility is disclosed for applications in the marking , micro - machining and drilling areas .