A Raman fiber laser source (RFLS) is configured with a feeding fiber delivering MM pump radiation to an inner cladding of double-clad MM Raman fiber laser. The MM pump radiation has a sufficient power to produce Raman scattering in the MM Raman fiber converting the pump radiation to a MM signal radiation at a Raman-shifted wavelength λram which is longer than a wavelength λpump of the pump radiation. The RFLS further has a pair of spaced reflectors defining therebetween a resonator for the signal radiation at a 1st Stokes wavelength and flanking at least part of the MM core of the Raman fiber which is provided with a central core region which is doped with impurities for enhancing Raman process. The reflectors and central core region are dimensioned to correspond to the fundamental mode of the MM signal radiation.