Beam quality of the Stokes output of continuous-wave Raman fiber amplifiers using multimode fiber

The beam quality of the Stokes output of a Raman fiber amplifier based on a long multimode graded-index fiber is examined. The beam quality of the Stokes output is modeled by considering the relative gain of the Stokes modes of the fiber; this model predicts that the beam quality of the coupled Stokes beam and the beam quality of the coupled pump beam determine the beam quality of the Stokes output. Experimental investigations validate this model by showing that the beam quality of the Stokes output is similar to the beam quality of the Stokes input. (C) 2008 Optical Society of America