Mechanisms of beam clean up at Raman conversion of multimode radiation in graded-index fibers

As shown recently, highly multimode (M2~30) radiation of laser diodes may be converted in graded-index fibers into high-quality (M2≤2) Raman beam at Stokes-shifted wavelength. Here we experimentally study output beam profiles for the transmitted pump and generated Stokes beams and develop an analytical balance model enabling theoretical beam profiles both near the Raman threshold and well above the threshold where significant depletion of the pump beam occurs due to the Raman conversion. Comparison of the theory and experiment allows for the identification of main mechanisms of the Raman beam clean up in this system, including linear and nonlinear effects.