Beam cleanup with stimulated Raman scattering

We describe the application of stimulated Raman scattering on the reduction of aberrations on optical beams. The gain of the stimulated Raman process depends only on the intensity of the pump beam integrated along a ray path of the Stokes beam. As a result phase structure on the pump is not transferred directly to the Stokes, although it can be if it is allowed to diffract into amplitude structure. Amplitude structure on the pump beam will not be transferred to the Stokes beam if it is effectively averaged out along a ray path of the Stokes beam in one e-folding distance of the Raman gain. As a result, in certain conditions amplification of a diffraction-limited Stokes beam can be accomplished without transferring the aberrations of the pump beam. We discuss the Raman beam cleanup process for various pump beam geometries, illustrating the different regimes of behavior. Experimental results are compared with those obtained from numerical analysis using a wave-optics code. The use of phase conjugation in the Raman amplifier is also described.