Passive Multiple Beam Combination in Optical Fibers via Stimulated Brillouin Scattering

Many active methods of scaling laser brightness have been demonstrated inrecent years. The goal of this research was to demonstrate the feasibility of passivelycombining multiple laser beams using Stimulated Brillouin Scattering (SBS) in a longmultimode optical fiber. This method of combination employed a "Gatling gun"fiber array that allowed several collimated beams to be focused by a lens into anoptical fiber. The retroreflected Stokes beam is passed through the center of the beamcombiner for analysis. In addition to experimental methodology and equipment used,the theoretical and historical background of SBS in optical fibers is provided.The ability to combine four single-mode off-axis input beams using a fiber arrayand a long multimode optical fiber was demonstrated. The Stokes beam generatedby the four off-axis input beams was found to be near Gaussian with an M2 value of: 1.08. The reflected Stokes beam intensity was erratic with no discernible period.The maximum average reflected power was approximately 50 mW with a conversionefficiency of 3.5% and a slope efficiency of 11.4%. The peak reflected power wasmeasured to be over 400 mW with a conversion efficiency of 42%. The thresholdrequired to generate SBS in this configuration was quadruple the predicted powerrequired to generate SBS using an on-axis pump beam.The deleterious effects of the addition of an off-axis side channel was demonstrated.