Suppressing Stimulated Brillouin Scattering with High Beam Quality by Selective Multimode Excitation in Optical Fibers

For many applications of high-power delivery through optical fibers and fiber amplifiers, high average power and high beam quality are both needed but difficult to meet simultaneously. This is because high-power operation requires a large fiber core to minimize nonlinear effects, but a large core usually supports many modes — it is a general belief that high output-beam quality requires single-mode operation. The power scaling for narrowband fiber amplifiers is often limited by the stimulated Brillouin scattering (SBS), which introduces high transmission loss and can induce strong backward-propagating Stokes light that damage upstream lasers. Various techniques have been developed to suppress SBS and strive to maintain single-mode operation to ensure beam quality simultaneously. If good beam quality can be attained, multimode fibers (MMF) are highly desirable because the SBS can be greatly suppressed (to be detailed next), and there is no need to maintain single-mode operation in a large core by fiber design. However, a major concern for using MMFs is the speckled output, often considered to have poor beam quality. In this work, we show experimentally and theoretically that dividing the coherent seed into many modes increases the SBS threshold to several times the threshold for fundamental-mode (FM)-only excitation. Moreover, we experimentally focus the output of an 80-mode MMF to a diffraction-limited spot by shaping the incident wavefront of a coherent narrowband seed with a spatial light modulator (SLM).