Multiple-stream instabilities and soliton turbulence in photonic plasma

We demonstrate weak and strong regimes of optical spatial turbulence by considering the nonlinear interaction of three partially coherent spatial beams. The geometry represents a multiple bump-on-tail instability, allowing an interpretation of nonlinear statistical light as a photonic plasma. For weak nonlinearity, we observe instability competition and sequential flattening of the bumps in momentum space, with no observable variations in position-space intensity. For strong nonlinearity, intensity modulations appear and the triple-hump spectrum merges into a single-peaked profile with an algebraic $k^{-2}$ inertial range. This spectrum, with its associated modulations, is a definitive observation of soliton, or Langmuir, turbulence.