  We demonstrate an all-optical bump-on-tail instability by considering the
nonlinear interaction of two partially-coherent spatial beams. For weak wave
coupling, we observe momentum transfer with no variation in intensity. For
strong wave coupling, modulations appear in intensity and evidence appears for
wave (Langmuir) collapse at large scales. Borrowing plasma language, these
limits represent regimes of weak and strong spatial optical turbulence. In both
limits, the internal spectral energy redistribution is observed by recording
and reconstructing a hologram of the evolving dynamics. The results are
universal and can appear in any wave-kinetic system with short-wave/long-wave
coupling.
