Incoherent branched flow of light

Waves traveling in weakly disordered media possessing long-range correlations experience a universal phenomenon known as branched flow, where the waves split and form channels (branches) of enhanced intensity that keeps dividing as the waves propagate. We explore this fascinating phenomenon in thin liquid soap membranes – soap bubbles. This phenomenon is naturally emerging in this system because soap films naturally have thickness variations whose parameters can be tuned to act as a weakly correlated potential, as required for branched flow. Thus far, branched flow effects have been studied experimentally in various systems, but always with coherent waves. Here, we present the first experimental observation of the branched flow of spatially-incoherent light. We show that the primary effect of branching occurs for both coherent and incoherent light, but secondary branching arises from interference effects and disappears when the waves become incoherent. The location of the first caustic, where the branches reach peak intensity, remains the same as the coherence is reduced, but a close look at the branch statistics reveals a very different distribution arising from the incoherence.