  We study the coherence of a disordered and interacting quantum light field
after propagation along a nonlinear optical fiber. Disorder is generated by a
cross-phase modulation with a randomized auxiliary classical light field, while
interactions are induced by self-phase modulation. When penetrating the fiber
from free space, the incoming quantum light undergoes a disorder and
interaction quench. By calculating the coherence function of the transmitted
quantum light, we show that the decoherence induced by the quench spreads in a
light-cone fashion in the nonequilibrium many-body quantum system, leaving the
latter prethermalize with peculiar features originating from disorder.
