  We investigate the turn-on process in a laser cavity where the roundtrip time
is several orders of magnitude greater than the active medium timescales. In
this long delay limit the electromagnetic field build-up can be mapped
experimentally roundtrip after roundtrip. We show how coherence settles down
starting from a stochastic initial condition. In the early stages of the
turn-on, we show that power drop-outs emerge, persist for several round-trips
and seed dark solitons. These latter structures exhibit a chaotic dynamics and
emit radiation that can lead to an overall turbulent dynamics depending on the
cavity dispersion.
