  Experimental evidence of an absorbing phase transition, so far associated
with spatio-temporal dynamics is provided in a purely temporal optical system.
A bistable semiconductor laser, with long-delayed opto-electronic feedback and
multiplicative noise shows the peculiar features of a critical phenomenon
belonging to the directed percolation universality class. The numerical study
of a simple, effective model provides accurate estimates of the transition
critical exponents, in agreement with both theory and our experiment. This
result pushes forward an hard equivalence of non-trivial stochastic,
long-delayed systems with spatio-temporal ones and opens a new avenue for
studying out-of-equilibrium universality classes in purely temporal dynamics.
