  Spatio-temporal chaos in parametrically driven waves is investigated in one
and two dimensions using numerical simulations of Ginzburg-Landau equations. A
regime is identified in which in one dimension the dynamics are due to double
phase slips. In very small systems they are found to arise through a Hopf
bifurcation off a mixed mode. In large systems they can lead to a state of
localized spatio-temporal chaos, which can be understood within the framework
of phase dynamics. In two dimensions the double phase slips are replaced by
bound defect pairs. Our simulations indicate the possibility of an unbinding
transition of these pairs, which is associated with a transition from ordered
to disordered defect chaos.
