  We present a functional renormalization group approach for the active to
inactive phase transition in directed percolation type systems, in which the
transition is approached from the active, finite density phase. By expanding
the effective potential for the density field around its minimum, we obtain a
background field action functional, which serves as a starting point for the
functional renormalization group approach. Due to the presence of the
background field, the corresponding non-perturbative flow equations yield
remarkably good estimates for the critical exponents of the directed
percolation universality class, even in low dimensions.
