Finite width and irreducible background effects to tbar t production at γγ Next Linear Colliders

We study the complete process \phphbbww\ using exact matrix element computations at tree-level, at a $\sqrt s=500$ GeV \eeb\ linear collider of the next generation. Incoming photons produced via back-scattering of laser light are considered. Sizable effects due to the finite width of the top quark as well as to the irreducible background to $t\bar t$ production and decay are predicted.