  Electron-positron pair production in an arbitrary polarized ultrastrong laser
field is investigated in the first order perturbation approximation in which
the Volkov states are used for convenient calculation of scattering amplitude
and cross section. It is found surprisingly that the optimal pair production
depends strongly on the polarization. For some cases of field parameters, the
optimal field is elliptically polarized or evenly circularly polarized one,
rather than the usual linear polarization as indicated by previous works. Some
insights into pair generation are given and some interesting unexpected
features are also discussed briefly.
