  The momentum spectrum and the number density of created electron-positron
pairs in a frequency modulated laser field are investigated using quantum
kinetic equation. It is found that the momentum spectrum presents obvious
interference pattern. This is an imprint of the frequency modulated field on
the momentum spectrum, because the momentum peaks correspond to the pair
production process by absorbing different frequency component photons.
Moreover, the interference effect can also be understood qualitatively by
analyzing turning point structures. The study of the pair number density shows
that the number density is very sensitive to modulation parameters and can be
enhanced by over two orders of magnitude for certain modulation parameters,
which may provide a new way to increase the number of created electron-positron
pairs in future experiments.
