  Investigation of laser matter interaction with electromagnetic codes requires
to implement sources for the electromagnetic fields. A way to do so is to
prescribe the fields at the numerical box boundaries in order to achieve the
desired fields inside the numerical box. Here we show that the often used
paraxial approximation can lead to unexpected field profiles with strong impact
on the laser matter interaction results. We propose an efficient numerical
algorithm to compute the required laser boundary conditions consistent with the
Maxwell's equations for arbitrarily shaped, tightly focused laser pulses.
