A computational model, which describes EM field formation in a pulsed laser from a randomly generated initial
spontaneous field inside the laser cavity has been developed. The model is based on a two-dimensional fast
Fourier transform and describes a real laser system taking into account a lensing and a diaphragm effect of the
laser rod. The laser cavity is described by five effective planes, which represent different laser cavity elements-the back and the front mirror, the Q-switch element and the laser rod. At each plane the EM field is calculated in
real space and propagation between the planes is achieved in Fourier space by multiplication with an appropriate
phase factor. The computational time needed for simulation of a realistic pulse formation is in order of minutes.
The model can predict the shape and the integral energy of the pulse, its transverse profile at different distances
from the front mirror (including near and far field) and beam divergence. The results of the model were found
to be in good agreement with measured parameters for a Q-switched ruby laser system running in stable as well
as unstable cavity configurations. The temporal shape of a laser pulse was measured and calculated not only for
the ruby laser, but also for a Nd:YAG laser. It was found that FWHM of a pulse produced by ruby laser is three
times longer than FWHM of a pulse produced by Nd:YAG laser.