Transient states analysis of a partially coherent laser beam propagating through a Gaussian cavity

The transient states of a laser beam propagating through a plane-parallel cavity with a Gaussian graded reflectivity mirror are numerically and experimentally investigated in the far-field, i.e., in the region around the focal plane of a positive lens, where variations in the beam are enhanced. The evolution of significant parameters (quality factor M², spot-size and focusing distance), from which all the others can be deduced, is studied from transit to transit. It is found that the type of evolution in the first cavity round-trips depends strongly on the initial values of beam parameters at the first emission from the active medium. It is also found that the laser system used in the experiment does not reach equilibrium during the pulse time. A satisfactory accordance between experimental and numerical data has been found