TRANSVERSE STRUCTURES IN A SODIUM-FILLED FABRY-PEROT RESONATOR .1. EXPERIMENTAL RESULTS - SYMMETRIES AND THE ROLE OF THE INCOUPLING CONDITIONS

Transverse patterns occurring in a sodium-filled Fabry-Perot resonator are studied in a carefully controlled experiment. The transitions amo ng them, observed as a function of cavity tuning, can be continuous or discontinuous (Transverse Optical Bistability), depending on the expe rimental parameters. For a variety of suitably chosen input conditions , in which only a few excited modes participate in the interaction, th e classification of the patterns according to their topological charac teristics reveals underlying symmetries in the sign of the nonlinearit y, the plane of observation, and, most importantly, the mode-matching conditions. We show that the crucial parameters controlling the struct ures are the phase coefficients of the resonator modes excited by the input beam. The properties of the nonlinear cavity spectrum are also u sed to characterize further the observations.