Phase measurements in surface nonlinear optics: The effect of laser beam quality

When the relative phase of a surface nonlinear polarizability is measured by interference techniques, the resultant interferogram may be of lower contrast than that expected for ideal waves. It is shown, both theoretically and experimentally that this degradation arises from the multimode character of the pulsed lasers used. Differences between longitudinal and transverse mode effects are demonstrated. The laser coherence time may be deduced from the fringe degradation caused by longitudinal modes.