Numerical simulations of the pulse build up in aQ-switched laser reveal a considerable change in the beam spatial profile during the pulse evolution, on a nanosecond time scale. Such changes were measured experimentally, and the measured temporal evolution of the beam profile is shown to be in good agreement with the theoretical results. The time evolution of the spatial profile translates into an evolution of the beam quality parameterM2; for typical experimental conditions, in a Nd:YAG laser utilizing a variable-reflectivity mirror resonator,M2is very close to a value of 1 at the beginning of the pulse, and increases smoothly throughout the pulse to reach a value of approximately 2 at the pulse trailing edge.
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