Designing pulsed laser amplifiers requires detailed knowledge about beam distortions at the amplifying medium. Time resolved experiments have been performed on fluoride phosphate and pure phosphate glass in order to study optical aberrations induced by pump beam irradiation. By means of a probe beam tunable in wavelength, it was possible to discriminate between thermally induced variations of the optical path and beam narrowing effects due to the radial gain distribution. The experimental results were analyzed theoretically regarding Fresnel diffraction. A quite different response was found for the two types of glass despite their similar nominative properties.