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Experimental observations of beam profile evolution and beam quality changes in an asymmetrical diode-end-pumped laser oscillator are presented. The beam intensity distribution and beam size show great differences between the two intra-cavity counter-propagating beams. Measurements also show that the beam quality changes when the beam passes through the gain medium. These phenomena are attributed to the thermally induced aberrations and soft aperture induced diffractions. The results present a novel clue of designing a high power laser oscillator with fundamental mode output.