the diode laser. In this thesis, the impact of the longitudinal distribution of internal intensity, temperature and gain on the lateral beam parameters is revealed. Numerical simulations demonstrate that homogenizing the longitudinal device temperature prevents near-field narrowing and accompanying far-field blooming, which is experimentally verified by symmetrizing the facet reflectivities. The second part of this work deals with the impact of spatially mismatched optical feedback on beam quality and lifetime. It is found that irradiance of solder and p-side metalization as well as the adjacent highly p-doped layers causes localized heating around the front facet, which strongly reduces the threshold for catastrophic optical damage. It was also found that lateral displacement of the feedback spot can lead to severe beam quality degradation for single- side emitting devices. This effect could be reduced by symmetrically out- coupling external-cavity configurations.