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In Parts I and II of this work , , the resonant modes of GaAs lasers, as predicted from a proposed model, are found to be in very good agreement with experimental results that emphasize the frequency domain. This paper, Part III, supplements the previous work by investigating the spatial field distributions of junction-laser beams. For the first time, the propagation characteristics of the beams are described mathematically in a manner that is consistent with detailed beamwidth measurements in the Fresnel and Fraunhofer regions of the diffraction field. This correlation between theory and experiment is made possible by the use of a lens-slit combination that ensures the generation of reproducible beams with rectangular symmetry. The astigmatic beam solutions describing the propagation characteristics beyond the slit include beam-broadening effects produced both by the lens and by phase variations at the laser mirror, along the junction plane. These latter variations have not been previously reported. Their existence is deduced from our measurements beyond the slit, which show a greater beam expansion along the junction plane than can be theoretically accounted for by the lens system alone. Near threshold, at a distance of one beamwidth from the center of the mirror illumination, the phase lag is found to have a current-dependent magnitude of a few degrees.