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An external dispersive cavity is used to map out the gain profile of single-heterostructure (SH), double-heterostructure (DH), and large-optical-cavity (LOC) lasers. The variations in gain profile with dopant and device operating conditions are reported. With certain cavity arrangements, long time delays in DH devices are observed; this shows that saturable absorption can be present in these as well as in all other types of GaAs diodes. A model combining saturable absorption with a breakdown in optical guiding makes it possible to consistently interpret all reported time-delay data for GaAs diffused-junction (DJ) and SH diode lasers, including -switching and abnormal-delay effects. Finally, experiments where two gratings are used to oscillate a diode simultaneously at two different, independently variable frequencies are described.