The results of a study of near-infrared (IR) and terahertz photoluminescence in doped GaAs layers placed in a near-IR optical waveguide are presented. Terahertz radiation under optical interband pumping is associated with transitions of nonequilibrium electrons from the conduction band to impurity states. Stimulated interband near-IR emission involving impurity states was obtained. An accelerated increase in the integral intensity of terahertz radiation has been demonstrated at pump intensities exceeding the threshold for stimulated emission in the near-IR range. The increase in intensity is associated with the accelerated depopulation of the ground state of the impurity by stimulated emission.