The recent development of semiconductor diode optical amplifiers and lasers having a laterally tapered gain region has changed the outlook for high-power semiconductor optical sources. For the first time, highbrightness, single-element, all-semiconductor sources which emit several watts of cw power in a nearly ideal, single-lobed, diffraction-limited beam have been demonstrated. As semiconductor sources these devices have the inherent advantages of high efficiency, small size, light weight, and reliability. The amplifier12 and all-semiconductor master-oscillator power-amplifier (MOPA) devices34 have gain regions linearly tapered from a few micrometers at the amplifier input to several hundred micrometers at the output. Device lengths are typically 2 mm or more. The angle of the taper is chosen to match the diffraction angle of the input beam which has its waist near the narrow end of the taper. Such a structure is shown schematically in Fig. 1 . The etched grooves have angled side walls and act as cavity spoilers, designed to prevent oscillation of the device as a broad-area laser. The devices are fabricated in single-quantum-well strained-layer InGaAs/AlGaAs graded-index separate-confinement heterostructure laser wafers grown by organometallic vapor phase epitaxy.5 The tapered devices also operate as laser oscillators6 by increasing the input facet reflectivity. For amplifiers, both the input and output facets are coated for low reflectivity (in Fig. 1 , Ri = R2 = 1%), but for oscillators, the input facet is left uncoated (R1 —30%). The oscillators also emit several watts of cw power into a nearly single-lobed, nearly diffraction-limited beam, though their beam quality is usually somewhat inferior to that obtained for amplifiers, particularly at the highest output powers. The lateral mode of the oscillator is similar to the modes described by Fox and Li7 for unstable resonators, except that the semiconductor medium has a significant effect on the self-consistent mode which oscillates. A beam propagation calculation has been carried out to model these effects, as described below. This paper includes a review of the properties of both tapered amplifiers and oscillators.