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Junction diodes made from certain types of semiconductors emit light efficiently when forward biased. Wavelengths between 8.5 µ and 0.64 µ have been observed, depending on the material. When the diode has cleaved facets normal to the junction, a resonant optical cavity is formed in the plane of the junction and. stimulated emission can be observed. The threshold current density for stimulated emission in GaAs is of the order of 1000 A/cm2at 77°K and increases as T8. The efficiency is of the order of 10 percent. For GaAs, electroluminescent emission occurs at about 8500 Â at 77°K, and the spontaneous linewidth is 200 Â. Above threshold sharp lines appear, the spacing of which is correctly predicted by cavity theory. The width of an individual mode line in GaAs is, at most, 10 Mc/s. A fan-shaped laser beam emerges from a Fabry-Perot cavity above threshold; the plane of the fan is normal to the junction and the dimensions are about 2° by 6°. Simulated emission occurs from filaments in the junction plane; the cross section is about 6 µ by 25 µ. Diode laser emission can be modulated simply by modulating the current. The stimulated optical decay time is at most 0.2 µs. Diodes have been made to amplify light, and various cooperative and quenching effects have been observed in optically-coupled chips. These effects suggest the use of diode lasers as computer elements. In general, diode lasers will find use as intense point light sources.