p-n Junction Lasers for Communication Systems

Secure ground-to-ground communication links, communication from satellites and space probes, and coherent optical logic in high-speed integrated computers lie within the realm of semiconductor single lasers or laser arrays. Lasers of this type have the advantage of extremely small size, ease of direct modulation, and high efficiency. Ruggedness and simplicity of pumping promise high reliability for military electronics. A discussion is presented on design considerations, technological problems, and performance of p-n junction lasers for short range communication systems. After a brief review of fundamental principles for lasing action in semiconductors, an account is presented of previously unpublished technological procedures required to achieve GaAs laser diodes with high output in the coherent beam at a high pulse rate required to operate a pulse code, modulated voice transmission system. A description is given of wafer preparation, diffusion of extremely planar p-n junctions, and the formation of ohmic, low-resistance area contacts. Measurement techniques to determine the performance characteristics of these lasers with respect to such factors as threshold current density, maximum output power, external quantum efficiency, spectral distribution, and linewidth of emitted radiation are discussed.