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Effective single-longitudinal-mode selection can be achieved in short-coupled-cavity (SCC) injection lasers consisting of a cleaved short-cavity laser (length , refractive index ) with a short external resonator (spacing d). The mode-selectivity is due to the combined advantages of large gain-rolloff for the adjacent modes in the short ( cavity, and resonator-loss-modulation in the coupled cavity. This I paper discusses the theoretical considerations and experimental studies for the SCC lasers. The results show that for good single-longitudinal-mode operation, the resonator-loss modulation period factor , given by the effective optical length ratio , should be between approximately three and eight, with typical short-cavity laser length in the m range and external resonator spacing also in the m range, depending on the specific .