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Injection locking of AlGaAs double-heterostructure (DH) lasers was studied with respect to locking bandwidth, required power, and coherence. The relation of the locking bandwidth versus the ratio of locked laser power to injected power was consistent with the classical analysis on injection locking phenomena reported by Adler . The measured maximum locking bandwidth was 5.8 GHz when the locking gain was 18 dB. A maximum gain of 40 dB was observed with a 500 MHz locking bandwidth. The power increase in the injected mode agrees well with theoretical values calculated with the van der Pol equation. The interference pattern was observed between an injecting beam and a locked laser beam. Visibility was the same as that obtained by the interference between forward and backward emitted beams in an identical free-running laser. Spurious mode suppression was observed when a single-frequency optical power is injected into an RF-modulated laser. Single longitudinal mode operation was obtained at a sufficiently high injecting level.