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Summary form only given. Optoelectronic generation of MM signals attracts increasing amount of interest in applications including wireless MM-wave communication systems and optical feeding of phased-array antennas. Among a number of optoelectronic techniques available for the generation of MM-wave signals such as using mode-locked lasers and using external modulators, heterodyning of two independent lasers is particularly suited for the generation of higher-frequency tunable signals because its operation is not restricted by the cavity resonant frequency or the bandwidth of modulators. A pair of grating-extended external-cavity semiconductor lasers were operated at 1.53 /spl mu/m. The MM-wave beat signal between them was detected by a photodiode, and was phase-locked to a low-frequency reference source using a portable spectrum analyzer as a harmonic mixer. When the loop was closed, a high purity MM-wave signal was generated and transmitted through a standard fiber. The typical RMS phase fluctuation of the signal was about 50 mrad. The signal frequency was easily tuned by controlling the stimulus. By using an auxiliary external mixer, the frequency was extended up to 70.5 GHz that is, to our knowledge, the highest offset frequency reported for an optical phase-locked loop. The technique reported should be useful for the investigation of MM-wave communication systems.