Beat-signal synchronisation for optical generation of millimetre-wave signals

High-spectral-purity millimetre-wave beat signals were optically generated at 40 to 120 GHz by heterodyning the outputs from two continuous-wave semiconductor lasers the beat signal of which was harmonically synchronised to an 8 GHz external optical clock. A four-wave-mixing signal generated from the two lasers and the clock was used to achieve the synchronisation. The phase fluctuations of the generated millimetre-wave signals were 0.1 to 0.27 rad.