Harmonic synchronization of Fourier-synthesized optical pulses to an external optical clock

The fifth-harmonic synchronization of Fourier-synthesized 40-GHz optical pulses to a slower 8-GHz mode-locked optical pulse train was demonstrated by means of an advanced optical phase-locked-loop technique. For the Fourier synthesis, three continuous-wave semiconductor lasers were phase locked to generate the 40-GHz optical pulses, where an optical single-sideband modulator was used to obtain arbitrary waveforms. For the synchronization, a modified clock-recovery technique based on optical four-wave mixing was developed, where the timing-error signal was fed back to one of the Fourier-component lasers in order to minimize the timing jitter with respect to the optical clock. Synchronization with an estimated root-mean-square timing jitter of 0.43 ps was, thus, achieved.