Figure 9: The spectra of the mode-locked optical pulse train modulated by a synchronized subharmonic tone. In this case, a 40.2 GHz optical pulse train is modulated by its 10.06 GHz subharmonic. The spectral intensity at the frequencies where the upper side bands and the lower side bands of neighboring longitudinal modes meet varies with the RF phase shifts imposed on the subharmonic tone only when the longitudinal modes have time-invariant phase relationship.