Past exam of the optics and electrodynamics course of the University of Cambridge / 2018 / Section B / B2 / Equally spaced spectral components

For $N$ equal-intensity lines of spacing $\Delta \omega$, sum their correlations as a geometric series:The interferogram of an equally spaced spectral multiplet has normalized visibilityIn the second original plot, full revivals occur at approximately $\tau =0,\pm 100,\pm 200\mathrm{ps}$, soBetween $0$ and $100\mathrm{ps}$ there are three zeros, near $25$, $50$ and $75\mathrm{ps}$, and two small subsidiary lobes. Since the model has $N-1$ zeros between adjacent revivals,The peak envelope of about $8$ and the intensity background of about $8$ are consistent with four equal components in the source's normalization, but the internal zero count is the useful shape-based check. The inference assumes the equal spectral weights represented by the graph; revival spacing alone does not fix the number of lines, and arbitrary unequal or hidden components would require more spectral information. Delayed self-interference measures spectral powers, not the relative phases between different source frequencies, so these revivals alone do not prove that the source emits a phase-locked pulse train.