Past exam of the optics and electrodynamics course of the University of Cambridge / 2018 / Section B / B2 / Two-line frequency separation

For equal line intensities, the correlation is proportional to $e^{-i{\omega }_1\tau }+e^{-i{\omega }_2\tau }=2e^{-i\bar{\omega }\tau }\cos (\Delta \omega \tau /2)$. The two-line coherence beating envelope is therefore $|\cos (\Delta \omega \tau /2)|$.

In the original first plot, the first visibility zeros are at approximately $\tau =\pm 100\mathrm{ps}$; the envelope revives at $0$ and about $\pm 200\mathrm{ps}$. HenceUse the envelope recurrence or its nulls, not the much faster carrier fringes. The visibility zeros indicate equal component intensities in the ideal model; unequal intensities would leave a nonzero minimum. The arbitrary vertical normalization does not affect the inferred separation.