= Delayed-fibre linewidth measurement
Two delayed copies of the same laser field probe its <first-order optical coherence>. With equal detected intensities and matched <polarizations>, the <interferometric visibility> is $V=|g^{(1)}(\tau)|$. A <Lorentzian line shape> of frequency full width at half maximum $\Delta\nu$ gives $V=e^{-\pi\Delta\nu|\tau|}$. For a fibre length difference $\Delta l$, the delay is $\tau=n_g\Delta l/c$, where $n_g$ is the <group index>. Hence $\Delta\nu=-\log V/(\pi|\tau|)$. Replacing $n_g$ by the effective phase index requires negligible dispersion. Polarization mismatch, unequal intensities and uncontrolled phase drift can reduce visibility independently of linewidth.
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