Asuming they are using some time domain reflectometry technique, I would
assume that it could be done on a live comms cable.

Fibre gyroscopes are popular on high end projects ... they measure
rotation in accuracies of the order of the number of wavelengths around
their circumference. they split the input light, the two signals going
in opposite direction around a circle. the recombination of the light
gives pulses with rotation ... I can't remember how they achieve
quadrature.

Mark
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