IIRC we measured the elongation of optic cable as it was being ploughed by
injecting a 200MHz modulation onto a laser & measuring the phase of the
reflecttion from the far end. Got better than 5mm resolultion over several
km of cable length. Of course, we had the advantage that the reference and
return signal were both available at the same location but there are ways
around this I think. (e.g multiple transmitters )
As above, all we used was a 200MHz sig gen and a vector voltmeter (& PC &
software), along with the required optical components. I think the part we
had the most trouble with was depositing a silver mirror onto the far end
of the fibre!
At 200Mhz,(in fibre), the wavelength is about 1.07m. 10mm (i.e 5mm increase
in cable length) then represents about  3.4 degrees of phase difference so
it is pretty easy to measure..

However, doing it 2400 times asecond for 3 different signal sources it a
somewhat different proposition!

RP

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