Patent ID: 8134696

Claim:
A method for measuring Brillouin backscattering from an optical fibre, comprising: launching a probe pulse of coherent light with a frequency f 0 into an optical fibre; receiving backscattered light from the optical fibre that includes at least one Brillouin spectral line at a frequency f B (t) shifted from f 0 by a Brillouin shift, the Brillouin spectral line varying with time along the optical fibre; producing a first signal that is representative of f B (t); generating a second signal at a frequency f 1 (t) that varies with time in the same manner as a Brillouin shift f B (t)−f 0 previously measured from the optical fibre, using information defining the previously measured Brillouin shift; mixing the first signal and the second signal to produce a difference signal at a difference frequency ΔF(t)=f B (t)−f 1 (t), where ΔF(t) has a nominally constant value with respect to time corresponding to the received backscattered light having a Brillouin shift that matches the previously measured Brillouin shift; acquiring the difference signal; and processing the difference signal to determine one or more properties of the Brillouin spectral line of the received backscattered light.