Measuring Brillouin backscatter from an optical fibre using a tracking signal

Apparatus for measuring Brillouin backscattering from an optical fibre comprising:

• an optical source operable to generate probe pulses of coherent light at a frequency f0 and launch the probe pulses into an optical fibre;
• a backscatter receiving component arranged to receive backscattered light from the optical fibre that includes at least one Brillouin spectral line at a frequency fB(t) shifted from f0 by a Brillouin shift and produce therefrom a first signal that is representative of fB(t);
• a programmable local oscillator;
• a memory array for storing information defining a Brillouin shift fB(t)−f0 previously measured from the optical fibre, the memory array connected to the programmable local oscillator such that the stored information is to be used by the programmable local oscillator to generate a second signal having a frequency f1(t) that varies with time in the same way as the previously measured Brillouin shift;
• a frequency mixer for receiving and mixing the first signal and the second signal to produce a difference signal at a difference frequency ΔF(t)=fB(t)−f1(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;
• an acquisition system operable to receive the difference signal from the frequency mixer and record the difference signal;
• and
• a processor operable to process the recorded difference signal to determine one or more properties of the Brillouin spectral line of the received backscattered light.