Apparatus for deriving measurement signals from fibre optic distributed sensors

A distributed fibre optic sensor apparatus comprising:

• an optical source that launches at least first and second optical pulse pairs of a series pulse pairs into an optical fibre, each pulse having a frequency, wherein each pulse pair has a frequency configuration such that the first pulses of the series of pulse pairs have the same frequency as one another and the second pulses of the series of pulse pairs have the same frequency as one another, the pulses generated such that the phase relationship of the pulses of the first pulse pair and the phase relationship of the pulses of the second pulse pair are different by a predetermined relative phase difference;
• and
• a detector that detects radiation which is backscattered within the optical fibre and a processor coupled to said detector to form at least one channel signal comprising samples of the backscatter radiation, the samples acquired at substantially the same time after launch of each pulse pair;
• and to demodulate said channel signal to determine a phase value for the relevant section of optical fibre wherein:
• the pulses of the first and second pulse pairs have a frequency difference between them;
• and
• the first and second pulse pairs are generated so as to be launched into the optical fibre at different times;
• such that the time between launch of the first and second pulse pairs corresponds to the time for a relative phase change in a signal at a frequency equal to the frequency difference between the pulses in a pair equal to said predetermined relative phase difference.