Apparatus for interferometric sensing

An apparatus for interferometric sensing, comprising a plurality of single-longitudinal mode laser sources to each provide radiation at a corresponding selected wavelength, and at least one control modulator to frequency or phase modulate the radiation from each laser source;

• a plurality of Fabry-Perot interferometers formed by Bragg Gratings written into optical fibre, each interferometer having a resonance wavelength corresponding to one of the selected wavelengths and being responsive to the modulated radiation at one of the selected wavelengths to each produce a reflected or transmitted optical output signal dependent on the corresponding interferometer path length;
• and one or more demodulators to demodulate the optical output signals and produce a plurality of respective measurement signals indicative of the optical path lengths of the respective interferometers.