Optical sensor and method of use
An interferometer apparatus for an optical fiber system and method of use is described. The interferometer comprises an optical coupler and optical fibers which define first and second optical paths. Light propagating in the first and second optical paths is reflected back to the optical coupler to generate an interference signal. First, second and third interference signal components are directed towards respective first, second and third photodetectors. The third photodetector is connected to the coupler via a non-reciprocal optical device and is configured to measure the intensity of the third interference signal component directed back towards the input fiber. Methods of use in applications to monitoring acoustic perturbations and a calibration method are described.
1. An apparatus, comprising:
a pulsed light source;
an optical fibre deployed in an environment to be monitored and arranged to receive pulses of light from the pulsed light source;
an interferometer arranged to receive light backscattered and/or reflected from along the length of the optical fibre as the pulses of light travel along the fibre and to generate an interference signal in dependence thereon, the light being backscattered and/or reflected in dependence on acoustic perturbations incident along the length of the optical fibre, and the interference signal comprising first, second, and third interference signal components having phase shifts therebetween;
a first photodetector arranged to measure an intensity of the first interference signal component to provide first intensity data;
a second photodetector arranged to measure an intensity of the second interference signal component to provide second intensity data;
a third photodetector arrange to measure an intensity of the third interference signal component to provide third intensity data; and
a processor time synchronised with the pulsed light source and arranged to:
i) receive the first, second and third intensity data and to determine therefrom any optical phase modulation in the received light, and thereby obtain optical phase modulation data from along the length of the optical fibre; and
ii) identify acoustic perturbations incident along the optical fibre in dependence on the determined optical phase modulation, wherein the optical phase modulation data obtained from along, the length of the optical fibre is synchronised to enhance signal sensitivity.
2. The apparatus of claim 1 , wherein the interferometer further comprises a fourth photodetector arranged to monitor the intensity of the input light from the pulsed light source.
3. The apparatus of claim 1 , comprising oversampling the outputs of the photodetectors to provide multiple optical phase angle data over a spatial resolution of the apparatus.
4. The apparatus of claim 3 , wherein the outputs of the photodetectors are sampled at least twice over the spatial resolution.
5. The apparatus of claim 3 , comprising:
determining a visibility factor from the combined outputs of the photodetectors at each sampled point; and
providing as weighted signal average, of optical phase angle data from multiple sample points over the spatial resolution of the apparatus in dependence on the visibility factor.
6. The apparatus of claim 5 , comprising:
determining a visibility factor from the combined outputs of the photodetectors at each sampled point; and
adjusting the timing of the digitised samples of the photodetector outputs in dependence on the visibility factor.
7. The apparatus of claim 6 , wherein the adjusting the timing of the digitised samples is performed by a digitiser with dynamically varying clock cycles.
8. The apparatus of claim 1 , wherein the pulsed light source is a plurality of laser light pulses with a plurality of wavelengths.
9. The apparatus of claim 8 , wherein the plurality of light pulses are time shifted with respect to each other to control the cross-phase modulation between the plurality of light pulses.
10. The apparatus of claim 1 , further comprising calibration circuitry arranged to determine electrical offsets for the photodetectors during a first calibration condition in which no light propagates in the interferometer.
11. The apparatus of claim 1 , further comprising calibration circuitry arranged to determine one or more of a photodetector offset, a relative photodetector gain, and/or a coupling ratio of optical paths in the interferometer from the outputs of the photodetectors during a calibration condition in which an incoherent light source inputs incoherent light to the interferometer.