Distributed sensing system

• US 6,285,446 B1
• Filed: 11/16/1999
• Issued: 09/04/2001
• Est. Priority Date: 05/19/1997
• Status: Expired due to Term

• Current Assignee: Sensornet Limited (Nova Holdings LLC)
• Original Assignee: Sensornet Limited (Nova Holdings LLC)
• Inventors: Farhadiroushan, Mahmoud
• Primary Examiner(s): Font, Frank G.
• Assistant Examiner(s): LEE, HWA S
• Application Number:
  US09/441,015
• Time in Patent Office: 658 Days
• Field of Search: 374/161, 356/35.5, 356/450
• US Class Current: 356/35.5
• CPC Class Codes: G01D 5/35383 using multiple sensor devices using multiplexing techniques
• CPC Class Codes: G01L 1/242 the material being an optical fibre

Abstract

A system for measuring the strain in a structure in which the temperature at the location the strain is measured is also measured in which an optic fiber is incorporated in the structure which has an interferometer positioned at the point or points where the strain is to be measured. A pulse of light is sent down the fiber and the backscattered light is split to separate the Raman Scattered Spectrum light. The interferometer is used to measure the strain and the Raman Scattered Spectrum light is used to measure temperature and loss.

Claims

1. A method for measuring strain and temperature at a location in a structure comprising:

providing a structure including optical fibre,

providing interferometer means positioned at a location along a length of the optical fibre where strain is to be measured,

transmitting a pulse of light along the optical fibre and reflecting the light with the interferometer means wherein the reflected light includes scattered Raman Scatter Spectrum light,

measuring the scattered Raman Scatter Spectrum light, and determining temperature from the measured Raman Scatter Spectrum light.

2. A method according to claim 1 wherein the interferometer means comprises a pair of reflective means with a path length between reflective means wherein the path length varies with strain in the fibre and temperature.

3. A method according to claim 2 wherein the pair of reflective means includes two reflective surfaces positioned apart wherein each reflective surface reflects a portion of the transmitted light.

4. A method according to claim 3 wherein the path length is greater than a coherence length of the transmitted light.

5. A method according to claim 3 wherein the reflective surfaces are splices in the fibre.

6. A method according to claim 2 further comprising converting path length variations in the interferometer means to an intensity modulation by comparison with a reference interferometer means which is not subject to strain.

7. A method according to claim 2 comprising detecting the amplitude of the scattered Raman Scatter Spectrum light and the reflected light.

8. A method according to claim 1 comprising detecting and measuring scattered Raman Scatter Spectrum light from a plurality of locations along the length of the optical fibre.

9. A method according to claim 4 comprising detecting and measuring scattered Raman Scatter Spectrum light from a plurality of locations along the length of the optical fibre.

10. A method according to claim 1 comprising detecting the amplitude of the scattered Raman Scatter Spectrum light and measuring anti-stokes and stokes components of the scattered Raman Scatter Spectrum light.

11. A method according to claim 4 comprising detecting the amplitude of the scattered Raman Scatter Spectrum light and measuring anti-stokes and stokes components of the scattered Raman Scatter Spectrum light.

12. A method according to claim 11 comprising conveying the reflected light to detection processing means comprising wavelength selection means, a reference interferometer means, detection means and processing means wherein the wavelength selection means selects and separates the stokes and anti-stokes components of the scattered Raman Scatter Spectrum light.

13. A method according to claim 1 comprising passing a portion of the reflected light through a reference interferometer and detecting an interference pattern.

14. A sensing network comprising:

a structure including at least one optical fibre,

interferometer means positioned at a location along the optical fibre where strain is to be measured,

means for transmitting a pulse of light along the optical fibre,

means for detecting and measuring light reflected back from the interferometer means wherein the reflected light includes scattered Raman Scatter Spectrum light wherein the means for detecting and measuring light includes different channels for receiving the reflected light and scattered Raman Scatter Spectrum light and means for determining temperature from scattered Raman Scatter Spectrum light.

15. A sensing network according to claim 14 wherein the optical fibers are selected from the group consisting of a single mode optical fibre, polarization maintaining optical fibre and multimode optical fibre and wherein the optical fibre comprises a plurality of sensing interferometers and further comprising means for converting magnitude of physical parameters to a change of optical path length of the sensing interferometers.

16. A sensing network according to claim 15 wherien the sensing interferometers are in-line interferometers and comprise reflective means.

17. A sensing network according to claim 14 wherein the wavelength selection means comprises directional couplers, optical gratings, optical filters and a monochromator.

18. A sensing means according to claim 14 wherein the optical fibre is high birefringence optical fibre and the sensing interferometer means includes pairs of polarization cross-coupling points along the length of high birefringence optical fibre and wherein an optical path length difference between polarization modes generated from said pairs of polarization cross-coupling points is greater than coherence of backscattered light and further comprising means for coupling a portion of light substantially in a first light polarization mode to light in another polarization mode orthogonal to the first polarization mode.

19. A sensing network according to claim 18 wherein the means for detecting includes a detector selected from the group consisting of photomultipliers, avalanche photodiodes, detector array amplifiers, multiplexers, routers and optical switches and wherein the means for detecting includes a digital electronic timing device.

20. A method for measuring strain and temperature at a location in a structure comprising:

providing an optical fibre,

providing an interferometer positioned at a location along a length of the optical fibre where strain is to be measured,

transmitting a pulse of light along the optical fibre and reflecting the light with the interferometer wherein the reflected light includes scattered Raman Scatter Secptrum light, and

measuring the scattered Raman Scatter Spectrum light and determining temperature from the measured Raman Scatter Spectrum light.

21. A sensing network comprising:

a structure including at least one optical fibre,

an interferometer positioned at a location along the optical fibre where strain is to be measured,

a light source which generates a pulse of light,

a detecting and measuring assembly which receives light reflected back from the interferometer wherein the reflected light includes scattered Raman Scatter Spectrum light and wherein the detecting and measuring assembly includes different channels for receiving the reflected light and scattered Raman Scatter Spectrum light and a processor for determining temperature from the scattered Raman Scatter Spectrum light.