Apparatus for deriving measurement signals from fibre optic distributed sensors

Patent metadata

Field	Value
Publication number	US-9909903-B2
Application number	US-201213441802-A
Country	US
Kind code	B2
Filing date	Apr 6, 2012
Priority date	Apr 8, 2011
Publication date	Mar 6, 2018
Grant date	Mar 6, 2018

Abstract

The application describes methods and apparatus for distributed fiber sensing, especially distributed acoustic/strain sensing. The method involves launching at least first and second pulse pairs into an optical fiber, the first and second pulse pairs having the same frequency configuration as one another and being generated such that the phase relationship of the pulses of the first pulse pair has a predetermined relative phase difference to the phase relationship of the pulses of the second pulse pair. In one embodiment there is a frequency difference between the pulses in a pulse pair which is related to the launch rate of the pulse pairs. In another embodiment the phase difference between the pulses in a pair is varied between successive launches. In this way an analytic version of the backscatter interference signal can be generated within the baseband of the sensor.

First claim

The invention claimed is:

1. A distributed fibre optic sensor apparatus comprising:
an optical source, the optical source generating at least first and second optical pulse pairs for launching into an optical fibre, each pulse having a frequency, wherein the first pulses of the pulse pairs have the same frequency and the second pulses of the pulse pairs have the same frequency, and
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 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.

2. A distributed fibre optic sensor apparatus as claimed in claim 1 wherein said predetermined relative phase difference has a magnitude of 90°.

3. A distributed fibre optic sensor apparatus as claimed in claim 1 further comprising:
at least one detector configured to detect any radiation backscattered from said first and second pulse pairs; and
processing circuitry coupled to said at least one detector, wherein the processing circuitry is configured to determine a phase value for at least one given section of optical fibre based on the detected backscatter radiation from said first and second pulse pairs.

4. A distributed fibre optic sensor apparatus as claimed in claim 3 wherein the optical source, detector and processing means are configured such that the bandwidth of the optical detection path is sufficient to represent substantially all of the frequency content of the backscattered radiation.

5. A distributed fibre optic sensor apparatus as claimed in claim 3 wherein said optical detection path bandwidth is sufficient to represent substantially all the pulse frequencies of the pulses of a pulse pair.

6. A distributed fibre optic sensor apparatus as claimed in claim 3 wherein the processing circuitry is configured to demodulate independent instantaneous detector samples from said first and second pulse pairs to determine said phase value.

7. A distributed fibre optic sensor apparatus as claimed in claim 3 wherein the processing circuitry determines said phase value using said predetermined relative phase difference.

8. A distributed fibre optic sensor apparatus as claimed in claim 1 wherein the optical source is configured to launch a series of first and second pulse pairs into the optical fibre at a launch rate such that the time between launch of any pulse pairs in said series corresponds to the time for a relative phase change in a signal at a frequency equal to said frequency difference equal to said predetermined relative phase difference.

9. A distributed fibre optic sensor apparatus as claimed in claim 8 wherein said frequency difference is lower than the launch rate.

10. A distributed fibre optic sensor apparatus as claimed in claim 9 wherein said frequency difference is less than half of the launch rate.

11. A distributed fibre optic sensor apparatus as claimed in claim 8 wherein said frequency difference is substantially equal to a quarter of the launch rate such that said predetermined phase difference is substantially 90°.

12. A distributed fibre optic sensor apparatus as claimed in claim 8 wherein the optical source is configured to generate the pulse pairs at a launch rate such that there is only one pulse from said series in a sensing part of the optical fibre at any time.

13. A distributed fibre optic sensor apparatus as claimed in claim 1 wherein said frequency difference is 10 kHz or less.

14. A distributed fibre optic sensor apparatus as claimed in claim 1 where said frequency difference is within the baseband of the sensor.

15. A distributed fibre optic sensor apparatus as claimed in claim 1 where said frequency difference is within the Nyquist band of the sensor.

16. A distributed fibre optic sensor apparatus as claimed in claim 1 wherein said optical source comprises at least one laser and a modulator for modulating the light from said laser to produce the first and second pulse pairs.

17. A distributed fibre optic sensor apparatus as claimed in claim 16 wherein said modulator comprises at least one acousto-optic modulator.

18. A distributed fibre optic sensor apparatus as claimed in claim 1 , further comprising:
at least one detector configured to detect any radiation backscattered from said first and second pulse pairs; and
processing circuitry coupled to said at least one detector, wherein the processing circuitry is configured to determine a phase value for at least one given section of optical fibre based on the detected backscatter radiation from said first and second pulse pairs wherein said processing circuitry is configured to:
form a signal comprising samples of the backscatter radiation, the samples acquired at substantially the same time after launch of each pulse pair; and
demodulate said signal at the frequency difference of the pulses in each pulse pair so to determine a phase value for at least one section of optical fibre.

19. A distributed fibre optic sensor apparatus as claimed in claim 1 wherein:
the pulses in the first pulse pair and in the second pulse pair all have the same frequency as one another; and
the phase difference between the pulses in the first pulse pair differs from the phase difference between the pulses in the second pulse pair by said predetermined relative phase difference.

20. A distributed fibre optic sensor apparatus as claimed in claim 19 , further comprising:
at least one detector configured to detect any radiation backscattered from said first and second pulse pairs; and
processing circuitry coupled to said at least one detector, wherein the processing circuitry is configured to determine a phase value for at least one given section of optical fibre based on the detected backscatter radiation from said first and second pulse pairs, wherein said predetermined phase difference is substantially 90° and wherein the processing circuitry is configured to use the detected backscatter signals received the same time after launch of the first and second pulse pairs as in-phase and quadrature phase components respectively so as to determine a phase value for at least one section of optical fibre.

21. A distributed fibre optic sensor apparatus as claimed in claim 20 wherein the processing circuitry is configured to low-pass filter the detected backscatter signals received the same time after launch of the first and second pulse pairs to provide In-phase and Quadrature values and to convert said In-phase and Quadrature values to a phase value by rectangular to polar conversion.

22. A distributed fibre optic sensor apparatus as claimed in claim 19 wherein said optical source is configured to continually generate pulse pairs to be launched into the optical fibre at a launch rate wherein each successive pulse pair has a relative phase difference between the pulses of that pair that differs from that of the previous pair by said predetermined phase difference.

23. A distributed fibre optic sensor apparatus as claim

Assignees

• Lewis Andrew Biggerstaff
• Russell Stuart John
• Optasense Holdings Ltd

Inventors

• Lewis Andrew Biggerstaff
• Russell Stuart John

Classifications

• G01D5/35387

  using wavelength division multiplexing · CPC title

• G01D5/35396

  using other forms of multiplexing · CPC title

• G01D5/35312

  using a Fabry Perot · CPC title

• G01H9/004 Primary

  using fibre optic sensors (light guides per se G02B6/00, acousto-optical devices specially adapted for gating or modulating in optical wave guides G02F1/125) · CPC title

• G01D5/3539

  using time division multiplexing · CPC title