Fibre optic distributed sensing

• US 9,945,717 B2
• Filed: 09/06/2016
• Issued: 04/17/2018
• Est. Priority Date: 04/08/2011
• Status: Active Grant

• Current Assignee: Optasense Holdings Limited (Luna Innovations, Inc.)
• Original Assignee: Optasense Holdings Limited (Luna Innovations, Inc.)
• Inventors: Lewis, Andrew Biggerstaff, Russell, Stuart John
• Primary Examiner(s): Stock, Jr., Gordon J
• Application Number:
  US15/257,188
• Publication Number:
  US 20170199075A1
• Time in Patent Office: 588 Days
• CPC Class Codes: G01D 1/00 Measuring arrangements giving results other than momentary value of variable, of general application G01D3/00 takes precedence; in tariff metering apparatus G01D4/00; transducers not specially adapted for a specific variable G01D5/00
• CPC Class Codes: G01D 1/02 giving mean values, e.g. root means square values measuring root mean square values of currents or voltages G01R19/02
• CPC Class Codes: G01D 5/35303 using a reference fibre, e.g. interferometric devices
• CPC Class Codes: G01D 5/35306 using an interferometer arrangement
• CPC Class Codes: G01D 5/35312 using a Fabry Perot
• CPC Class Codes: G01D 5/35329 using interferometer with two arms in transmission, e.g. Mach-Zender interferometer
• CPC Class Codes: G01D 5/35345 using Amplitude variations to detect the measured quantity
• CPC Class Codes: G01D 5/35358 using backscattering to detect the measured quantity
• CPC Class Codes: G01D 5/35361 using elastic backscattering to detect the measured quantity, e.g. using Rayleigh backscattering
• CPC Class Codes: G01D 5/35367 using reflected light other than backscattered to detect the measured quantity
• CPC Class Codes: G01D 5/35387 using wavelength division multiplexing
• CPC Class Codes: G01D 5/3539 using time division multiplexing
• CPC Class Codes: G01D 5/35393 using frequency division multiplexing
• CPC Class Codes: G01D 5/35396 using other forms of multiplexing
• CPC Class Codes: G01H 9/004 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

Abstract

The application describes methods and apparatus for distributed fiber sensing, especially distributed acoustic/strain sensing. The method involves launching interrogating radiation in to an optical fiber and sampling radiation backscattered from within said fiber at a rate so as to acquire a plurality of samples corresponding to each sensing portion of interest. The plurality of samples are divided into separate processing channels and processed to determine a phase value for that channel. A quality metric is then applied to the processed phase data and the data combined to provide an overall phase value for the sensing portion based on the quality metric. The quality metric may be a measure of the degree of similarity of the processed data from the channels. The interrogating radiation may comprise two relatively narrow pulses separated by a relatively wide gap and the sampling rate may be set such that a plurality of substantially independent diversity samples are acquired.

Claims

1. A distributed fibre optic sensor apparatus comprising an optical source configured to, in use, repeatedly launch interrogating radiation into an optical fibre wherein the interrogating radiation comprises a first pulse followed a second pulse, wherein a time period between a first pulse and a second pulse is greater than a pulse duration of the first and/or second pulse and further comprising a sampling detector configured to acquire, in use, a plurality of samples in a duration equal to a time between the pulses and wherein, in use, said plurality of samples are acquired such that, for each sample, each contributing section of optical fibre from which a backscatter signal could be received at the detector from a pulse of interrogating radiation is substantially independent of the corresponding contributing section of a majority of the rest of the plurality of samples.

2. A distributed fibre optic sensor apparatus according to claim 1 which the plurality of samples are samples of radiation which is Rayleigh backscattered from said pulses within the optical fibre.

3. A distributed fibre optic sensor apparatus as claimed in claim 1 wherein the duration between the first and second pulses is at least twice the pulse duration of the first pulse and/or second pulse.

4. A distributed fibre optic sensor apparatus as claimed in claim 1 wherein the optical source is configured such that the first and second pulses have a frequency difference between them.

5. A distributed fibre optic sensor apparatus as claimed in claim 1 wherein the sampling detector is configured such that a time difference between any two samples in said plurality of samples is at least equal to a duration of the first pulse.

6. A distributed fibre optic sensor apparatus as claimed in claim 1 wherein a sampling rate of the detector is such that a series of successive samples can be used directly as the plurality of samples corresponding to a sensing portion of interest.

7. The distributed fibre optic sensor apparatus of claim 1 wherein the sampling detector acquires a plurality of samples from a sensing portion of interest and, wherein the distributed fibre optic sensor apparatus includes a processor configured to process said samples in separate diversity channels and to combine a result from said channels based on a similarity of resulting data from the separate diversity channels.

8. A distributed fibre optic sensor apparatus as claimed in claim 7 wherein said processor is configured to:

take a plurality of samples corresponding to radiation backscattered from at least part of a spatial sensing portion of interest;

divide said plurality of samples into the diversity channels;

process at least some of said channels to determine at least phase data for said channels; and

wherein the processor is configured to correlate the phase data from one channel with the phase data from the other channels.

9. A distributed fibre optic sensor apparatus as claimed in claim 7 wherein said processor is configured to:

take a plurality of samples corresponding to radiation backscattered from at least part of a spatial sensing portion of interest;

divide said plurality of samples into the diversity channels;

process at least some of said channels to determine at least phase data for said channels;

apply a quality metric to said phase data from said channels and determine results of applying said quality metric; and

combine the determined phase data for said channels into an overall phase value for said spatial sensing portion of interest based on the results of applying said quality metric, wherein the processor is further configured to applying a weighting applied to the phase data from each channel based on said quality metric and combine the phase data from the channels to provide said overall phase value according to said weighting.

10. A distributed fibre optic sensor apparatus as claimed in claim 7 wherein said processor is configured to:

take a plurality of samples corresponding to radiation backscattered from at least part of a spatial sensing portion of interest;

divide said plurality of samples into the diversity channels;

process at least some of the channels to determine at least phase data for said channels; and

wherein the processor is configured to determine a degree of similarity between the phase data from two channels by determining, for each channel, a degree of variation of current phase value from an average phase value for that channel.

11. A distributed fibre optic sensor apparatus as claimed in claim 7 wherein said processor is configured to:

take a plurality of samples corresponding to radiation backscattered from at least part of a spatial sensing portion of interest;

divide said plurality of samples into the separate diversity channels;

process at least some of said channels to determine at least phase data for said channels; and

wherein determining the similarity comprises a determination as to a degree of similarity between the phase data for a given channel and phase data from the other processed channels.

13. A distributed fibre optic sensor apparatus as claimed in claim 11 wherein said processor is further configured to:

apply a quality metric based on the degree of similarity to said phase data from said channels by analysing the phase data from said channels to determine whether results from one channel are similar to results from the other channels and determine results of applying said quality metric; and

combine the determined phase data for said channels into an overall phase value for said spatial sensing portion of interest based on the results of applying said quality metric.

14. A distributed fibre optic sensor apparatus as claimed in claim 11 wherein the processor is further configured to determine a score based on how similar the phase data from one channel is to the phase data from another channel.

15. A distributed fibre optic sensor apparatus as claimed in claim 10 wherein the processor is configured to determine, for each combination of two channels, a first metric of the form:

M1(A,B)=(A −

<

A>

)·

(B −

<

B>

),

where A and B are current data values from the channels and <

A> and

<

B>

are average values of the data from the channels.

16. A distributed fibre optic sensor apparatus as claimed in claim 15 wherein the processor is configured to determine a measurement of a magnitude of a difference between the phase data in two channels by determining, for each combination of two channels a second metric of the form:

M2(A,B)=((A −

<

A>

)−

(B −

<

B>

))².

17. A distributed fibre optic sensor apparatus as claimed in claim 16 wherein the processor is configured to determine an overall quality metric for each combination of two channels by determining a difference between said first and second metrics.

12. A distributed fibre optic sensor apparatus as claimed in 9 wherein said processor is configured such that phase data from channels which are similar to one another are given higher weightings than processed phase values that are less similar to one another.