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WO1989002067A1 - Improvements relating to optical sensing systems - Google Patents

Improvements relating to optical sensing systems Download PDF

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Publication number
WO1989002067A1
WO1989002067A1 PCT/GB1988/000681 GB8800681W WO8902067A1 WO 1989002067 A1 WO1989002067 A1 WO 1989002067A1 GB 8800681 W GB8800681 W GB 8800681W WO 8902067 A1 WO8902067 A1 WO 8902067A1
Authority
WO
WIPO (PCT)
Prior art keywords
optical fibre
sensor
splices
reflective
elements
Prior art date
Application number
PCT/GB1988/000681
Other languages
French (fr)
Inventor
Jack Marcus Keen
Michael Laurence Henning
Original Assignee
Plessey Overseas Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Plessey Overseas Limited filed Critical Plessey Overseas Limited
Publication of WO1989002067A1 publication Critical patent/WO1989002067A1/en

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
    • G01D5/35383—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using multiple sensor devices using multiplexing techniques
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
    • G01H9/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means
    • G01H9/004—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means using fibre optic sensors
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/264—Optical coupling means with optical elements between opposed fibre ends which perform a function other than beam splitting

Definitions

  • This invention relates to optical sensing systems and relates more specifically to optical fibre sensor arrays for use in such systems and comprising optical fibre sensors defined by an optical fibre provided along its length with a plurality of spaced partially reflective discontinatureties which effectively divide the optical fibre into a plurality of discrete optical fibre sensing elements and which serve to reflect a proportion of light signals propagating along the optical fibre sensor back along the sensor so that interference takes place between light reflected from successive discontinuities along the fibre or between light signals reflected from the discontinuities and reference signals to produce time displaced electrical signals in • opto-electric detector means indicative of the changes in state of the individual optical fibre sensor elements due to environmental changes such as due to pressure or temperature.
  • the partially reflective discontinuities spaced along the optical fibre sensor may be provided by various fibre splicing techniques to achieve the requisite level of reflection at the discontinuities.
  • One problem experienced with these reflectometric optical fibre sensors is the possibility of multiple light reflections occurring within the optical fibre giving rise to cross-talk between the optical fibre sensing elements and thereby tending to limit the performance of the sensors, especially those having a large number of sensor elements.
  • the reflection coefficients of the discontinuties or splices between sensing elements need to be as low as possible, a reflection coefficient of about 0.5% being a typical value.
  • Some improvement in the level of cross-talk may be achieved by reducing the reflection coefficients of the reflective splices to a lower value, say 0.1% but this lowers the signal levels within the sensor which introduces noise problems.
  • the problem of multiple reflections and consequential cross-talk in optical fibre sensors divided into a plurality of optical fibe sensor elements by reflective discontinuities or splices is alleviated without causing noise problems by appropriately varying the reflection coefficients of the partially reflective splices or discontinuities along the fibre sensor.
  • the reflection coefficients of successive reflective splices along the sensor may be gradually increased whereby the light signals arriving at successive optical fibre sensor elements along the sensor will be improved relative to any cross-talk signals which can only be received from previous sensing elements and is dependent upon the reflection coefficients of the reflective splices in front of the sensing element concerned.
  • the sharper the increase in the reflection coefficient of a splice the lower will be the level of cross-talk.
  • the reflective discontinuities or splices in the optical fibre sensor may be provided by a Fabry Perot etalon interposed between the ends of two optical fibre sensing elements with the gap being filled with a subsequently cured epoxy of different refractive index to the glass of the fibre elements and the length of the gap being adjusted to provide the desired reflection.
  • a Fabry Perot etalon interposed between the ends of two optical fibre sensing elements with the gap being filled with a subsequently cured epoxy of different refractive index to the glass of the fibre elements and the length of the gap being adjusted to provide the desired reflection.
  • suitable splicing arrangements may be used.
  • Figures 1 and 2 show respective optical fibre sensors comprising a plurality of optical fibre sensing elements separated by reflective splices having different patterns of reflection coefficients.
  • this shows an optical fibre sensor 1 comprising a plurality of sensing elements 2, 3, - 4, 5 and 6 separated by splices 7, 8, 9, 10 and 11.
  • cross-talk within the sensing elements may be reduced by gradually increasing the reflection coefficient of the splices along the optical fibre.
  • the increase in the coefficient is goemetric increasing progressively from B4r to r.
  • the minimum reflection coefficient ie. the reflection coefficient of the splice 7
  • the maximum ie. the reflection coefficient of splice 11
  • Calculations have shown that noise signals due to first order multiple reflections at the far end sensing elements of a twenty five sensing element sensor array is reduced by a factor of four.
  • the first three splices 18, 19 and 20 at the input end of the fibre have a constant reflection coefficient of low level (e.g. 0.1%) where the light level is highest and then rising geometrically as in the Figure 1 arrangement from splice 20 to splice 23.
  • the first eight splices may have a reflection coefficient of 0.1% then rising geometrically to 1% at the far end to provide an improvement in cross -talk of approximately sixteen times over a corresponding array having uniform reflection coefficient splices throughout the fibre.
  • the relationship between the reflection coefficients of the splices can be predetermined in such a way as to minimise the multiple reflection cross-talk problems experienced with conventional sensor arrays.

Landscapes

  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Optical Transform (AREA)
  • Measurement Of Mechanical Vibrations Or Ultrasonic Waves (AREA)

Abstract

The problem of multiple reflections and consequential cross-talk in optical fibre sensors divided into a plurality of optical fibre sensor elements by reflective discontinuities or splices is alleviated without causing noise problems by appropriately varying the reflection coefficients of the partially reflective splices or discontinuities along the fibre sensor.

Description

IMPROVEMENTS RELATING TO OPTICAL SENSING SYSTEMS
This invention relates to optical sensing systems and relates more specifically to optical fibre sensor arrays for use in such systems and comprising optical fibre sensors defined by an optical fibre provided along its length with a plurality of spaced partially reflective discontiunuities which effectively divide the optical fibre into a plurality of discrete optical fibre sensing elements and which serve to reflect a proportion of light signals propagating along the optical fibre sensor back along the sensor so that interference takes place between light reflected from successive discontinuities along the fibre or between light signals reflected from the discontinuities and reference signals to produce time displaced electrical signals in • opto-electric detector means indicative of the changes in state of the individual optical fibre sensor elements due to environmental changes such as due to pressure or temperature.
Our British Patent No. 2126820B to which attention is hereby directed describes and claims an optical sensing system of the above form which may be used for example in hydrophones for sensing the changes in length of the optical fibre elements due to the impingement of acoustic waves on the optical fibre sensor.
The partially reflective discontinuities spaced along the optical fibre sensor may be provided by various fibre splicing techniques to achieve the requisite level of reflection at the discontinuities. One problem experienced with these reflectometric optical fibre sensors is the possibility of multiple light reflections occurring within the optical fibre giving rise to cross-talk between the optical fibre sensing elements and thereby tending to limit the performance of the sensors, especially those having a large number of sensor elements. To minimise this cross-talk the reflection coefficients of the discontinuties or splices between sensing elements need to be as low as possible, a reflection coefficient of about 0.5% being a typical value.
Some improvement in the level of cross-talk may be achieved by reducing the reflection coefficients of the reflective splices to a lower value, say 0.1% but this lowers the signal levels within the sensor which introduces noise problems.
According to the present invention the problem of multiple reflections and consequential cross-talk in optical fibre sensors divided into a plurality of optical fibe sensor elements by reflective discontinuities or splices is alleviated without causing noise problems by appropriately varying the reflection coefficients of the partially reflective splices or discontinuities along the fibre sensor.
In carrying out the invention the reflection coefficients of successive reflective splices along the sensor may be gradually increased whereby the light signals arriving at successive optical fibre sensor elements along the sensor will be improved relative to any cross-talk signals which can only be received from previous sensing elements and is dependent upon the reflection coefficients of the reflective splices in front of the sensing element concerned. The sharper the increase in the reflection coefficient of a splice the lower will be the level of cross-talk.
The reflective discontinuities or splices in the optical fibre sensor may be provided by a Fabry Perot etalon interposed between the ends of two optical fibre sensing elements with the gap being filled with a subsequently cured epoxy of different refractive index to the glass of the fibre elements and the length of the gap being adjusted to provide the desired reflection. However, other suitable splicing arrangements may be used.
By way of example, the present invention will now be described with reference to the accompanying drawings in which:
Figures 1 and 2 show respective optical fibre sensors comprising a plurality of optical fibre sensing elements separated by reflective splices having different patterns of reflection coefficients.
Referring Figure 1, this shows an optical fibre sensor 1 comprising a plurality of sensing elements 2, 3, - 4, 5 and 6 separated by splices 7, 8, 9, 10 and 11. According to the invention cross-talk within the sensing elements may be reduced by gradually increasing the reflection coefficient of the splices along the optical fibre. As can be seen in the Figure the increase in the coefficient is goemetric increasing progressively from B4r to r. The minimum reflection coefficient (ie. the reflection coefficient of the splice 7) may be 0.1% whereas the maximum (ie. the reflection coefficient of splice 11) may be 0.5%. Calculations have shown that noise signals due to first order multiple reflections at the far end sensing elements of a twenty five sensing element sensor array is reduced by a factor of four. In the arrangement where the reflection coefficients of the splices are constant throughout the sensor the weighting is unity but this weighting is reduced in the non -uniform reflection coefficient arrangements according to the invention. Referring now to Figure 2 this shows an optical fibre sensor having elements 12, 13, 14, 15,16, and 17 separated by splices 18, 19, 20, 21, 22 and 23. In this embodiment of the invention the first three splices 18, 19 and 20 at the input end of the fibre have a constant reflection coefficient of low level (e.g. 0.1%) where the light level is highest and then rising geometrically as in the Figure 1 arrangement from splice 20 to splice 23. Using this principle in a twenty five element array the first eight splices may have a reflection coefficient of 0.1% then rising geometrically to 1% at the far end to provide an improvement in cross -talk of approximately sixteen times over a corresponding array having uniform reflection coefficient splices throughout the fibre.
As will be apparent from the foregoing the relationship between the reflection coefficients of the splices can be predetermined in such a way as to minimise the multiple reflection cross-talk problems experienced with conventional sensor arrays.

Claims

CLAIMS :-
1. An optical fibre sensor divided into a plurality of optical fibre sensor elements by reflective discontinuities or splices, in which multiple reflections and consequential cross-talk in the sensor is alleviated without causing noise problems by appropriately varying the reflection coefficients of the partially reflective splices or discontinuities along the fibre sensor.
2. An optical fibre sensor as claimed in claim 1, in which the reflection coefficients of successive reflective splices along the sensor are gradually increased whereby the light signals arriving at successive optical fibre sensor elements along the sensor will be improved relative to any cross-talk signals which can only be received from previous sensing elements and is dependent upon the reflection coefficients of the reflective splices in front of the sensing element concerned.
3. An optical fibre sensor as claimed in claim 1 or claim 2, in which the reflective discontinuites or splices in the optical fibre are provided by a Fabry Perot etalon interposed between the ends of two optical fibre sensing elements with the gap being filled with a subsequently cured epoxy of different refractive index to the gloss of the fibre elements and the length of the gaps being adjusted to provide the desired reflection.
PCT/GB1988/000681 1987-09-01 1988-08-17 Improvements relating to optical sensing systems WO1989002067A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB8720526 1987-09-01
GB8720526A GB2209212A (en) 1987-09-01 1987-09-01 Optical sensing systems

Publications (1)

Publication Number Publication Date
WO1989002067A1 true WO1989002067A1 (en) 1989-03-09

Family

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Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/GB1988/000681 WO1989002067A1 (en) 1987-09-01 1988-08-17 Improvements relating to optical sensing systems

Country Status (2)

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GB (1) GB2209212A (en)
WO (1) WO1989002067A1 (en)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0713077A1 (en) * 1994-11-17 1996-05-22 Alcatel Cable Detection and/or measurement procedure for physical entities using a distributed sensor
US5557400A (en) * 1995-02-15 1996-09-17 Hewlett-Packard Company Multiplexed sensing using optical coherence reflectrometry
WO2016142695A1 (en) * 2015-03-06 2016-09-15 Silixa Ltd. Method and apparatus for optical sensing
US10935418B2 (en) 2018-01-12 2021-03-02 Ap Sensing Gmbh High-rate fiber-optical distributed acoustic sensing

Families Citing this family (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2262803A (en) * 1991-12-24 1993-06-30 Marconi Gec Ltd An optical fibre sensor array
GB0302434D0 (en) * 2003-02-03 2003-03-05 Sensor Highway Ltd Interferometric method and apparatus for measuring physical parameters
CN101995227B (en) * 2010-09-29 2012-06-06 哈尔滨工程大学 Optical path autocorrelator for distributed optical fiber strain sensing measurement

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1984004439A1 (en) * 1983-04-26 1984-11-08 Central Electr Generat Board Measuring apparatus and method
DE3525314A1 (en) * 1985-07-16 1987-01-22 Hdw Elektronik Gmbh Measuring device for detecting measured quantities by means of optical sensors

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1984004439A1 (en) * 1983-04-26 1984-11-08 Central Electr Generat Board Measuring apparatus and method
DE3525314A1 (en) * 1985-07-16 1987-01-22 Hdw Elektronik Gmbh Measuring device for detecting measured quantities by means of optical sensors

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
A Gard conference, Preprint no. 383, Guided Optical Structures in the Military environment, Turkey, 23-27 September 1985, J.P. Dakin et al.: "Progress with multiplexed sensor arrays based on reflection at spliced joints between sensors", pages 5-1 - -5 *
Electronics Letters, volume 20, no. 1, 5 January 1984, (Stevenage, Herts., GB), J.P. Dakin et al.: "Novel optical fibre hydrophone array using a single laser source and detector", pages 53-54 *

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0713077A1 (en) * 1994-11-17 1996-05-22 Alcatel Cable Detection and/or measurement procedure for physical entities using a distributed sensor
US5557400A (en) * 1995-02-15 1996-09-17 Hewlett-Packard Company Multiplexed sensing using optical coherence reflectrometry
WO2016142695A1 (en) * 2015-03-06 2016-09-15 Silixa Ltd. Method and apparatus for optical sensing
GB2550774A (en) * 2015-03-06 2017-11-29 Silixa Ltd Method and apparatus for optical sensing
CN107567575A (en) * 2015-03-06 2018-01-09 希里克萨有限公司 Method and apparatus for optical sensing
US10883861B2 (en) 2015-03-06 2021-01-05 Silixa Ltd. Method and apparatus for optical sensing
GB2550774B (en) * 2015-03-06 2021-08-04 Silixa Ltd Method and apparatus for optical sensing
US11719560B2 (en) 2015-03-06 2023-08-08 Silixa Ltd. Method and apparatus for optical sensing
US10935418B2 (en) 2018-01-12 2021-03-02 Ap Sensing Gmbh High-rate fiber-optical distributed acoustic sensing

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Publication number Publication date
GB2209212A (en) 1989-05-04
GB8720526D0 (en) 1987-10-07

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