WO2015110177A1 - Optical distributed sensing device and method for measurements over extended ranges - Google Patents

Optical distributed sensing device and method for measurements over extended ranges Download PDF

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Publication number
WO2015110177A1
WO2015110177A1 PCT/EP2014/051515 EP2014051515W WO2015110177A1 WO 2015110177 A1 WO2015110177 A1 WO 2015110177A1 EP 2014051515 W EP2014051515 W EP 2014051515W WO 2015110177 A1 WO2015110177 A1 WO 2015110177A1
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Prior art keywords
optical
distributed
distributed sensing
sensing device
sensing elements
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PCT/EP2014/051515
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French (fr)
Inventor
Etienne Rochat
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Omnisens Sa
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Priority to PCT/EP2014/051515 priority Critical patent/WO2015110177A1/en
Priority to EP14705291.4A priority patent/EP3100005A1/en
Publication of WO2015110177A1 publication Critical patent/WO2015110177A1/en

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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
    • 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
    • G01D5/35338—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 other arrangements than interferometer arrangements
    • G01D5/35354—Sensor working in reflection
    • G01D5/35358—Sensor working in reflection using backscattering to detect the measured quantity
    • G01D5/35364—Sensor working in reflection using backscattering to detect the measured quantity using inelastic backscattering to detect the measured quantity, e.g. using Brillouin or Raman backscattering

Definitions

  • the invention relates to an optical distributed sensing device allowing measurements over extended ranges.
  • the invention relates also to a method for doing such measurements.
  • the field of the invention is, but not limited to, distributed optical temperature and strain measurements devices and methods.
  • Optical distributed sensing techniques are widely used for collecting temperature and/or strain information along long-distance paths, or for monitoring large structures.
  • sensing elements which comprise optical fibers embedded in the structure or along the paths to monitor, so as to be submitted to surrounding temperature and/or strain environmental conditions. These environmental conditions modify locally the condition of propagation of the light into the sensing fibers, in a way which may be detected .
  • Optical Time Domain Reflectometry For instance, Optical Time Domain Reflectometry (OTDR) techniques are known. A pulsed wave is injected into the sensing fiber. A scattered wave is collected at the same end of that fiber, which results from Raleigh scattering of the incident wave on local inhomogeneities into the fiber. The scattered wave comprises terms which depend on the temperature and the strain along the sensing fiber, and which may be detected using a coherent detection scheme.
  • OTDR Optical Time Domain Reflectometry
  • BOTDR Brillouin Optical Time Domain Reflectometers
  • narrow pulses of light are injected into a sensing optical fiber of the distributed sensor.
  • a backscattered optical signal is collected on the same end of the fiber.
  • the optical signal comprises spectral components due to spontaneous Brillouin scattering generated along the sensing fiber by the propagation of the light pulses. The analysis of these spectral components provides information on the temperature and/or strain conditions along the measurement path.
  • BOTDA Brillouin Optical Time Domain Analysers
  • narrow pulses of light are injected into a sensing optical fiber of the distributed sensor.
  • a continuous probe optical wave is also injected into the sensing fiber, in the direction opposite to the light pulses.
  • the frequency of the probe optical wave is varied over a frequency range covering the frequency range of the spontaneous Brillouin scattering generated along the sensing fiber by the propagation of the light pulses.
  • a resonance condition is established, leading to the efficient stimulation of the Brillouin scattering . So, temperature and/or strain may be measured efficiently along the sensing fiber.
  • the forth and back time-of-flight of the optical signal into the distributed sensor establishes a limit in the repetition rate of the measurements. Otherwise signals due to successive measurements mix up. So long measurement ranges are not compatible with fast measurement rates.
  • a distributed optical sensing device for doing distributed measurements along a measurement path, comprising a plurality of optical fiber based distributed sensing elements arranged along said measurement path,
  • first optical linking means connected to a first end of said distributed sensing elements, and able to transfer separately measurement optical signals emerging from different distributed sensing elements towards detection means.
  • the device of the invention may further comprise at least one optical routing means connected to the first end of a distributed sensing element, and able to direct a first optical signal into said distributed sensing element, and to direct the measurement optical signal emerging from said distributed sensing element into a first optical linking means.
  • the optical routing means may comprise an optical circulator.
  • the sensing device of the invention may comprise at least one distributed sensing element having a second end optically connected through an optical routing means to the first end of another distributed sensing element.
  • It may further comprise at least one optical amplifier inserted between the second end of a distributed sensing element and an optical routing means.
  • the sensing device of the invention may further comprise at least one additional optical routing means, connected to a second end of a distributed sensing element, and able to direct a first optical signal emerging from said distributed sensing element towards an optical routing means, and to direct a second optical signal to the second end of said distributed sensing element.
  • the additional optical routing means may comprise an optical circulator.
  • the sensing device of the invention may further comprise second optical linking means for transferring a second optical signal to or from a second end of the distributed sensing elements.
  • the sensing device of the invention may comprise distributed sensing elements comprising two parallel optical branches arranged in a Mach-Zehnder interferometer configuration operating in transmission, with reflecting elements arranged so as to build with said branches also a Michelson interferometer operating in reflection.
  • the sensing elements may be similar to those described in the document US 2012/0224182.
  • the sensing device of the invention may then further comprise measurement optical routing means for transferring the measurement signals emerging from the second side of the sensing element towards the respective detection means.
  • the measurement signals in transmission and in reflection emerging respectively from the two sides of the sensing elements may be directed to the corresponding detection means.
  • the sensing device of the invention may implement an Optical Time-Domain Reflectometer (OTDR) scheme, and further comprise :
  • OTDR Optical Time-Domain Reflectometer
  • the sensing device of the invention may implement a Brillouin Optical Time-Domain Reflectometer (BOTDR) scheme, and further comprise :
  • the sensing device of the invention may implement a Brillouin Optical Time-Domain Analyser (BOTDA) scheme, and further comprise :
  • it may further comprise two laser sources for generating respectively the optical pulsed signal and the optical probe wave.
  • the detection means may comprise one detection channel per distributed sensing element.
  • a distributed sensing method for doing distributed measurements along a measurement path, using a plurality of optical fiber based distributed sensing elements arranged along said measurement path,
  • - Fig. 1 shows a first mode of realization of the invention
  • FIG. 2 shows a second mode of realization of the invention
  • FIG. 3 shows a third mode of realization of the invention
  • FIG. 4 shows a fourth mode of realization of the invention.
  • Fig . 1 to Fig. 4 show different modes of realization of the invention which have several characteristics in common.
  • the devices of the invention may include any other necessary components, such as amplifiers, isolators, polarizers...
  • a device of the invention basically comprises a distributed optical sensor 19 and an interrogation system 15, which allows doing measurements with the distributed optical sensor 19.
  • the interrogation system 15 may implement any known and applicable detection scheme for achieving optical distributed sensing .
  • the distributed optical sensor 19 uses optical fibers as sensing elements. It allows measuring for instance temperature and/or strain along a measurement path which may be several tens of kilometers long . It may also be used for detecting and locating intrusions, leaks, or any event which may be detected with optical fibers.
  • the distributed optical sensor 19 comprises a plurality (or at least two) optical fiber-based distributed sensing elements 10. These distributed sensing elements 10 are arranged along the measurement path . They are preferably evenly distributed along that measurement path so as to allow measurements at any position along the path.
  • the distributed sensing elements 10 are preferably made in a way similar to the way a classical single distributed optical sensor would be done for use with the detection scheme implemented in the interrogation system 15. They may for instance comprise single-mode optical fibers subjected to temperature and strain, or enclosed in protective casings so as to be protected from strain .
  • At least a first optical signal 12 is launched into the distributed sensor, and a measurement signal 13 is collected for further processing .
  • the first optical signal 12 is transferred to all the distributed sensing elements 10.
  • the first optical signal 12 is transferred to the respective distributed sensing elements 10 with a relative time delay which corresponds substantially to the difference in position along the measurement path of these distributed sensing elements 10.
  • the first optical signal 12 propagates in the different distributed sensing elements 10 in a time frame which is consistent with, or which corresponds to, their location along the measurement path .
  • the measurement signals 13 emerging from the different distributed sensing elements 10 are collected and processed separately.
  • each measurement signal 13 corresponds to a measurement path which is limited to one distributed sensing element 10. This brings several advantages :
  • the repetition rate of the measurement is usually limited by the forth and back travel time of the optical waves along the distributed sensor. If repetition rates higher than the forth and back travel time are used, the return signals corresponding to successive acquisitions may mix up. So, advantageously, in the invention the repetition rate is only limited by the length of the distributed sensing element 10.
  • Fig . 1 shows a mode of realization of the invention in which the distributed sensing elements 10 are connected in parallel .
  • a first optical signal 12 arising from the interrogation system 15 is directed to a first side of the distributed sensing elements 10, through an optical circulator 11.
  • the measurement signals 13 emerging from the distributed sensing elements 10 are directed by the optical circulator 11 towards distinct detection means 18 of the interrogation system 15.
  • the optical circulator 11 is a well-known optical device which allows :
  • An optical amplifier 14 may be inserted before the optical circulator 11 for amplifying the first optical signal 12 and compensate the losses due to the path in the optical fibers from the entrance side of the distributed sensor 19.
  • the length of the connection lines between the entrance side of the distributed sensor and the distributed sensing elements 10 is adjusted so as to introduce a time delay on the first optical signal 12 which is consistent with, or which corresponds to, the location of the distributed sensing elements 10 along the measurement path .
  • optical circulators 11 may be located at the level of the interrogation system 15 (at least when no optical amplifier 14 is used) so that the embedded part of the distributed sensor 19 comprises only passive and compact elements.
  • Fig . 2 shows a mode of realization of the invention in which the distributed sensing elements 10 are connected in series, or cascaded along a single optical line.
  • the first optical signal 12 arising from the interrogation system 15 is directed to a first side of a first distributed sensing element 10, through an optical circulator 11. After emerging from the second side of that distributed sensing element 10, it reaches a second optical circulator 11 for being transferred to the first side of a second distributed sensing element 10.
  • Several distributed sensing element 10 may be cascaded that way.
  • Optical amplifiers 14 may be inserted before the optical circulators 11 for restoring the first optical signal 12 emerging from the second sides of the distributed sensing elements 10.
  • the measurement signals 13 emerging respectively from the distributed sensing elements 10 are directed by the optical circulators 11 each towards a detection means 18 of the interrogation system 15.
  • each measurement signals 13 transmitted towards a detection means 18 corresponds to a single sensing element 10.
  • the modes of realization of Fig . 1 and Fig . 2 are adapted to interrogation systems 15 which allow measurements from a single end of the distributed sensor 19.
  • the interrogation systems 15 may implement for instance a Brillouin Optical Time Domain Reflectometer (BOTDR) scheme, as shown in Fig . 1. Of course, the same interrogation systems 15 may be used with the mode of realization of Fig . 2.
  • BOTDR Brillouin Optical Time Domain Reflectometer
  • the first optical signal 12 is then a pulsed pump signal 12 which is generated by a laser source 16 and a pulse generator 17.
  • the laser source 16 comprises a distributed feedback laser diode (DFB-DFB-DFB-DFB-DFB-DFB-DFB-DFB-DFB-DFB-DFB-DFB-DFB-DFB
  • LD which generates a continuous wave in the infrared or the near-infrared range.
  • the pulse generator 17 comprise for instance a semiconductor optical amplifier (SOA) driven by an electrical pulsed signal .
  • SOA semiconductor optical amplifier
  • a pulsed pump signal 12 comprising narrow pulses of light is injected into the distributed sensor 19 so as to travel through all the sensing elements 10.
  • a plurality of measurement signals 13, each corresponding to a backscattered optical signal generated into a distributed sensing element 10, are respectively acquired and processed by different detection and processing means 18.
  • These measurement signals 13 comprise spectral components due to spontaneous Brillouin scattering generated respectively along the distributed sensing elements 10 by the propagation of the light pulses.
  • These spectral components comprise Brillouin Stokes and anti-Stokes spectrums located at about ⁇ 11 GHz of the central frequency of the laser source, with a spectral width of about 30 M Hz.
  • the Stokes spectrum comprises frequency components at frequencies lower than the central frequency of the laser source and the anti-Stokes spectrum comprises frequency components at frequencies lower than that central frequency.
  • the measurement signal 13 is optically filtered (using Bragg bandpass filter) for selecting the Stokes or the anti-Stokes spectral components. It is then demodulated using an optical heterodyne detection in which it is mixed with the continuous optical signal of the laser source 16 used as local oscillator. A time-frequency analysis of the Brillouin spectrum is then performed to obtain information on the temperature and/or strain distribution over the respective distributed sensing elements 10.
  • a global temperature and/or strain distribution profile over the whole distributed sensor may then be obtained by combining the information from the detection and processing means 18.
  • the interrogation system 15 used with the modes of realization of Fig . 1 or Fig . 2 may also implement a coherent optical time domain reflectometer scheme (OTDR) .
  • OTDR coherent optical time domain reflectometer scheme
  • the first optical signal 12 is then a pulsed signal 12 which is generated by a laser source 16 and a pulse generator 17.
  • the laser source 16 comprises a distributed feedback laser diode (DFB- LD) which generates a continuous wave in the infrared or the near-infrared range.
  • DFB- LD distributed feedback laser diode
  • the pulse generator 17 comprise for instance a semiconductor optical amplifier (SOA) driven by an electrical pulsed signal .
  • SOA semiconductor optical amplifier
  • the device further comprises a source modulator (not shown) for varying the frequency of the pulsed signal 12. That source modulator is inserted between the laser source 16 and the pulse generator 17.
  • the source modulator comprises an electro-optic modulator configured so as to modulate the intensity of the incoming signal according to a Dual Side Band with Suppressed Carrier (DBS-SC) modulation scheme.
  • the generated optical signal comprises two spectral components located symmetrically relative to the frequency of the laser source 16. The frequency of these spectral components may be varied by varying the control signal applied to the electro-optic modulator.
  • the electro-optic modulator is preferably a lithium niobate electro-optic modulator based on a Mach-Zehnder architecture.
  • a control signal is applied, which comprises :
  • the other generated sideband is filtered out using an optical filter (for instance based on a Bragg grating or dielectric layers), so as to generate a sing le-sideband modulation scheme (SSB) .
  • an optical filter for instance based on a Bragg grating or dielectric layers
  • the so-generated pulsed signal 12 is injected into the d istributed sensor 17 so as to travel throug h all the sensing elements 10.
  • the pulsed sig nals 12 While propagating into the sensing elements 10, the pulsed sig nals 12 are partial ly scattered by inhomogeneities or other scatterers, and reverse propagating Raleig h scattering waves are generated .
  • the intensity of the measurement sig nals 13 comprises essential ly two terms :
  • a second interference term correspond ing to the interferences of optical waves which have been scattered several times on several consecutive scatterers along the sensing element 10.
  • the interference term depends on the phase of the interferences, or in other words on the optical path along the sensing element 10 between the scatterers. So, it depends on the respective ind ices of refraction of the fiber of the sensing element 10.
  • the interference term depends also of these parameters.
  • the freq uency of the pulsed sig nal 12 is varied using the source mod ulator, and several measurements are made with several pulses having different freq uencies for each acq uisition time .
  • the scatterers are due to physical inhomogeneities in the fibers of the sensing element 10, their d istribution pattern along the fiber is not supposed to change much between consecutive acq uisition times, at least for smal l variations of environmental cond itions.
  • the interference terms acqu ired at d ifferent frequencies d uring these two acq uisition times respectively are correlated, and the freq uency d ifferences which allow the best local matching or correlation of these interference terms are determined .
  • a g lobal temperature and/or strain d istribution profile over the whole d istributed sensor may then be obtained by combining the information from the detection and processing means 18.
  • Fig . 3 and Fig . 4 show some other modes of real ization of the invention which are adapted for being used with interrogations systems 15 which req aries an access to both side of the d istributed sensing elements 10.
  • They comprise linking means for transferring a second optical sig nal 30 to a second end of the distributed sensing elements 10.
  • Fig . 3 shows a mode of realization of the invention in which the d istributed sensing elements 10 are connected in parallel .
  • Fig . 1 It is similar to the mode of real ization of Fig . 1 , except that it further comprises optical fiber lines for conveying a second optical sig nal 30 to the second ends of the d istributed sensing elements 10.
  • Fig . 4 shows a mode of realization of the invention in which the d istributed sensing elements 10 are connected in series, or cascaded along a sing le optical line.
  • Optical amplifiers 14 may be inserted between the additional optical circulators 41 and the optical circulators 11 for restoring the first optical signal 12 emerging from the second sides of the distributed sensing elements 10.
  • Optical amplifiers 42 may also be added on the path of the second optical signal 30 before the additional optical circulators 41, for restoring that second optical signal 30 along the distributed sensor 19.
  • Fig . 3 and Fig . 4 allow for instance using an interrogation systems 15 implementing a stimulated Brillouin Optical Time Domain Analyser (BOTDA) scheme, as shown in Fig . 3.
  • BOTDA stimulated Brillouin Optical Time Domain Analyser
  • the same interrogation systems 15 may be used with the mode of realization of Fig . 4.
  • the interrogation system 15 comprises a light source 16 which is used for generating all necessary optical signals.
  • This light source 16 comprises a distributed feedback laser diode (DFB-LD) with a wavelength around 1.5 prn, which generates a continuous wave.
  • DFB-LD distributed feedback laser diode
  • a source coupler 31 directs a part of the light issued from the source 16 towards a pulse generator 17 for generating a first optical signal 12 which is an optical pulsed pump wave 12.
  • the pulse generator 17 comprises a semiconductor optical amplifier
  • SOA optical gating device
  • the source coupler 31 directs also a part of the light issued from the source 16 towards a probe modulator 32 for generating a second optical signal 30 which is a probe wave 30.
  • the probe modulator 32 comprises an electro-optic modulator configured so as to modulate the intensity of the incoming signal according to a Dual Side Band with Suppressed Carrier (DBS-SC) modulation scheme. So, the generated optical probe signal comprises two spectral components located symmetrically relative to the frequency of the laser source 16. The frequency of these spectral components may be varied by varying the control signal applied to the electro-optic modulator.
  • DBS-SC Dual Side Band with Suppressed Carrier
  • the electro-optic modulator is preferably a lithium niobate electro-optic modulator based on a Mach-Zehnder architecture.
  • a control signal is applied, which comprises:
  • the first optical signal 12 (the pulsed pump wave) and the second optical signal 30 (the probe wave) propagate in each distributed sensing element 10 in an opposite direction.
  • the conditions are met to generate a stimulated Brillouin scattering signal resulting from the interactions in the sensing element 10 of the backward-propagating optical probe wave 30 with the forward-propagating optical pulsed pump wave 12.
  • a plurality of measurement signals 13, each corresponding to a stimulated Brillouin scattering signal generated into a distributed sensing element 10, are then respectively acquired and processed by different detection and processing means 18.
  • the optical pulsed pump wave 12 comprises an optical frequency v PU which corresponds to the optical frequency of the laser source 16.
  • the optical probe wave 30 comprises two spectral components of optical frequencies v PR+ and v PR- . These spectral components are located symmetrically relative to the optical frequency v PU of the pulsed signal.
  • the propagation in the forward direction of the optical pulsed wave in the sensing elements 10 generates Brillouin scattering.
  • the spectrum of that Brillouin scattering comprises two spectral components, including a Stokes component around a center frequency v sB s lower than the pulsed signal optical frequency v PU and an anti-Stokes component around a center frequency v sBa s higher than the pulsed signal optical frequency v PU .
  • the spontaneous Brillouin scattering depends on the local conditions along the fibers of the sensing element 10
  • the Brillouin spectrum may also vary along the fiber depending on the local conditions of temperature and strain.
  • the frequency of the optical probe wave 30 is varied using the probe mod ulator 32 so as to scan the freq uency ranges where Brillouin scattering may appear.
  • the optical freq uency of the backward-propagating optical probe wave 30 is scanned over the spectral range of the spontaneous Brillouin scattering generated by the optical pulsed wave in the sensing element 10, an energy transfer occurs between both sig nals, which mod ifies the amplitude of the backward-propagating optical probe wave.
  • the optical freq uency of the probe wave at which the maximum mod ification of the probe wave amplitude occurs is defined as the Bril louin freq uency.
  • the energy transfer ind uces a gain in the Stokes reg ion of the Brillouin spectrum and a loss in the anti- Stokes reg ion .
  • the measurement sig nal 13 is optically filtered (using Bragg band pass filter) for selecting the anti-Stokes region, whose intensity is measured with a photodetector.
  • the Bril louin scattering spectrum may be sampled in frequency for any location along the sensing element 10.
  • a g lobal temperature and/or strain d istribution profile over the whole d istributed sensor may then be obtained by combining the information from the detection and processing means 18.

Abstract

The present invention concerns a distributed optical sensing device for doing distributed measurements along a measurement path, comprising a plurality of optical fiber based distributed sensing elements (10) arranged along said measurement path, and first optical linking means connected to a first end of said distributed sensing elements (10) and able to transfer separately measurement optical signals (13) emerging from different distributed sensing elements (10) towards detection means (18). The present invention concerns also a method for doing distributed measurements along a measurement path device.

Description

« Optical distributed sensing device and method for measurements over extended ranges »
Field of the invention
The invention relates to an optical distributed sensing device allowing measurements over extended ranges. The invention relates also to a method for doing such measurements.
The field of the invention is, but not limited to, distributed optical temperature and strain measurements devices and methods.
Background of the invention
Optical distributed sensing techniques are widely used for collecting temperature and/or strain information along long-distance paths, or for monitoring large structures.
They use sensing elements which comprise optical fibers embedded in the structure or along the paths to monitor, so as to be submitted to surrounding temperature and/or strain environmental conditions. These environmental conditions modify locally the condition of propagation of the light into the sensing fibers, in a way which may be detected .
Several measurement techniques are known.
For instance, Optical Time Domain Reflectometry (OTDR) techniques are known. A pulsed wave is injected into the sensing fiber. A scattered wave is collected at the same end of that fiber, which results from Raleigh scattering of the incident wave on local inhomogeneities into the fiber. The scattered wave comprises terms which depend on the temperature and the strain along the sensing fiber, and which may be detected using a coherent detection scheme.
Techniques based on Brillouin scattering are also known.
In Brillouin Optical Time Domain Reflectometers (BOTDR), narrow pulses of light are injected into a sensing optical fiber of the distributed sensor. A backscattered optical signal is collected on the same end of the fiber. The optical signal comprises spectral components due to spontaneous Brillouin scattering generated along the sensing fiber by the propagation of the light pulses. The analysis of these spectral components provides information on the temperature and/or strain conditions along the measurement path. In Brillouin Optical Time Domain Analysers (BOTDA), narrow pulses of light are injected into a sensing optical fiber of the distributed sensor. A continuous probe optical wave is also injected into the sensing fiber, in the direction opposite to the light pulses. The frequency of the probe optical wave is varied over a frequency range covering the frequency range of the spontaneous Brillouin scattering generated along the sensing fiber by the propagation of the light pulses. When the frequency of the probe optical wave falls within the frequency range of the spontaneous Brillouin scattering generated by the pulsed optical wave, a resonance condition is established, leading to the efficient stimulation of the Brillouin scattering . So, temperature and/or strain may be measured efficiently along the sensing fiber.
We know also the document US 2012/0224182 which discloses a distributed sensor comprising a combination of a Mach-Zehnder interferometer and a Michelson interferometer, for detecting and locating events along the measurement path.
All these distributed sensing techniques have in common to be limited in range, even if such range may for some of them extend over 100 Km.
Extended ranges give rise to two kinds of problems:
- Excessive power losses due to the propagation of the optical waves in the fibers impact the signal to noise ratio and limit the reachable range. And there are also some limitations in the optical power which may be injected into the sensing fibers, due to non-linear effects which degrade the measurements. So the maximum power on one side and the signal to noise ratio on the other side establish a limit in the possible measurement range;
- For time resolved methods, the forth and back time-of-flight of the optical signal into the distributed sensor establishes a limit in the repetition rate of the measurements. Otherwise signals due to successive measurements mix up. So long measurement ranges are not compatible with fast measurement rates.
It is an object of the invention to provide an optical distributed sensing device and method which allows measurements over extended ranges.
It is also an object of the invention to provide an optical distributed sensing device and method which allows fast measurement rates, even for measurements over extended ranges.
Summary of the invention Such objects are accomplished through a distributed optical sensing device for doing distributed measurements along a measurement path, comprising a plurality of optical fiber based distributed sensing elements arranged along said measurement path,
characterized in that it further comprises first optical linking means, connected to a first end of said distributed sensing elements, and able to transfer separately measurement optical signals emerging from different distributed sensing elements towards detection means.
The device of the invention may further comprise at least one optical routing means connected to the first end of a distributed sensing element, and able to direct a first optical signal into said distributed sensing element, and to direct the measurement optical signal emerging from said distributed sensing element into a first optical linking means.
The optical routing means may comprise an optical circulator.
According to some modes of realization, the sensing device of the invention may comprise at least one distributed sensing element having a second end optically connected through an optical routing means to the first end of another distributed sensing element.
It may further comprise at least one optical amplifier inserted between the second end of a distributed sensing element and an optical routing means.
The sensing device of the invention may further comprise at least one additional optical routing means, connected to a second end of a distributed sensing element, and able to direct a first optical signal emerging from said distributed sensing element towards an optical routing means, and to direct a second optical signal to the second end of said distributed sensing element.
The additional optical routing means may comprise an optical circulator.
According to some modes of realization, the sensing device of the invention may further comprise second optical linking means for transferring a second optical signal to or from a second end of the distributed sensing elements.
According to some modes of realization, the sensing device of the invention may comprise distributed sensing elements comprising two parallel optical branches arranged in a Mach-Zehnder interferometer configuration operating in transmission, with reflecting elements arranged so as to build with said branches also a Michelson interferometer operating in reflection. In that case, the sensing elements may be similar to those described in the document US 2012/0224182.
The sensing device of the invention may then further comprise measurement optical routing means for transferring the measurement signals emerging from the second side of the sensing element towards the respective detection means. Thus, the measurement signals in transmission and in reflection emerging respectively from the two sides of the sensing elements may be directed to the corresponding detection means.
According to some modes of realization, the sensing device of the invention may implement an Optical Time-Domain Reflectometer (OTDR) scheme, and further comprise :
- means for generating a first optical signal corresponding to a probe signal ;
- detection means and processing means for detecting and processing the measurement optical signals respectively due to Raleigh scattering within the distributed sensing elements.
It may implement any relevant Optical Time-Domain Reflectometer (OTDR) scheme, such as for instance Correlation based OTDR, Coherence based OTDR, or Polarization based OTDR.
According to some modes of realization, the sensing device of the invention may implement a Brillouin Optical Time-Domain Reflectometer (BOTDR) scheme, and further comprise :
- means for generating a first optical signal corresponding to a pulsed pump wave,
- detection means and processing means for detecting and processing the measurement optical signals respectively due to Brillouin scattering within the distributed sensing elements.
It may then further comprise a continuous laser source and an amplitude modulator or a gating device for generating the pulsed pump wave.
According to some modes of realization, the sensing device of the invention may implement a Brillouin Optical Time-Domain Analyser (BOTDA) scheme, and further comprise :
- means for generating a first optical signal corresponding to a pulsed pump wave, - means for generating a second optical signal corresponding to a continuous probe wave,
- detection means and processing means for detecting and processing the measurement optical signals respectively due to stimulated Brillouin scattering interactions between the pulsed pump wave and the probe wave within the distributed sensing elements.
It may then further comprise a laser source used for generating the optical pulsed signal and the optical probe wave.
Alternatively, it may further comprise two laser sources for generating respectively the optical pulsed signal and the optical probe wave.
In the sensing device of the invention, the detection means may comprise one detection channel per distributed sensing element.
According to another aspect, it is proposed a distributed sensing method for doing distributed measurements along a measurement path, using a plurality of optical fiber based distributed sensing elements arranged along said measurement path,
characterized in that it comprises steps of:
- directing a first optical signal into said distributed sensing elements,
- transferring separately measurement optical signals emerging from different distributed sensing elements towards detection means.
Description of the drawings
The methods according to embodiments of the present invention may be better understood with reference to the drawings, which are given for illustrative purposes only and are not meant to be limiting. Other aspects, goals and advantages of the invention shall be apparent from the descriptions given hereunder.
- Fig. 1 shows a first mode of realization of the invention,
- Fig. 2 shows a second mode of realization of the invention,
- Fig. 3 shows a third mode of realization of the invention,
- Fig. 4 shows a fourth mode of realization of the invention.
Detailed description of the invention
Fig . 1 to Fig. 4 show different modes of realization of the invention which have several characteristics in common.
For sake of clarity, only the components which are relevant for the description of the invention are mentioned . Of course, the devices of the invention may include any other necessary components, such as amplifiers, isolators, polarizers...
A device of the invention basically comprises a distributed optical sensor 19 and an interrogation system 15, which allows doing measurements with the distributed optical sensor 19.
The interrogation system 15 may implement any known and applicable detection scheme for achieving optical distributed sensing .
The distributed optical sensor 19 uses optical fibers as sensing elements. It allows measuring for instance temperature and/or strain along a measurement path which may be several tens of kilometers long . It may also be used for detecting and locating intrusions, leaks, or any event which may be detected with optical fibers.
According to the invention, the distributed optical sensor 19 comprises a plurality (or at least two) optical fiber-based distributed sensing elements 10. These distributed sensing elements 10 are arranged along the measurement path . They are preferably evenly distributed along that measurement path so as to allow measurements at any position along the path.
The distributed sensing elements 10 are preferably made in a way similar to the way a classical single distributed optical sensor would be done for use with the detection scheme implemented in the interrogation system 15. They may for instance comprise single-mode optical fibers subjected to temperature and strain, or enclosed in protective casings so as to be protected from strain .
In most of the known techniques, such as Brillouin-based techniques or reflectometry techniques (using Raleigh scattering), at least a first optical signal 12 is launched into the distributed sensor, and a measurement signal 13 is collected for further processing .
According to the invention, the first optical signal 12 is transferred to all the distributed sensing elements 10.
In a preferred mode of realization, the first optical signal 12 is transferred to the respective distributed sensing elements 10 with a relative time delay which corresponds substantially to the difference in position along the measurement path of these distributed sensing elements 10. In other words, the first optical signal 12 propagates in the different distributed sensing elements 10 in a time frame which is consistent with, or which corresponds to, their location along the measurement path .
This allows achieving an accurate localization of the measured events along the measurement path, without having to know accurately the location of the distributed sensing elements 10 along that path .
According to the invention, the measurement signals 13 emerging from the different distributed sensing elements 10 are collected and processed separately.
So, each measurement signal 13 corresponds to a measurement path which is limited to one distributed sensing element 10. This brings several advantages :
- It is possible by using several distributed sensing elements 10 to do measurements on path lengths which are much longer than what is achievable using a single distributed sensor, because of the attenuation and the losses in the optical fibers ;
- It is also possible to do measurements with a much faster repetition rate. This is because the repetition rate of the measurement is usually limited by the forth and back travel time of the optical waves along the distributed sensor. If repetition rates higher than the forth and back travel time are used, the return signals corresponding to successive acquisitions may mix up. So, advantageously, in the invention the repetition rate is only limited by the length of the distributed sensing element 10.
Fig . 1 shows a mode of realization of the invention in which the distributed sensing elements 10 are connected in parallel .
A first optical signal 12 arising from the interrogation system 15 is directed to a first side of the distributed sensing elements 10, through an optical circulator 11.
The measurement signals 13 emerging from the distributed sensing elements 10 are directed by the optical circulator 11 towards distinct detection means 18 of the interrogation system 15.
The optical circulator 11 is a well-known optical device which allows :
- directing an optical signal incident on a first branch (label "1") of the circulator to a second branch (label "2"), and - directing an optical signal incident on the second branch of the circulator to a third branch (label "3") .
An optical amplifier 14 may be inserted before the optical circulator 11 for amplifying the first optical signal 12 and compensate the losses due to the path in the optical fibers from the entrance side of the distributed sensor 19.
Preferably, the length of the connection lines between the entrance side of the distributed sensor and the distributed sensing elements 10 is adjusted so as to introduce a time delay on the first optical signal 12 which is consistent with, or which corresponds to, the location of the distributed sensing elements 10 along the measurement path .
It should be noted that the optical circulators 11 may be located at the level of the interrogation system 15 (at least when no optical amplifier 14 is used) so that the embedded part of the distributed sensor 19 comprises only passive and compact elements.
Fig . 2 shows a mode of realization of the invention in which the distributed sensing elements 10 are connected in series, or cascaded along a single optical line.
The first optical signal 12 arising from the interrogation system 15 is directed to a first side of a first distributed sensing element 10, through an optical circulator 11. After emerging from the second side of that distributed sensing element 10, it reaches a second optical circulator 11 for being transferred to the first side of a second distributed sensing element 10. Several distributed sensing element 10 may be cascaded that way.
Optical amplifiers 14 may be inserted before the optical circulators 11 for restoring the first optical signal 12 emerging from the second sides of the distributed sensing elements 10.
The measurement signals 13 emerging respectively from the distributed sensing elements 10 are directed by the optical circulators 11 each towards a detection means 18 of the interrogation system 15.
It should be noted that, thanks to the optical circulators 11, the measurements signals 13 emerging from the sensing elements 10 do not travel through previous sensing elements 10 along the line. So, again, each measurement signals 13 transmitted towards a detection means 18 corresponds to a single sensing element 10. The modes of realization of Fig . 1 and Fig . 2 are adapted to interrogation systems 15 which allow measurements from a single end of the distributed sensor 19.
The interrogation systems 15 may implement for instance a Brillouin Optical Time Domain Reflectometer (BOTDR) scheme, as shown in Fig . 1. Of course, the same interrogation systems 15 may be used with the mode of realization of Fig . 2.
The first optical signal 12 is then a pulsed pump signal 12 which is generated by a laser source 16 and a pulse generator 17.
The laser source 16 comprises a distributed feedback laser diode (DFB-
LD) which generates a continuous wave in the infrared or the near-infrared range.
The pulse generator 17 comprise for instance a semiconductor optical amplifier (SOA) driven by an electrical pulsed signal .
A pulsed pump signal 12 comprising narrow pulses of light is injected into the distributed sensor 19 so as to travel through all the sensing elements 10.
A plurality of measurement signals 13, each corresponding to a backscattered optical signal generated into a distributed sensing element 10, are respectively acquired and processed by different detection and processing means 18.
These measurement signals 13 comprise spectral components due to spontaneous Brillouin scattering generated respectively along the distributed sensing elements 10 by the propagation of the light pulses. These spectral components comprise Brillouin Stokes and anti-Stokes spectrums located at about ± 11 GHz of the central frequency of the laser source, with a spectral width of about 30 M Hz. The Stokes spectrum comprises frequency components at frequencies lower than the central frequency of the laser source and the anti-Stokes spectrum comprises frequency components at frequencies lower than that central frequency.
In the detection and processing means 18, the measurement signal 13 is optically filtered (using Bragg bandpass filter) for selecting the Stokes or the anti-Stokes spectral components. It is then demodulated using an optical heterodyne detection in which it is mixed with the continuous optical signal of the laser source 16 used as local oscillator. A time-frequency analysis of the Brillouin spectrum is then performed to obtain information on the temperature and/or strain distribution over the respective distributed sensing elements 10.
A global temperature and/or strain distribution profile over the whole distributed sensor may then be obtained by combining the information from the detection and processing means 18.
The interrogation system 15 used with the modes of realization of Fig . 1 or Fig . 2 may also implement a coherent optical time domain reflectometer scheme (OTDR) . This mode of realization allows measuring the strain and the temperature along the sensing elements 10 using Raleigh scattering .
The first optical signal 12 is then a pulsed signal 12 which is generated by a laser source 16 and a pulse generator 17.
The laser source 16 comprises a distributed feedback laser diode (DFB- LD) which generates a continuous wave in the infrared or the near-infrared range.
The pulse generator 17 comprise for instance a semiconductor optical amplifier (SOA) driven by an electrical pulsed signal .
The device further comprises a source modulator (not shown) for varying the frequency of the pulsed signal 12. That source modulator is inserted between the laser source 16 and the pulse generator 17.
The source modulator comprises an electro-optic modulator configured so as to modulate the intensity of the incoming signal according to a Dual Side Band with Suppressed Carrier (DBS-SC) modulation scheme. So, the generated optical signal comprises two spectral components located symmetrically relative to the frequency of the laser source 16. The frequency of these spectral components may be varied by varying the control signal applied to the electro-optic modulator.
The electro-optic modulator is preferably a lithium niobate electro-optic modulator based on a Mach-Zehnder architecture. In order to generate the Dual Side Band with Suppressed Carrier (DSB-SC) modulation, a control signal is applied, which comprises :
- a bias voltage establishing a destructive interferences condition between the optical waves in both arms of the interferometer (extinction condition), and, - a mod ulation freq uency corresponding to the desired freq uency sh ift of the spectral components of the pulsed sig nal 12 relative to the optical frequency of the laser source 16.
As only one optical freq uency or only one sideband is necessary for the measurements, the other generated sideband is filtered out using an optical filter (for instance based on a Bragg grating or dielectric layers), so as to generate a sing le-sideband modulation scheme (SSB) .
The so-generated pulsed signal 12 is injected into the d istributed sensor 17 so as to travel throug h all the sensing elements 10.
While propagating into the sensing elements 10, the pulsed sig nals 12 are partial ly scattered by inhomogeneities or other scatterers, and reverse propagating Raleig h scattering waves are generated .
A plurality of measurement sig nals 13, each correspond ing to a backscattered optical sig nal generated into a d istributed sensing element 10, are respectively acq uired and processed by d ifferent detection and processing means 18.
The intensity of the measurement sig nals 13 comprises essential ly two terms :
- a first term corresponding to a sum of time-delayed intensities of the optical waves generated by the scatterers al l along the corresponding sensing element 10. This term, which corresponds to an echo term does not bring much information ;
- a second interference term correspond ing to the interferences of optical waves which have been scattered several times on several consecutive scatterers along the sensing element 10.
The interference term depends on the phase of the interferences, or in other words on the optical path along the sensing element 10 between the scatterers. So, it depends on the respective ind ices of refraction of the fiber of the sensing element 10.
As the index of refraction of the fiber is sensitive to the temperature and the strain, the interference term depends also of these parameters.
For measuring the variations of index of refraction along a sensing element 10 between two acqu isition times, the freq uency of the pulsed sig nal 12 is varied using the source mod ulator, and several measurements are made with several pulses having different freq uencies for each acq uisition time . As the scatterers are due to physical inhomogeneities in the fibers of the sensing element 10, their d istribution pattern along the fiber is not supposed to change much between consecutive acq uisition times, at least for smal l variations of environmental cond itions.
In order to determine the local variations of ind ices of refraction along the sensing element 10 between two acq uisition times, the interference terms acqu ired at d ifferent frequencies d uring these two acq uisition times respectively are correlated, and the freq uency d ifferences which allow the best local matching or correlation of these interference terms are determined .
The effect of a freq uency variation on an interference term being fairly eq uivalent to the effect of a variation of index of refraction , the correspond ing local variations of index of refraction are determined .
So, a g lobal temperature and/or strain d istribution profile over the whole d istributed sensor may then be obtained by combining the information from the detection and processing means 18.
Fig . 3 and Fig . 4 show some other modes of real ization of the invention which are adapted for being used with interrogations systems 15 which req uire an access to both side of the d istributed sensing elements 10.
They comprise linking means for transferring a second optical sig nal 30 to a second end of the distributed sensing elements 10.
Fig . 3 shows a mode of realization of the invention in which the d istributed sensing elements 10 are connected in parallel .
It is similar to the mode of real ization of Fig . 1 , except that it further comprises optical fiber lines for conveying a second optical sig nal 30 to the second ends of the d istributed sensing elements 10.
Fig . 4 shows a mode of realization of the invention in which the d istributed sensing elements 10 are connected in series, or cascaded along a sing le optical line.
It is similar to the mode of real ization of Fig . 2, except that it further comprises add itional optical circulators 41 for conveying a second optical sig nal 30 to the second ends of the d istributed sensing elements 10.
These add itional optical circulators 41 are inserted between the second end of the d istributed sensing elements 10 and the circulators 11 for conveying the first optical signals 12. So, they convey the second optical sig nal 30 to the second end of a preceding d istributed sensing element 10, and the first optical signals 12 emerging from that distributed sensing element 10 towards the next distributed sensing element 10.
Optical amplifiers 14 may be inserted between the additional optical circulators 41 and the optical circulators 11 for restoring the first optical signal 12 emerging from the second sides of the distributed sensing elements 10.
Optical amplifiers 42 may also be added on the path of the second optical signal 30 before the additional optical circulators 41, for restoring that second optical signal 30 along the distributed sensor 19.
The modes of realization of Fig . 3 and Fig . 4 allow for instance using an interrogation systems 15 implementing a stimulated Brillouin Optical Time Domain Analyser (BOTDA) scheme, as shown in Fig . 3. Of course, the same interrogation systems 15 may be used with the mode of realization of Fig . 4.
The interrogation system 15 comprises a light source 16 which is used for generating all necessary optical signals. This light source 16 comprises a distributed feedback laser diode (DFB-LD) with a wavelength around 1.5 prn, which generates a continuous wave.
A source coupler 31 directs a part of the light issued from the source 16 towards a pulse generator 17 for generating a first optical signal 12 which is an optical pulsed pump wave 12.
The pulse generator 17 comprises a semiconductor optical amplifier
(SOA) driven by an electrical pulsed signal . It is used as an optical gating device for generating the optical pulsed pump wave 12.
The source coupler 31 directs also a part of the light issued from the source 16 towards a probe modulator 32 for generating a second optical signal 30 which is a probe wave 30.
The probe modulator 32 comprises an electro-optic modulator configured so as to modulate the intensity of the incoming signal according to a Dual Side Band with Suppressed Carrier (DBS-SC) modulation scheme. So, the generated optical probe signal comprises two spectral components located symmetrically relative to the frequency of the laser source 16. The frequency of these spectral components may be varied by varying the control signal applied to the electro-optic modulator.
The electro-optic modulator is preferably a lithium niobate electro-optic modulator based on a Mach-Zehnder architecture. In order to generate the Dual Side Band with Suppressed Carrier (DSB-SC) modulation, a control signal is applied, which comprises:
- a bias voltage establishing a destructive interferences condition between the optical waves in both arms of the interferometer (extinction condition), and,
- a modulation frequency corresponding to the desired frequency shift of the spectral components of the optical probe signal relative to the optical frequency of the incoming optical signal.
As a result, the first optical signal 12 (the pulsed pump wave) and the second optical signal 30 (the probe wave) propagate in each distributed sensing element 10 in an opposite direction. Provided that their respective frequencies are correctly set as explained later, the conditions are met to generate a stimulated Brillouin scattering signal resulting from the interactions in the sensing element 10 of the backward-propagating optical probe wave 30 with the forward-propagating optical pulsed pump wave 12.
A plurality of measurement signals 13, each corresponding to a stimulated Brillouin scattering signal generated into a distributed sensing element 10, are then respectively acquired and processed by different detection and processing means 18.
The optical pulsed pump wave 12 comprises an optical frequency vPU which corresponds to the optical frequency of the laser source 16.
The optical probe wave 30 comprises two spectral components of optical frequencies vPR+ and vPR-. These spectral components are located symmetrically relative to the optical frequency vPU of the pulsed signal.
The propagation in the forward direction of the optical pulsed wave in the sensing elements 10 generates Brillouin scattering. The spectrum of that Brillouin scattering comprises two spectral components, including a Stokes component around a center frequency vsBs lower than the pulsed signal optical frequency vPU and an anti-Stokes component around a center frequency vsBas higher than the pulsed signal optical frequency vPU. As the spontaneous Brillouin scattering depends on the local conditions along the fibers of the sensing element 10, the Brillouin spectrum may also vary along the fiber depending on the local conditions of temperature and strain. For performing measurements, the frequency of the optical probe wave 30 is varied using the probe mod ulator 32 so as to scan the freq uency ranges where Brillouin scattering may appear.
When the optical freq uency of the backward-propagating optical probe wave 30 is scanned over the spectral range of the spontaneous Brillouin scattering generated by the optical pulsed wave in the sensing element 10, an energy transfer occurs between both sig nals, which mod ifies the amplitude of the backward-propagating optical probe wave. The optical freq uency of the probe wave at which the maximum mod ification of the probe wave amplitude occurs is defined as the Bril louin freq uency. The energy transfer ind uces a gain in the Stokes reg ion of the Brillouin spectrum and a loss in the anti- Stokes reg ion .
In the detection and processing means 18, the measurement sig nal 13 is optically filtered (using Bragg band pass filter) for selecting the anti-Stokes region, whose intensity is measured with a photodetector.
So, for a given probe freq uency vPR+, we obtain at the output of the photodetector an electrical sig nal whose time profile is representative of the Bril louin scattering along the correspond ing sensing element 10 at that probe freq uency vPR+. Knowing the speed of light in the fiber, the time profile may be converted in d istance profile. The resolution in d istance or time of the measurements depends on the pulse d uration of the optical pulsed sig nal .
Then, by scanning the probe freq uency vPR+ over the freq uency ranges where Bril louin spectrum may appear, the Bril louin scattering spectrum may be sampled in frequency for any location along the sensing element 10.
A g lobal temperature and/or strain d istribution profile over the whole d istributed sensor may then be obtained by combining the information from the detection and processing means 18.
While this invention has been described in conjunction with a number of embodiments, it is evident that many alternatives, modifications and variations wou ld be or are apparent to those of ord inary skil l in the appl icable arts. Accord ing ly, it is intended to embrace al l such alternatives, modifications, eq uivalents and variations that are within the spirit and scope of this invention .

Claims

1. A distributed optical sensing device for doing distributed measurements along a measurement path, comprising a plurality of optical fiber based distributed sensing elements (10) arranged along said measurement path,
characterized in that it further comprises first optical linking means, connected to a first end of said distributed sensing elements (10), and able to transfer separately measurement optical signals (13) emerging from different distributed sensing elements (10) towards detection means (18).
2. The sensing device of claim 1, which further comprises at least one optical routing means (11) connected to the first end of a distributed sensing element (10), and able to direct a first optical signal (12) into said distributed sensing element (10), and to direct the measurement optical signal (13) emerging from said distributed sensing element (10) into a first optical linking means.
3. The sensing device of claim 2, wherein the optical routing means (11) comprise an optical circulator.
4. The sensing device of claim 2 or 3, which comprises at least one distributed sensing element (10) having a second end optically connected through an optical routing means (11) to the first end of another distributed sensing element (10).
5. The sensing device of claim 4, which further comprises at least one optical amplifier (14) inserted between the second end of a distributed sensing element (10) and an optical routing means (11).
6. The sensing device of claims 4 or 5, which further comprises at least one additional optical routing means (41), connected to a second end of a distributed sensing element (10), and able to direct a first optical signal (12) emerging from said distributed sensing element (10) towards an optical routing means (11), and to direct a second optical signal (30) to the second end of said distributed sensing element (10).
7. The sensing device of claim 6, wherein the additional optical routing means (41) comprise an optical circulator.
8. The sensing device of any of claims 1 to 3, which further comprises second optical linking means for transferring a second optical signal (30) to or from a second end of the distributed sensing elements (10).
9. The sensing device of claim 8, which comprises distributed sensing elements (10) comprising two parallel optical branches arranged in a Mach- Zehnder interferometer configuration operating in transmission, with reflecting elements arranged so as to build with said branches also a Michelson interferometer operating in reflection.
10. The sensing device of any of claims 2 to 5, implementing an Optical Time-Domain Reflectometer (OTDR) scheme, which further comprises:
- means (16, 17) for generating a first optical signal (12) corresponding to a probe signal;
- detection means (18) and processing means for detecting and processing the measurement optical signals (13) respectively due to Raleigh scattering within the distributed sensing elements (10).
11. The sensing device of any of claims 2 to 5, implementing a Brillouin
Optical Time-Domain Reflectometer (BOTDR) scheme, which further comprises:
- means (16, 17) for generating a first optical signal (12) corresponding to a pulsed pump wave,
- detection means (18) and processing means for detecting and processing the measurement optical signals (13) respectively due to Brillouin scattering within the distributed sensing elements (10).
12. The sensing device of claim 11, which further comprises a continuous laser source (16) and an amplitude modulator (17) for generating the pulsed pump wave (12).
13. The sensing device of any of claims 6 to 8, implementing a Brillouin
Optical Time-Domain Analyser (BOTDA) scheme, which further comprises:
- means (16, 17) for generating a first optical signal (12) corresponding to a pulsed pump wave,
- means (16, 31, 32) for generating a second optical signal (30) corresponding to a continuous probe wave,
- detection means (18) and processing means for detecting and processing the measurement optical signals (13) respectively due to stimulated Brillouin scattering interactions between the pulsed pump wave (12) and the probe wave (30) within the distributed sensing elements (10).
14. The sensing device of claim 13, which comprises a laser source (16) used for generating the optical pulsed signal (12) and the optical probe wave (30).
15. The sensing device of any of claims 9 to 14, wherein the detection means (18) comprise one detection channel per distributed sensing element (10).
16. A distributed sensing method for doing distributed measurements along a measurement path, using a plurality of optical fiber based distributed sensing elements (10) arranged along said measurement path,
characterized in that it comprises steps of:
- directing a first optical signal (12) into said distributed sensing elements (10),
- transferring separately measurement optical signals (13) emerging from different distributed sensing elements (10) towards detection means
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