WO2014177197A1 - Procédé et dispositif de détection à fibre optique distribuée de diffusion brillouin stimulée par double pompage - Google Patents

Procédé et dispositif de détection à fibre optique distribuée de diffusion brillouin stimulée par double pompage Download PDF

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Publication number
WO2014177197A1
WO2014177197A1 PCT/EP2013/059022 EP2013059022W WO2014177197A1 WO 2014177197 A1 WO2014177197 A1 WO 2014177197A1 EP 2013059022 W EP2013059022 W EP 2013059022W WO 2014177197 A1 WO2014177197 A1 WO 2014177197A1
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optical
wave
brillouin
stokes
spectrum
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PCT/EP2013/059022
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English (en)
Inventor
Luc Thevenaz
Miguel GONZALES HERRAEZ
Sébastien LEFLOCH
Chin SANGHOON
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Omnisens Sa
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Priority to PCT/EP2013/059022 priority Critical patent/WO2014177197A1/fr
Publication of WO2014177197A1 publication Critical patent/WO2014177197A1/fr

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    • 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
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
    • G01K11/00—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00
    • G01K11/32—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
    • G01K11/00—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00
    • G01K11/32—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres
    • G01K11/322—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres using Brillouin scattering
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
    • G01M11/30—Testing of optical devices, constituted by fibre optics or optical waveguides
    • G01M11/39—Testing of optical devices, constituted by fibre optics or optical waveguides in which light is projected from both sides of the fiber or waveguide end-face

Definitions

  • the invention relates to a stimulated Brillouin optical distributed sensing method which allows fast measurement rates and improved detection sensitivity.
  • the invention relates also to a device implementing the method.
  • the field of the invention is, but not limited to, distributed temperature and/or strain sensing using Brillouin scattering .
  • Brillouin scattering in optical fibers is a well-known technique for performing measurements of temperature and/or strain along long distances.
  • Brillouin scattering occurs when a light wave propagating in a medium
  • acoustic waves such as an optical fiber
  • the light wave interacts with these variations of index of refraction and a fraction of the light is scattered accordingly. Since acoustic waves propagate at the speed of sound in the medium, deflected light is also subjected to a Doppler shift, so its frequency changes.
  • the speed of sound in the medium depends on the temperature of the medium or on the strain. So, a variation of any of these parameters induces a variation of the frequency shift of the scattered light due to Brillouin scattering, and so may be measured.
  • an intense beam such as a laser beam travels in a medium such as an optical fiber
  • the variations in the electric field of the beam itself may produce acoustic vibrations in the medium.
  • the beam may undergo Brillouin scattering from these vibrations, usually in opposite direction to the incoming beam.
  • Brillouin optical time domain instruments have been done on the basis of this principle. They allow measuring the temperature and/or the strain along distributed sensors based on single-mode optical fibers which may be several kilometers long.
  • the applications relate main ly to the domains of geosciences, mining , oil exploitation, energy transportation, and civil eng ineering for the monitoring of large structures.
  • the d istributed sensors are embedded in the environment or the structures to mon itor. So, the optical fibers of these d istributed sensors are subjected to the variations of temperature and strain of the environment along their path .
  • BOTDA Bril louin Optical Time Domain Analyzers
  • An optical pulsed pump wave is launched into a sensing optical fiber of the d istributed sensor.
  • a continuous optical probe wave is launched into the same sensing fiber from the opposite end , so that the pump and the probe signals travel in the sensing fiber in opposite directions. So, the pump and the probe sig nals can interfere to activate electrostriction and mutually interact throug h the stimulated Bril louin scattering process (SBS) . Due to the pulsed nature of the pump, the SBS interaction between the pump and the probe takes place at different position along the fiber at different time.
  • SBS stimulated Bril louin scattering process
  • the SBS interaction results in an energy transfer between the pulsed pump wave and the probe wave.
  • the probe wave emerg ing from the sensing fiber is detected and processed so as to determine the SBS interaction and thus the variations of temperature and/or strain along the sensing fiber.
  • the Stokes spontaneous Brillouin spectrum is located at frequencies about 1 1 G Hz lower than the freq uency of the pump wave, while the anti-Stokes spontaneous Bril louin spectrum is located at frequencies about 11 G Hz higher than the freq uency of the pump wave .
  • Both Stokes and anti-Stokes Bril louin spectra have a Lorentzian shape, with the intrinsic bandwidth of about 30 MHz at full width at half maximum (FWHM).
  • the respective peak frequencies where the Brillouin gain and the Brillouin loss are the largest are usually defined as Brillouin frequencies. In standard conditions these Brillouin frequencies have a linear relationship with temperature and strain, typically of 1 MHz/K or 1 ⁇ /20 ⁇ respectively, ⁇ being the axial elongation or compression of the fiber.
  • measurements are usually done in loss mode.
  • the frequency of the probe wave is varied step-by- step across a frequency range covering the Brillouin loss spectrum.
  • a signal is acquired in the time domain whose local amplitude depends on the Brillouin interaction strength at the corresponding location along the sensing fiber.
  • the measurements at different frequencies are combined to reconstruct the shape of the Brillouin loss interaction spectrum and to compute the corresponding Brillouin frequency.
  • the probe wave is positioned at a frequency which is shifted relative to the Brillouin frequency (in pre-defined conditions of strain and temperature) so as to fall in a region of the Brillouin gain spectrum where the variability is high, such as for instance the frequency corresponding to the half maximum of the spectrum or the -3dB frequency.
  • the Brillouin gain spectrum shifts locally in frequency with variations of temperature and/or strain, the amplitude of the probe wave varies accordingly. Under these conditions, the Brillouin frequency may be obtained directly from the amplitude of the probe wave at a single frequency around the -3dB frequency.
  • the measurement range in terms of Brillouin frequency shift is limited to about the full width at half maximum (FWHM) of the Brillouin gain spectrum, which is about 30 MHz ( ⁇ 15 MHz) for the intrinsic gain spectrum.
  • the measurement range in terms of temperature and/or strain is limited accordingly;
  • the probe signal which is detected is a DC signal with low-frequency amplitude variations containing the Brillouin scattering information. So the noise perturbations, due for instance to the optical amplifiers and the intensity noise of the probe wave itself, have a direct impact on the sensitivity and the accuracy of the measurements.
  • Another drawback of the known BOTDA methods is that the systems require a tunable RF signal generator with a tuning range of ⁇ 1 GHz at 11 GHz, which is expensive.
  • the conventional BOTDA sensing systems preferably use a single laser to provide a pump signal and probe signal.
  • the laser is split to two branches. Light in one branch is sent to a pulse generator, generating pump pulse. Light in the other branch is sent to a frequency shifter and the frequency-shifted light delivered by the frequency shifter is used as probe signal.
  • an external electro-optic modulator EOM
  • RF radiofrequency
  • Phase-lock loop (PLL) circuit is in general used to generate such RF signal, but the inevitable phase noise caused in the PLL adds random fluctuation in amplitude of the probe signal when it is detected. It is an object of the invention to propose a Brillouin sensing method which allows mitigating the drawbacks of the prior art.
  • the first and the second optical pulsed pump waves and the optical probe wave are spectrally arranged so that, at least for a given set of temperature and/or strain conditions along at least a part of the sensing optical fiber, the optical probe wave comprises a probe spectral component located within the Stokes Brillouin spectrum of the first optical pulsed pump wave and a probe spectral component located within the anti- Stokes Brillouin spectrum of the second optical pulsed pump wave.
  • the first and the second optical pulsed pump waves may comprise:
  • the optical probe wave may comprise a modulated signal with a carrier signal and at least two spectral sidebands
  • the first and the second optical pulsed pump waves and the optical probe wave may be spectrally arranged so that, at least for a given set of temperature and/or strain conditions along at least a part of the sensing optical fiber, only the carrier signal or only the sidebands of the probe wave may interact with the Stokes Brillouin spectrum of the first optical pulsed pump wave and/or with the anti-Stokes Brillouin spectrum of the second optical pulsed pump wave.
  • the Stokes Brillouin spectrum and the anti-Stokes Brillouin spectrum may overlap at least partially in an overlap range comprising the average frequency located halfway between the first and second pump frequencies;
  • the optical probe wave may comprises a carrier signal spectrally located at said average frequency and sidebands located outside the Stokes Brillouin spectrum and the anti-Stokes Brillouin spectrum;
  • the optical probe wave may comprise a signal modulated in intensity by a modulation wave at a modulation frequency
  • the method of the invention may further comprise a step of measuring the amplitude of modulation of the stimulated Brillouin scattering signal at the modulation frequency.
  • the method of the invention may further comprise steps of:
  • the method of the invention may alternatively further comprise a step of calculating the Brillouin frequency of the Stokes Brillouin spectrum and/or the anti-Stokes Brillouin spectrum using a pre-established relation between said amplitude of modulation and said Brillouin frequency which takes into account a modulation of the index of refraction in the sensing optical fiber at the average frequency due to the stimulated Brillouin scattering interaction.
  • the optical probe wave may alternatively comprise a signal modulated in phase.
  • the optical probe wave may comprise a signal modulated in phase by a modulation wave at a modulation frequency, with (i) a carrier signal spectrally located at an average frequency halfway between the first and second pump frequencies and outside the Stokes Brillouin spectrum and the anti-Stokes Brillouin spectrum, and (ii) two sidebands located respectively, at least for a given set of temperature and/or strain conditions along at least a part of the sensing optical fiber, within the Stokes Brillouin spectrum and within the anti- Stokes Brillouin spectrum;
  • the method of the invention may further comprise a step of measuring the amplitude of modulation of the stimulated Brillouin scattering signal at the modulation frequency.
  • the method of the invention may further comprise steps of:
  • the method of the invention may alternatively further comprise a step of measuring the amplitude of modulation of the stimulated Brillouin scattering signal at the modulation frequency, and calculating the Brillouin frequency of the Stokes Brillouin spectrum and/or the anti-Stokes Brillouin spectrum using a pre-established relation between said amplitude of modulation and said Brillouin frequency which takes into account the gains and the losses respectively undergone by the sidebands through their respective stimulated Brillouin scattering interactions.
  • the optical probe wave may comprise a signal modulated in intensity by a modulation wave at a modulation frequency, with (i) a carrier signal spectrally located at an average frequency halfway between the first and second pump frequencies and outside the Stokes Brillouin spectrum and the anti-Stokes Brillouin spectrum, and (ii) two sidebands located respectively, at least for a given set of temperature and/or strain conditions along at least a part of the sensing optical fiber, within the Stokes Brillouin spectrum and within the anti-Stokes Brillouin spectrum,
  • the method of the invention may further comprise a step of measuring the amplitude of modulation of the stimulated Brillouin scattering signal at the modulation frequency.
  • the Stokes Brillouin spectrum and the anti-Stokes Brillouin spectrum may overlap at least partially in an overlap range comprising the average frequency located halfway between the first and second pump frequencies;
  • the optical probe wave may comprise a main probe spectral component spectrally located at a frequency which is different, but close to within a half spectral width of the Stokes or the anti-Stokes Brillouin spectrum, from said average frequency.
  • the main probe spectral component may be spectrally located at a frequency which differs from the average frequency of an amount corresponding to :
  • the method of the invention may further comprise steps of:
  • a Brillouin optical distributed sensing device comprising:
  • - pump generation means for providing a first optical pulsed pump wave at a first pump frequency and a second optical pulsed pump wave at a second pump frequency
  • - probe generation means for providing an optical probe wave comprising at least one probe spectral component
  • optical routing means for directing said first and second optical pulsed pump waves and said optical probe wave to a sensing optical fiber so that the optical probe wave propagates in said sensing optical fiber in a direction opposite to said first and second optical pulsed pump waves
  • the pump generation means and the probe generation means are able to arrange spectrally the first and the second optical pulsed pump waves and the optical probe wave so that, at least for a given set of temperature and/or strain conditions along at least a part of the sensing optical fiber, the optical probe wave comprises a probe spectral component located within the Stokes Brillouin spectrum of the first optical pulsed pump wave and a probe spectral component located within the anti-Stokes Brillouin spectrum of the second optical pulsed pump wave.
  • the pump generation means may be able to provide first and second optical pulsed pump waves comprising light pulses of the same duration .
  • the pump generation means may comprise: - a pump modulator for applying an intensity modulation with suppressed carrier scheme to a continuous incoming light wave at an average frequency located halfway between the first and second pump frequencies, and
  • - a pulse generator for generating first and second optical pulsed pump wave with pulses of the same duration.
  • the probe generation means may comprise a probe modulator able to modulate in phase and/or in intensity a continuous incoming light wave at the average frequency with a modulation signal at a modulation frequency lower than 1 GHz.
  • Fig. 1 shows a first optical set-up of the invention
  • FIG. 2 shows a second optical set-up of the invention
  • - Fig. 3 shows the spectral arrangement of the signals in a first mode of realization of the invention for a first measurement situation
  • - Fig. 4 shows the spectral arrangement of the signals in the first mode of realization of the invention for a second measurement situation
  • - Fig . 5 shows the measurement range in the first mode of realization of the invention for a first configuration of the probe wave
  • FIG. 6 shows the measurement range in the first mode of realization of the invention for a second configuration of the probe wave
  • - Fig . 7 shows the spectral arrangement of the signals in a second mode of realization of the invention for a first measurement situation
  • Fig. 8 shows the spectral arrangement of the signals in the second mode of realization of the invention for a second measurement situation
  • - Fig 9 shows a measurement signal corresponding to the first measurement situation in the second mode of realization of the invention
  • - Fig 10 shows a measurement signal corresponding to the second measurement situation in the second mode of realization of the invention
  • - Fig. 11 shows the spectral arrangement of the signals in a third mode of realization of the invention
  • - Fig. 14 shows the modulation amplitude of the detected signal in function of the Brillouin frequency shift in the third mode of realization of the invention.
  • Fig. 1 and Fig. 2 are very similar to well-known setups based on single-mode optical fibers for implementing BOTDA systems. So, for the sake of clarity, only the components which are relevant for the description of the invention and which differ from well-known set-ups are shown on Fig. 1 and Fig . 2. Of course, the device of the invention may include any other necessary components.
  • the sensing device of the invention comprises a light source 1 which is used for generating all necessary optical signals.
  • This light source 1 comprises a distributed feedback laser diode (DFB-LD) with a wavelength around 1.5 prn, which generates a continuous wave at frequency v L .
  • DFB-LD distributed feedback laser diode
  • a source coupler 2 directs a part of the light issued from the source 1 towards a pump modulator 3 and a pulse generator 4 for generating optical pulsed pump waves.
  • the pump modulator 3 is preferably an electro-optic modulator 3 configured so as to modulate the intensity of the incoming signal according to a Dual Side Band with Suppressed Carrier (DBS-SC) modulation scheme. It generates at its output an optical signal which comprises two spectral components with respective frequencies v P1 and v P2 located symmetrically relative to the frequency v L of the laser source 1. The frequency of these spectral components may be varied by varying the control signal applied to the electro-optic modulator 3.
  • DBS-SC Dual Side Band with Suppressed Carrier
  • the electro-optic modulator 3 is preferably a lithium niobate electro-optic modulator based on a Mach-Zehnder architecture.
  • a control signal is applied, which comprises :
  • the pulse generator 4 comprises a semiconductor optical amplifier (SOA) driven by an electrical pulsed signal . It is used as an optical gating device for generating simultaneously two optical pulsed pump waves 20, 23 with respective frequencies v P1 and v P2 located symmetrically relative to the frequency v L of the laser source 1, as shown for instance in Fig . 3. So, the pulses of the two pump waves 20, 23 are synchronous in time and have the same duration .
  • SOA semiconductor optical amplifier
  • the source coupler 2 directs also a part of the light issued from the source 1 towards a probe modulator 5 for generating a probe wave.
  • the probe modulator 5 may be different for the different modes of realization of the invention, so it will be described below in relation with these modes of realization .
  • the probe wave issued from the probe modulator 5 is then directed to a sensing fiber 6 of a distributed sensor connected to the sensing device.
  • That distributed sensor may be for instance embedded in a structure 7 for monitoring the strain and/or the temperature in that structure 7, which may be up to several kilometers long .
  • the device of the invention further comprises an optical circulator 8.
  • optical circulator 8 is a well-known optical component which allows : - d irecting an optical sig nal incident on a first branch (label " 1") of the circulator to a second branch (label "2”), and
  • the optical circulator 8 is arranged so as to :
  • the pulse generator 4 may comprise an electro-optic modulator or any optical component suitable for generating the two optical pulsed pump waves 20, 23.
  • the device of the invention may comprise polarization means such as polarizer(s) and/or polarization control ler(s) arranged so as to allow generating pumps waves propagating in the fibers with orthogonal polarizations.
  • the pu mp waves may for instance be linearly polarized along polarization axis shifted of about ⁇ 90 deg rees from each other.
  • orthogonal polarization allows limiting the four wave mixing between the waves, and thus may el iminate the need for polarization scrambl ing .
  • the device of the invention comprises a first laser source 10 for generating the optical pu lsed sig nals and a second laser source 11 for generating the optical probe sig nal .
  • a first laser source 10 for generating the optical pu lsed sig nals
  • a second laser source 11 for generating the optical probe sig nal .
  • the frequencies of the two d istinct lasers 10, 11 may be locked onto each other, so that the relative freq uency between the two lasers is wel l secured and stable, but can be scanned to interrogate the d istributed Brillouin gain/loss spectru m;
  • the two lasers 10, 11 may be free-running with control led variation of the mean frequency so that the relative freq uency between the two lasers may be recorded and scanned . Then the recorded relative freq uency may be used to compensate any possible d istortion imposed onto the spectrum of the Brillouin gain/loss spectrum caused by the free-running feature of the two lasers.
  • the device of the mode of realization shown in Fig. 2 may be used and operated in the same way as the device of the mode of realization shown in Fig. 1, for similar results. So, everything which has been explained in relation with the mode of realization of Fig. 1 applies to the mode of realization of Fig. 2, except for the differences in the laser sources.
  • the device of the invention may comprise two laser sources 10 for generating the two optical pump signals;
  • the device of the invention may comprise laser sources 10, 11 fulfilling the functions of the pump modulator 3, the pulse generator 4, and/or the probe modulator 5, in which case the corresponding modulator or pulse generator may not be present.
  • the pump modulator 3 is configured so as to generate a first pulsed pump wave 20 and a second pulsed pump wave 23 with respective frequencies v P1 and v P2 located symmetrically relative to the frequency v L of the laser source 1.
  • Each of the pulsed pump waves 20, 23 may generate a Stokes Brillouin spectrum and an anti-Stokes Brillouin spectrum by propagating in the sensing fiber 6.
  • a Stokes Brillouin spectrum and an anti-Stokes Brillouin spectrum by propagating in the sensing fiber 6.
  • the maximum value 22 of the Stokes Brillouin spectrum is shifted towards the lower frequencies, relative to the pulsed pump frequency v P1 , of an amount corresponding to a Brillouin frequency shift v B1 ;
  • the maximum value 25 of the anti-Stokes Brillouin spectrum is shifted towards the upper frequencies, relative to the pulsed pump frequency v P2 , of an amount corresponding to a Brillouin frequency shift v B2 ;
  • the maxima 22, 25 of the Brillouin spectra are usual ly referred to as Bril louin freq uencies. They depend on the Bril louin freq uency shift v B which depends on the temperature and the strain along the sensing fiber 6.
  • this stimulation ind uces an energy transfer from the probe optical wave 26 to the pulsed optical wave 23 and an attenuation of the probe optical wave (loss mode) .
  • the resulting optical sig nal emerg ing from the sensing optical fiber 6 corresponds essentially to the probe optical wave 26 whose amplitude varies in function of the resonance cond itions met along the fiber at the freq uencies of the respective spectral components of that probe wave.
  • the Stokes Bril louin spectrum 21 may be considered as a gain spectrum and the anti-Stokes Brillou in spectrum 24 may be considered as a loss spectrum .
  • the gains and the losses cancel out mutually to their respective mag nitudes.
  • the measured stimulated Brillouin sig nal del ivers differential information about the gains and losses encountered by the probe optical wave 26 or some of its spectral components along the path .
  • the pump modulator 3 is config ured so as to generate a first pulsed pump wave 20 and a second pulsed pump wave 23 which are shifted in frequency, relative to the freq uency v L of the laser source 1 , of an amou nt substantial ly corresponding to the Brillouin freq uency shift v B for a g iven set of temperatu re and strain cond itions along the sensing fiber 6 (correspond ing to reference conditions) .
  • the Stokes Brillouin spectrum 21 of the first pulsed pump wave 20 and the anti-Stokes Bril louin spectrum 24 of the second pulsed pump wave 23 overlap at least partially in an overlap area.
  • the frequency shift introduced by the pump modulator 3 corresponds exactly to the Briiiouin frequency shift v B
  • the Stokes Briiiouin spectrum 21 and the anti-Stokes Briiiouin spectrum 24 overlap completely, and their respective Briiiouin frequencies 22, 25 correspond to the frequency v L of the laser source 1. That situation is illustrated in Fig. 3.
  • the Briiiouin frequency shift v B varies accordingly.
  • the respective frequencies v P1 and v P2 of the first pulsed pump wave 20 and the second pulsed pump wave 23 being held constant, the corresponding Stokes Briiiouin spectrum 21 and anti-Stokes Briiiouin spectrum 24 shift in frequency in opposite directions, symmetrically relative to the frequency v L of the laser source 1. In that case, they only partially overlap as illustrated in Fig. 4, until they separate completely, which may happen for large variations of Briiiouin frequency shift v B .
  • the probe wave is generated by shifting in frequency of an amount f M the light of the laser source 1. This frequency shift is done by the probe modulator 5 which may comprise for instance an acousto-optic modulator.
  • the shape of the Briiiouin spectra is known, it is possible to deduce directly from the variations of amplitude of the stimulated Briiiouin signal at the frequency v PR of the probe wave 26 the variations of Briiiouin frequency shift v B . So it is possible to perform real-time measurements of the Briiiouin frequency shift v B (and thus of temperature and/or strain variations) without the need for scanning the Briiiouin spectrum by varying the frequency of the probe wave, as in the prior art methods.
  • the frequency shift f M of the probe wave 26 may be chosen so as to correspond substantially to the half maximum frequency of the Brillouin spectrum (or the -3dB frequency), which is about 15 MHz for an intrinsic Brillouin spectrum, or about 30MHz for an effective Brillouin spectrum corresponding to a spatial resolution in the order of 3 meters. This is the situation illustrated in Fig. 3 and Fig. 5 for an effective Brillouin spectrum.
  • Fig. 5 shows the variations of the Brillouin gain at the frequency v PR of the probe wave 26 with respect to the variations of the Brillouin frequency shift v B , in these conditions.
  • the Brillouin net gain accounts for the gains and the losses due to the stimulated Brillouin interactions of the probe wave 26 with the first pulsed pump wave 20 and the second pulsed pump wave 23, respectively.
  • the amplitude of the stimulated Brillouin signal at the frequency v PR of the probe wave 26 depends on that Brillouin net gain. So the curve of Fig. 5 is also representative of the variations of amplitude of the stimulated Brillouin signal such as measured on the photo-detector 9.
  • the reference Brillouin frequency shift v B0 is the Brillouin frequency shift in the reference conditions, when the Stokes spectrum 21 and the anti-Stokes spectrum 24 are entirely superposed, as illustrated in Fig. 3. In these conditions, if the first pulsed pump wave 20 and the second pulsed pump wave 23 are well balanced, the Brillouin net gain is zero.
  • the Brillouin net gain varies also in a well-defined and unambiguous manner in a measurement range. So the determination of that Brillouin net gain allows determining directly the Brillouin frequency shift v B .
  • the frequency shift f M of the probe wave 26 corresponds substantially to the frequency at half maximum (-3dB frequency) of the Brillouin spectrum (which is assumed to have a Lorentzian shape), as illustrated in Fig. 5, the Brillouin net gain variations are fairly linear with respect to the Brillouin frequency shift v B in a measurement range which corresponds to about two times the -3dB frequency, hence about 60 MHz ( ⁇ 30 MHz) for an effective Brillouin spectrum corresponding to a spatial resolution in the order of 3 meters, showing the capability of temperature or strain variation measurement of about ⁇ 30 °C or ⁇ 600 ⁇ , respectively.
  • the measurement range may be further extended by shifting in frequency the probe wave 26 of an amount f M larger than the -3dB frequency of the Brillouin spectrum.
  • Fig. 6 shows the variations of the Brillouin net gain at the frequency v PR of the probe wave 26 with respect to the variations of the Brillouin frequency shift v B , for a frequency shift f M corresponding to two times the -3dB frequency of the Brillouin spectrum.
  • the measurement range is extended to about four times the -3dB frequency, hence about 120 MHz ( ⁇ 60 MHz) for an effective Brillouin spectrum corresponding to a spatial resolution in the order of 3 meters, showing the capability of temperature or strain variation measurement of about ⁇ 60 °C or ⁇ 1200 ⁇ , respectively.
  • the Brillouin net gain variation is less linear than in the case of Fig. 5, but such response nonlinearity can be compensated by accurate pre-calibration process, which means the precise measurement of Brillouin net gain with respect to the Brillouin frequency v B .
  • the method of the invention may be used for instance to monitor rapid variations of temperature or strain in the vicinity of reference values, or to do high speed acquisitions.
  • the measurements can be done as follows.
  • the respective frequencies v P1 and v P2 of the first pulsed pump wave 20 and the second pulsed pump wave 23 are adjusted using the pump modulator 3 so as to superpose the Stokes Brillouin spectrum 21 and the anti-Stokes Brillouin spectrum 24 over at least a short section of the sensing fiber 6. This can be achieved by detecting a null net gain condition on the stimulated Brillouin signal (Fig. 3 and Fig. 5).
  • the reference Brillouin frequency shift v B0 is then known as it corresponds to the frequency of the modulation signal applied to the pump modulator 3 for generating the pulsed wave frequencies v P1 and v P2 .
  • a stimulated Brillouin signal at the frequency v PR of the probe wave 26 is acquired;
  • a time variation of the Brillouin frequency shift v B is deduced from the amplitude variation of that stimulated Brillouin signal at the frequency v PR ;
  • the reference Brillouin frequency shift v B0 can be re-adjusted periodically to take into account slower variations of temperature and/or strain in the sensing fiber 6 and to keep the measurements in the available measurement range.
  • the pump modulator 3 is configured so as to generate a first pulsed pump wave 20 and a second pulsed pump wave 23 with respective frequencies v P1 and v P2 located symmetrically relative to the frequency v L of the laser source 1.
  • the pump modulator 3 is configured so as to generate a first pulsed pump wave 20 and a second pulsed pump wave 23 which are shifted in frequency, relative to the frequency v L of the laser source 1, of respective amounts (v P1 -v L ) and (v L -v P2 ) which are different from the Brillouin frequency shift v B , and which ensure that the Stokes Brillouin spectrum 21 of the first pulsed pump wave 20 and the anti- Stokes Brillouin spectrum 24 of the second pulsed pump wave 23 do not overlap. So the Brillouin spectra 21, 24 do not cross or do not comprise the source frequency v L .
  • frequency shifts (v P1 -v L ) and (v L -v P2 ) of the first pulsed pump wave 20 and of the second pulsed pump wave 23 may be larger or smaller than the Brillouin frequency shift v B .
  • the description of this mode of realization will be done for frequency shifts (v P1 -v L ) and (v L -v P2 ) larger than the Brillouin frequency shift v B .
  • the Brillouin frequency shift v B varies accordingly.
  • the respective frequencies v P1 and v P2 of the first pulsed pump wave 20 and the second pulsed pump wave 23 being held constant, the corresponding Stokes Brillouin spectrum 21 and anti-Stokes Brillouin spectrum 24 shift in frequency in opposite directions, symmetrically relative to the frequency v L of the laser source 1. This situation is illustrated in Fig. 7 and Fig. 8 for two different values of Brillouin frequency shift v B .
  • the probe wave is generated by modulating in phase the light of the laser source 1.
  • This phase modulation is done by the probe modulator 5 which comprises a phase modulator driven by a continuous wave radiofrequency (RF) signal at modulation frequency f s .
  • RF radiofrequency
  • This phase modulator may for instance be done using an electro-optic lithium-niobate modulator.
  • the spectrum of the probe wave comprises then three spectral components: a carrier 30 at the frequency v L of the laser source 1, and two symmetric sidebands 31 around respective frequencies v L ⁇ f s .
  • the sidebands 31 have equal amplitude but opposite polarity ( ⁇ -phase shifted).
  • Fig. 9 shows the electrical signal 35 delivered by the photodetector 9 in the configuration of Fig. 7, when the sidebands 31 are not in the Brillouin spectra 21, 24.
  • the electrical signal 35 shows no modulation and only contains a DC component. This is due to the fact that the two sidebands 31 produce on the photodetector 9 a beating with the carrier 30 that shows equal amplitude and opposite phase, so that they exactly cancel out.
  • the signal incident on the photodetector 9 corresponds essentially to the phase-modulated probe wave which emerges from the sensing fiber 6 without having been subjected to any stimulated Brillouin scattering .
  • the modulation frequency f s and the Brillouin frequency shift v B are such that the sidebands 31 are positioned respectively within the Stokes Brillouin spectrum 21 and anti-Stokes Brillouin spectrum 24 as illustrated in Fig . 8, the sidebands 31 interacts respectively with the first pulsed pump wave 20 and the second pulsed pump wave 23.
  • One sideband 31 which interacts with the anti-Stokes Brillouin spectrum 24 is attenuated, while the other sideband 31 which interacts with the Stokes Brillouin spectrum 21 is amplified .
  • the respective beatings of the sidebands 31 with the carrier 30 on the photodetector 9 do not have the same amplitude and they no longer cancel exactly. This effect is often referred to as FM-IM conversion .
  • Fig . 10 shows the electrical signal 35 delivered by the photodetector 9 in that configuration, corresponding to Fig . 8.
  • This electrical signal 35 comprises now a signal at the modulation frequency f s , with an amplitude which is proportional to the amount of gain and loss resulting from the stimulated Brillouin scattering at the respective frequencies of the sidebands 31.
  • the stimulated Brillouin scattering information is coded in the amplitude of an AC signal 35 at the modulation frequency f s .
  • This configuration is much more favorable in terms of signal to noise characteristics than the prior art schemes in which the Brillouin information is coded in the low-frequency amplitude variations of the detected DC signal . It allows rejecting the low-frequency noise contributions which are important in practice, due for instance to the source optical noise and the optical amplifiers.
  • This detection scheme corresponds to the classical methods with a scan in frequency. So it has the drawback of being quite slow.
  • the frequency shift required for scanning the Briiiouin spectra must be in the order of the Briiiouin frequency shift v B ( ⁇ 11 GHz) . It requires expensive generators.
  • the modulation frequency f s can be made much lower than that. It is limited towards the low frequencies only by the fact that the Briiiouin spectra 21, 24 must not overlap. So, in practice the modulation frequency f s is chosen in the range from 0.1 to 1 GHz. This allows using much less expensive generators, much less expensive generator using a more mature and developed technology, with more advanced capabilities.
  • the amplitude of the modulation at frequency f s of the detected signal 35 depends in an unambiguous way of the gain and the loss encountered by the sidebands 31, which in turn depend in an unambiguous way of the frequency location of the sidebands 30 relative to the respective maxima 22, 25 of the Briiiouin spectra 21, 24.
  • the shape of the Briiiouin spectra it is possible to determine a relation between the amplitude of the modulation of the detected signal 35 and the Briiiouin frequency shift v B corresponding to the respective maxima 22, 25 of the
  • the method of the invention may be used for instance to monitor rapid variations of temperature or strain in the vicinity of reference values, or to perform high speed acquisitions.
  • a reference Briiiouin frequency shift v B0 may be determined by scanning the Briiiouin spectra in reference conditions of temperature and/or strain, for instance as described in the first detection scheme.
  • a corresponding reference modulation frequency f so and maximum modulation amplitude may also be determined .
  • the modulation frequency f s may then be adjusted so as to position the sidebands 31 in the ascending or descending slopes of the Brillouin spectra, where the variations are unambiguous. It may for instance be chosen around the frequency at half maximum of these Brillouin spectra (-3dB frequency), in order to maximize the unambiguous measurement range.
  • the modulation frequency f s can be re-adjusted periodically to take into account slower variations of temperature and/or strain in the sensing fiber 6 and to keep the measurements in the available measurement range.
  • the probe wave may be generated by modulating in intensity the light of the laser source 1.
  • This intensity modulation may be done by the probe modulator 5 which comprises an intensity modulator driven by a continuous wave radiofrequency (RF) signal at modulation frequency f s .
  • This intensity modulator may for instance be done using an electro-optic lithium-niobate modulator.
  • the spectrum of the probe wave comprises then three spectral components: a carrier 30 at the frequency v L of the laser source 1, and two symmetric sidebands 31 around respective frequencies v L ⁇ f s .
  • the sidebands 31 have equal amplitude and same phase.
  • the electrical signal 35 shows a modulation with a maximum amplitude, corresponding to the intensity modulation introduced by the probe modulator 5.
  • the modulation frequency f s and the Brillouin frequency shift v B are such that the sidebands 31 are positioned respectively within the Stokes Brillouin spectrum 21 and anti-Stokes Brillouin spectrum 24 as illustrated in Fig. 8, the sidebands 31 interacts respectively with the first pulsed pump wave 20 and the second pulsed pump wave 23.
  • One sideband 31 which interacts with the anti-Stokes Brillouin spectrum 24 is attenuated, while the other sideband 31 which interacts with the Stokes Brillouin spectrum 21 is amplified.
  • both sidebands 31 experience a phase shift.
  • the amplitude of modulation of the electrical signal 35 depends on the amount of Brillouin gain and loss. It reaches a minimum value when the frequency of the sidebands 31 corresponds to the frequencies of the respective maxima 22, 25 of gain and loss of the Brillouin spectra 21, 24.
  • the stimulated Brillouin scattering information is coded in the amplitude of an AC signal 35 at the modulation frequency f s , and the Brillouin frequency shift v B may be determined in the same ways as what has been explained in relation with the phase modulation variant.
  • the pump modulator 3 is configured so as to generate a first pulsed pump wave 20 and a second pulsed pump wave 23 with respective frequencies v P1 and v P2 located symmetrically relative to the frequency v L of the laser source 1.
  • the pump modulator 3 is configured so as to generate a first pulsed pump wave 20 and a second pulsed pump wave 23 which are shifted in frequency, relative to the frequency v L of the laser source 1, of an amount substantially corresponding to the Brillouin frequency shift v B for a given set of temperature and strain conditions along the sensing fiber 6 (corresponding to reference conditions). So, the Stokes Brillouin spectrum 21 of the first pulsed pump wave 20 and the anti-Stokes Brillouin spectrum 24 of the second pulsed pump wave 23 overlap at least partially in an overlap area.
  • the frequency shift introduced by the pump modulator 3 corresponds exactly to the Brillouin frequency shift v B
  • the Stokes Brillouin spectrum 21 and the anti-Stokes Brillouin spectrum 24 overlap completely, and their respective Brillouin frequencies 22, 25 corresponds to the frequency v L of the laser source 1.
  • the Brillouin frequency shift v B varies accordingly.
  • the respective frequencies v P1 and v P2 of the first pulsed pump wave 20 and the second pulsed pump wave 23 being held constant, the corresponding Stokes Brillouin spectrum 21 and anti-Stokes Brillouin spectrum 24 shift in frequency in opposite directions, symmetrically relative to the frequency v L of the laser source 1. In that case, they only partially overlap as illustrated in Fig . 11, until they separate completely, which may happen for large variations of Brillouin frequency shift v B .
  • the probe wave is generated by modulating in intensity the light of the laser source 1.
  • This intensity modulation is done by the probe modulator 5 which comprises an intensity modulator driven by a continuous wave radiofrequency (RF) signal at modulation frequency f s .
  • RF radiofrequency
  • This intensity modulator may for instance be done using an electro-optic lithium-niobate modulator based on Mach-Zehnder interferometer architecture adjusted such as to work for instance around the 1/2 or the 2/3 constructive interference point.
  • the spectrum of the probe wave comprises then three spectral components : a carrier 30 at the frequency v L of the laser source 1, and two symmetric sidebands 31 around respective frequencies v L + fs-
  • the sidebands 31 are arranged so that to remain outside the Brillouin spectra 21, 24. So, stimulated Brillouin scattering may occur only at the frequency v L of the carrier 30.
  • Fig . 12 and Fig . 13 show a detailed view of the spectra around the frequency v L .
  • Fig . 12 shows an enlarged view of the Stokes Brillouin gain spectrum 21.
  • the arrow 43 indicates the direction of the frequency shift of that spectrum when the Brillouin frequency shift v B increases.
  • the refractive index of the sensing optical fiber 6 is modulated over the spectral region where the SBS resonance is present.
  • the curve 40 depicts the modulation of the refractive index ⁇ over the gain resonance 21 generated by the first pump pulsed wave 20. As can be seen, the refractive index 40 increases in frequency along the central part of that spectral region .
  • Fig . 12 shows an enlarged view of the anti-Stokes Brillouin loss spectrum 24.
  • the arrow 43 indicates the direction of the frequency shift of that spectrum when the Brillouin frequency shift v B increases.
  • the curve 41 depicts the modulation of the refractive index ⁇ over the loss resonance 24 generated by the second pump pulsed wave 23. As can be seen, the refractive index 41 decreases in frequency along the central part of that spectral region .
  • the carrier 30 is at the frequency v L of the laser source 1 , it does not experience any amplitude modulation because the Brillouin gain spectrum 21 and the Brillouin loss spectrum 24 are always of the same and opposite value at that central frequency v L .
  • the respective modulation 40, 41 of the refractive index ⁇ shift in the same direction, and thus their effects cumulate. So the carrier 30 experiences an optical phase shift ⁇ 5 ⁇ 5 whose amount depends on the Brillouin frequency shift v B .
  • the electric signal at the output of the photodetector 9 comprises a spectral component U fs at the modulation frequency f s whose amplitude depends on the cosine of the optical phase shift o SBS :
  • Fig . 14 shows the amplitude 42 of that modulation component U fs at frequency f s in function of the variation of the Brillouin frequency shift v B :
  • the amplitude 42 of the modulation at frequency f s is at the maximum value 41 ;
  • the amplitude of the modulation 42 at frequency f s returns to its maximum value. This corresponds to the fact that when the Brillouin spectra 21, 24 do not overlap anymore, no more stimulated Brillouin scattering occurs and the measured electric signal corresponds to the modulated probe wave.
  • the stimulated Brillouin scattering information is coded in the amplitude 42 of a signal at the modulation frequency f s .
  • This configuration is much more favorable in terms of signal to noise characteristics than the prior art schemes in which the Brillouin information is coded in the low- frequency amplitude variations of the detected signal. It allows rejecting all the low-frequency noise contributions which are important in practice, due for instance to the source optical noise and the optical amplifiers.
  • This detection scheme corresponds to the classical methods with a scan in frequency. So it has the drawback of being quite slow.
  • the amplitude 42 of the modulation at frequency f s of the detected signal depends in an unambiguous way of the index modulation introduced in the sensing fiber 6 by the Brillouin resonances at the frequency v L of the carrier 30 of the probe wave, which in turn depend in an unambiguous way of the Brillouin frequency shift v B corresponding to the respective maxima 22, 25 of the Brillouin spectra 21, 24.
  • the method of the invention may be used for instance to monitor rapid variations of temperatu re or strain in the vicinity of reference val ues, or to perform hig h speed acquisitions.
  • the respective frequencies v P1 and v P2 of the first pulsed pump wave 20 and the second pulsed pump wave 23 are adjusted using the pump modulator 3 so as to superpose the Stokes Bril louin spectrum 21 and the anti-Stokes Brillouin spectrum 24 over at least a short section of the sensing fiber 6.
  • This can be achieved by detecting the frequencies v P1 and v P2 for which the modulation amplitude 42 of the detected sig nal is the largest, which corresponds to the case where the gain and loss Bril louin spectra 21 , 24 overlap, and the respective Brillouin frequencies 22, 25 are eq ual to the freq uency v L of the laser source 1 ;
  • the reference Brillouin freq uency shift v B0 is then known as it corresponds to the freq uency of the mod ulation sig nal applied to the pump mod ulator 3 for generating the pulsed wave freq uencies v P1 and v P2 .
  • the respective freq uencies v P1 and v P2 of the first pulsed pump wave 20 and the second pulsed pump wave 23 must then be shifted by an offset freq uency shift so that the mod ulation ampl itude 42 of the detected sig nal for that reference Brillouin freq uency shift v B0 is the fast varying area 44 of the curve 42. Th is ensures that the detected sig nal varies unambig uously with the variations of the Bril louin frequency shift v B .
  • This offset freq uency shift may for instance correspond to the half width at half maximum (-3d B) freq uency shift of the Brillouin spectra 21 , 24. Th is situation is ill ustrated in Fig . 11.
  • the modulation freq uency f s can be re-adjusted period ically to take into account slower variations of temperature and/or strain in the sensing fiber 6 and to keep the measurements in the available measurement range .
  • the probe wave may be generated by modulating in phase the light of the laser source 1. The detection schemes which have been described in relation with the intensity modulation variant may be easily adapted to that variant.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Optical Transform (AREA)
  • Length Measuring Devices By Optical Means (AREA)

Abstract

La présente invention concerne un procédé de détection à fibre optique distribuée de diffusion Brillouin, comprenant les étapes suivantes : (i) la fourniture d'une première et d'une seconde onde pulsées de pompe optique (20, 23) et d'une onde de sonde optique comportant au moins une composante spectrale de sonde (30, 31); (ii) la détection d'un signal de diffusion de Brillouin résultant des interactions dans ladite fibre optique de détection (6) de l'onde de sonde optique avec les première et seconde ondes pulsées de pompe optique (20, 23), les première et seconde ondes pulsées de pompe optique (20, 23) et l'onde de sonde optique étant dans un agencement spectral tel que, au moins pour une partie d'un ensemble donné de conditions de température et/ou déformation le long d'au moins une partie de la fibre optique de détection (6), l'onde de sonde optique comporte une composante spectrale de sonde (26, 30, 31) située à l'intérieur du spectre Brillouin Stokes (21) de la première onde pulsée de pompe optique (20) et d'une composante spectrale de sonde (26, 30, 31) à l'intérieur du spectre Brillouin anti-Stokes (24) de la seconde de pulsée de pompe optique (23). La présente invention concerne également un dispositif de mise en œuvre du procédé.
PCT/EP2013/059022 2013-04-30 2013-04-30 Procédé et dispositif de détection à fibre optique distribuée de diffusion brillouin stimulée par double pompage WO2014177197A1 (fr)

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WO2016198728A1 (fr) * 2015-06-11 2016-12-15 Nokia Technologies Oy Communication à fibres optiques sur la base de la lumière décalée en fréquence codée
CN111707299A (zh) * 2020-06-08 2020-09-25 太原理工大学 一种基于泵浦光扫频获取增益损耗谱的botda装置
CN113776687A (zh) * 2021-08-17 2021-12-10 华中科技大学 一种基于微波光子测量结合光学测量的温度传感系统
CN116231437A (zh) * 2023-05-08 2023-06-06 之江实验室 泵浦增益系统和信号整形系统

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WO2008047329A2 (fr) * 2006-10-19 2008-04-24 University Of Johannesburg Procédé et dispositif de détection distribuée au moyen d'un laser de référence à verrouillage de stokes
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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2016198728A1 (fr) * 2015-06-11 2016-12-15 Nokia Technologies Oy Communication à fibres optiques sur la base de la lumière décalée en fréquence codée
US10805074B2 (en) 2015-06-11 2020-10-13 Nokia Technologies Oy Fibre-optic communication based on encoded frequency-shifted light
CN111707299A (zh) * 2020-06-08 2020-09-25 太原理工大学 一种基于泵浦光扫频获取增益损耗谱的botda装置
CN113776687A (zh) * 2021-08-17 2021-12-10 华中科技大学 一种基于微波光子测量结合光学测量的温度传感系统
CN116231437A (zh) * 2023-05-08 2023-06-06 之江实验室 泵浦增益系统和信号整形系统
CN116231437B (zh) * 2023-05-08 2023-07-25 之江实验室 泵浦增益系统和信号整形系统

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