WO2014067548A1 - Procédé de mesure optoélectronique de brillouin - Google Patents

Procédé de mesure optoélectronique de brillouin Download PDF

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Publication number
WO2014067548A1
WO2014067548A1 PCT/EP2012/071389 EP2012071389W WO2014067548A1 WO 2014067548 A1 WO2014067548 A1 WO 2014067548A1 EP 2012071389 W EP2012071389 W EP 2012071389W WO 2014067548 A1 WO2014067548 A1 WO 2014067548A1
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brillouin
monochromatic
frequency
monochromatic wave
wave
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PCT/EP2012/071389
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English (en)
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Senghoon CHIN
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Omnisens Sa
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Priority to EP12780485.4A priority Critical patent/EP2912412A1/fr
Priority to PCT/EP2012/071389 priority patent/WO2014067548A1/fr
Publication of WO2014067548A1 publication Critical patent/WO2014067548A1/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
    • 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 present invention concerns a Brillouin optoelectronic measurement method, and in particular, but not exclusively a Brillouin optoelectronic measurement method which uses a pump signal which comprises two or more different frequencies so as to achieve partial overlapping of generated Brillouin loss spectrum with a generated Brillouin gain spectrum, so that the width of the Brillouin gain or loss spectrum is reduced.
  • measuring apparatuses In many fields of application, like pipeline, power cables or subsea, the use of measuring apparatuses to monitor continuously structural and/or functional parameters is well known.
  • the measuring apparatuses can be applied also to the civil engineering sector, and in particular in the field of the construction of structures of great dimensions.
  • the measuring apparatuses are commonly used to control the trend over time of the temperature or of the strain, i.e. of the geometrical measure of the deformation or elongation resulting from stresses and defining the amount of stretch or compression along the fibre, of the respective structure.
  • these measuring apparatuses are suitable to give information of local nature, and they can be therefore used to monitor, as a function of the time, the temperature or the strain associated with a plurality of portions and/or of components of the engineering structure to be monitored, providing useful information on leak, ground movement, deformation, etc. of the structure.
  • the optoelectronic devices based upon optical fibres have a great significance. In particular, these
  • apparatuses normally comprise an electronic measuring device, provided with an optical fibre probe which is usually in the order of a few tens of kilometres.
  • this optical fibre is coupled stably to, and maintained substantially into contact with, portions or components of the engineered structure, whose respective physical parameters shall be monitored.
  • this optical fibre can run along the pipes of an oil pipeline, or it can be immersed or integral to a concrete pillar of a building, so that it can be used to display the local trend of the temperature or of the strain of these structures.
  • these optoelectronic devices comprise fibre optical sensors, i.e. sensors using the optical fibre as the sensing element. Fibre optical sensors can be:
  • optical fibre is a long uninterrupted linear sensor
  • the optical fibre is a single-mode fibre (SMF), i.e. a fibre designed for carrying a single ray of light (mode) only, then only forward and backward scattering (backscattering) are relevant since the scattered light in other directions is not guided. Backscattering is of particular interest since it propagates back to the fibre end where the laser light was originally launched into the optical fibre.
  • SMF single-mode fibre
  • Backscattering is of particular interest since it propagates back to the fibre end where the laser light was originally launched into the optical fibre.
  • Scattering processes originate from material impurities (Raleigh scattering), thermally excited acoustic waves (Brillouin scattering) or atomic or molecular vibrations (Raman scattering).
  • RAYLEIGH SCATTERING is the interaction of a light pulse with material impurities. It is the largest of the three backscattered signals in silica fibres and has the same wavelength as the incident light. Rayleigh scattering is the physical principle behind Optical Time Domain
  • BRILLOUIN SCATTERING is the interaction of a light pulse with thermally excited acoustic waves (also called acoustic phonons). Acoustic waves, through the elasto-optic effect, slightly, locally and periodically modify the index of refraction. The corresponding moving grating reflects back a small amount of the incident light and shifts its frequency (or wavelength) due to the Doppler Effect. The shift depends on the acoustic velocity in the fibre while its sign depends on the propagation direction of the travelling acoustic waves. Thus, Brillouin backscattering is created at two different frequencies around the incident light, called the Stokes and the Anti-Stokes components. In silica fibres, the Brillouin frequency shift is in the 10 GHz range (0.1 nm in the 1 550 nm wavelength range) and is temperature and strain dependent.
  • RAMAN SCATTERING is the interaction of a light pulse with thermally excited atomic or molecular vibrations (optical phonons) and is the smallest of the three backscattered signals in intensity.
  • Raman scattering exhibits a large frequency shift of typically 13 THz in silica fibres, corresponding to 100 nm at a wavelength of 1 550 nm.
  • the Raman Anti- Stokes component intensity is temperature dependent whereas the Stokes component is nearly temperature insensitive.
  • Figure 1 schematically shows a spectrum of the backscattered light generated at every point along the optical fibre when a laser light is launched in the optical fibre.
  • the so-called Stokes components and the so-called anti-Stokes components are the peaks at the right and left side of the Rayleigh peak.
  • the anti-Stokes Raman peak originated from atomic or molecular vibrations, has an amplitude depending on the temperature T.
  • the Brillouin shift (wavelength position with respect to the original laser light) is an intrinsic physical property of the fibre material and provides important information about the strain and temperature distribution experienced by an optical fibre.
  • the frequency information of Brillouin backscattered light can be exploited to measure the local temperature or strain information along an optical fibre.
  • Standard or special single-mode telecommunication fibres and cables can be used as sensing elements.
  • the technique of measuring the local temperature or strain is referred to as a frequency-based technique since the temperature or strain information is contained in the Brillouin frequency shift. It is inherently more reliable and more stable than any intensity-based technique, based on the Raman effect, which are sensitive to drifts, losses and variations of attenuations. As a result, the Brillouin based technique offers long term stability and large immunity to
  • SPBS Spontaneous Brillouin Scattering
  • the Brillouin scattering process has the particularity that it can be stimulated by a second optical signal - called the probe signal.
  • the probe signal is provided in addition to the first optical signal - called the pump signal.
  • the probe signal can stimulate backscattering of the pump signal provided that the probe signal fulfils specific conditions. This property is especially interesting for sensing applications and can be achieved by the use of a probe signal counter propagating with respect to the pump signal. Stimulation is maximized when pump and probe frequencies (or
  • the pump signal is composed by one or more nanoseconds long optical pulses and the probe by a Continuous Wave - CW light.
  • SBS Stimulated Brillouin Backscattering
  • BOTDA Brillouin Optical Time Domain Analysers
  • SPBS spontaneous Brillouin backscattering
  • An optoelectronic measurement device based on BOTDA normally performs a frequency domain analysis and a time domain analysis.
  • Frequency domain analysis the temperature/strain information is coded in the Brillouin frequency shift. Scanning the frequency of the probe signal with respect to the pump signal while monitoring the intensity of the backscattered signal allows to find the Brillouin gain peak, and thus the corresponding Brillouin shift, from which the temperature or the strain can be computed. This is achieved by using two optical sources, e.g. lasers, or a single optical source from which both the pump signal and the probe signal are created. In this case, an optical modulator (typically a
  • telecommunication component is used to scan the probe frequency in a controlled manner.
  • Time domain analysis due to the pulsed nature of the pump signal, the pump/probe signal interaction takes place at different location along the fibre at different times. For any given location, the portion of probe signal which interacted with the pump arrives on a detector after a time delay equal to twice the travelling time from the fibre input to the specified location.
  • monitoring the backscattered intensity with respect to time while knowing the speed of light in the fibre, provides information on the position where the scattering took place.
  • Figure 2 illustrates how a distributing sensing technique using Brillouin backscattering is performed, by a Brillouin analyser 3, to detect temperature and stain along a structure 1.
  • An optical fibre is attached to the structure 1 .
  • a pump signal 7 is passed through a first end 5 of the optical fibre 5.
  • a probe signal 9 is passed through an opposite end of the optical fibre 5.
  • the probe and pump signals 9,7 will propagate in opposite directions along the optical fibre 5.
  • the frequency of the probe signal 9 is scanned and the intensity of the stimulated Brillouin backscattered signal is measured by a signal processor 2.
  • the frequency at which the intensity is the greatest is the Brillouin frequency.
  • Figure 3 is an exemplary graph of the intensity of simulated Brillouin backscattering measured as the frequency of the probe signal is scanned. The maximum intensity occurs at the Brillouin frequency V B .
  • the shift in Brillouin frequency can be used to derive the temperature of stain change which has occurred in the structure.
  • time domain analysis of the simulated Brillouin backscattering the location on the structure, where the temperature or strain change has occurred can be determined.
  • the resolution (i.e. spatial resolution) of the temperate and strain measurement i.e. the accuracy with which the temperature and strain of the structure 1 can be measured, of the quantity of temperature change to be accurately detected, e.g. a 0.5° C difference can be detected
  • the spectral width of the generated simulated Brillouin backscattering is inversely proportional to the duration of the pulse signal 7.
  • a typical pulse signal 7 is illustrated in figure 4. This pulse signal has duration of 30ns.
  • the resulting spectral width of the generated simulated Brillouin backscattering is given by a convolution product of spectral width of the pump and intrinsic Brillouin linewidth, typically 30 MHz in standard single mode optical fibers.
  • the intrinsic Brillouin linewidth is defined as spectral width of Brillouin scattering generated by two counter-propagating continuous pump and probe waves. Therefore, for 30ns pump pulse, the resulting spectral width of the generated Brillouin back-scattering is quite narrow as can be seen from figure 5. Since the pulse signal 7 has a 30ns duration it provides a spatial resolution of 3m.
  • the duration of the pulse signal can be decreased to 1 ns, for example, as is shown in figure 6.
  • the spectral width of the generated simulated Brillouin backscattering is broad. With such a broad spectral width it is difficult to unambiguously detect the peak of the generated simulated Brillouin backscattering; accordingly it is difficult to detect a shift in the Brillouin frequency, and thus difficult to detect when changes in temperature or strain of the structure 1 have occurred.
  • the duration of pulse signal must be decreased.
  • a decrease in the duration of the pump signal has the disadvantage that the spectral width of the generated simulated Brillouin backscattering is broadened so that it becomes impossible, or at least difficult, to detect shifts in the Brillouin frequency.
  • the pump signal comprising at least a first and second monochromatic wave, such that the first monochromatic wave can generate a first Brillouin gain spectrum and the second monochromatic wave can generate a first Brillouin loss spectrum, wherein the frequency of the first and second monochromatic waves are different, and wherein the frequencies of the first and second monochromatic waves are such that a frequency range of the first Brillouin loss spectrum generated by the second monochromatic wave only partly overlaps a frequency range of the first Brillouin gain spectrum generated by the first monochromatic wave.
  • monochromatic waves are such that a part of a range of a Brillouin loss spectrum generated by the second monochromatic wave overlaps a part of a range of a Brillouin gain spectrum generated by the first monochromatic wave, and a part of a range of a Brillouin loss spectrum generated by the second monochromatic wave does not overlap a part of a range of a Brillouin gain spectrum generated by the first monochromatic wave. [0032] This is done so that the spectral width of the Brillouin gain spectrum is narrowed.
  • the frequency range of the first Brillouin loss spectrum generated by the second monochromatic wave only partly overlaps a frequency range of a Brillouin gain spectrum generated by the first monochromatic wave, part of the Brillouin gain spectrum will be cancelled by the Brillouin loss spectrum; thus the range of the resulting Brillouin gain spectrum is decreased as a result and thus the spectral width of the Brillouin gain spectrum is narrowed. Consequently, the effective Brillouin gain spectrum (i.e.
  • the resulting Brillouin gain spectrum which results when the Brillouin loss spectrum compensates for or cancels part of the Brillouin gain spectrum) through simulated Brillouin backscattering has a narrower spectral width compared to the spectral width of the Brillouin gain spectrum. Therefore, the trade-off between pulse duration
  • determining spatial resolution and Brillouin backscattering spectral width determining measurement accuracy can be compensated or mitigated.
  • this allows to improve the measurement accuracy and/or measurement resolution while preserving the spatial resolution.
  • the pump signal may comprise a third monochromatic wave which generates a second Brillouin loss spectrum, wherein the frequency of the third monochromatic wave is different to the frequencies of the first and second monochromatic waves, wherein the frequency of the third monochromatic waves is such that a frequency range of the second
  • Brillouin loss spectrum generated by the third monochromatic wave only partly overlaps a frequency range of a Brillouin gain spectrum generated by the first monochromatic wave.
  • the frequency of the third monochromatic waves may be such that a frequency range of the second Brillouin loss spectrum generated by the third monochromatic wave overlaps an upper part of the frequency range of the Brillouin gain spectrum generated by the first monochromatic wave, and wherein the frequency of the second monochromatic waves is such that a frequency range of the first Brillouin loss spectrum generated by the second monochromatic wave overlaps a lower part of the frequency range of the Brillouin gain spectrum generated by the first monochromatic wave. So that the width of the Brillouin gain spectrum is decreased from both the upper end and lower end of the Brillouin gain spectrum.
  • the upper part and lower part are of equal size, so the Brillouin gain spectrum is narrowed by the same amount from opposite ends of the Brillouin gain spectrum.
  • the frequency of the first monochromatic wave may be such that it generates a Brillouin gain spectrum which has a maximum gain located at a frequency which is equal to a frequency of the probe signal.
  • the pump signal may comprise a third monochromatic wave which generates a second Brillouin gain spectrum, wherein the frequency of the third monochromatic wave is different to the frequencies of the first and second monochromatic waves, wherein the frequency of the third monochromatic waves is such that a frequency range of the second
  • Brillouin gain spectrum generated by the third monochromatic wave only partly overlaps a frequency range of a Brillouin loss spectrum generated by the second monochromatic wave.
  • monochromatic waves are such that a part of a range of a Brillouin gain spectrum generated by the third monochromatic wave overlaps a part of a range of a Brillouin loss spectrum generated by the second monochromatic wave, and a part of a range of a Brillouin gain spectrum generated by the third monochromatic wave does not overlap a part of a range of a Brillouin loss spectrum generated by the second monochromatic wave. [0039] This is done so that the spectral width of the Brillouin loss spectrum is narrowed.
  • the frequency range of the first Brillouin gain spectrum generated by the third monochromatic wave only partly overlaps a frequency range of a Brillouin loss spectrum generated by the second monochromatic wave, part of the Brillouin loss spectrum will be cancelled by the Brillouin gain spectrum; thus the range of the resulting Brillouin loss spectrum is decreased as a result and thus the spectral width of the
  • Brillouin loss spectrum is narrowed. Consequently, the effective Brillouin loss spectrum (i.e. the resulting Brillouin loss spectrum which results when the Brillouin gain spectrum compensates for or cancels part of the Brillouin loss spectrum) through simulated Brillouin backscattering has a narrower spectral width compared to the spectral width of the Brillouin loss spectrum. Brillouin loss spectrum can be used in the same manner as a Brillouin gain spectrum for performing Brillouin optoelectronic
  • the pump signal may comprise a third monochromatic wave which generates a second Brillouin gain spectrum, wherein the frequency of the third monochromatic wave is different to the frequencies of the first and second monochromatic waves, wherein the frequency of the third monochromatic waves is such that a frequency range of the second
  • Brillouin gain spectrum generated by the third monochromatic wave only partly overlaps a frequency range of a Brillouin loss spectrum generated by the second monochromatic wave.
  • the frequency of the third monochromatic waves may be such that a frequency range of the second Brillouin gain spectrum generated by the third monochromatic wave overlaps an upper part of the frequency range of the Brillouin loss spectrum generated by the second
  • the monochromatic wave is such that a frequency range of the first Brillouin gain spectrum generated by the first monochromatic wave overlaps a lower part of the frequency range of the Brillouin loss spectrum generated by the second monochromatic wave. So that the width of the Brillouin loss spectrum generated by the second monochromatic wave is decreased from both the upper end and lower end of the Brillouin loss spectrum. It will be understood that in this manner the Brillouin loss spectrum can be narrowed symmetrically from opposite ends of the spectrum.
  • the upper part and lower part may be of equal size, so the
  • Brillouin loss spectrum is narrowed by the same amount from opposite ends of the Brillouin loss spectrum.
  • the frequency of the second monochromatic wave may be such that it generates a Brillouin loss spectrum which has a maximum loss located at a frequency which is equal to a frequency of the probe signal.
  • the pump signal may be a single pulse.
  • the pump signal may comprise a plurality of pulses.
  • the pump signal may comprise a first pulse which comprises the first monochromatic wave and a second pulse which comprises the second monochromatic wave.
  • the pump signal may comprise a third pulse which comprises the third monochromatic wave.
  • the duration of each of the plurality of pulses may be equal.
  • the duration of each of the plurality of pulses may be different.
  • Each of the plurality of pulses may have different durations, but temporally synchronized by rising or falling edges.
  • Each of the plurality of pulses may have the same amplitude.
  • Each of the plurality of pulses may have different amplitudes. Different amplitudes can give different amplitude of Brillouin loss or gain spectrums, so it is a way to further tailor the spectral profile of effective Brillouin loss or gain spectrums.
  • the probe signal may comprise a single frequency.
  • the probe signal may comprise a single monochromatic wave only.
  • monochromatic wave may have a single frequency.
  • a single frequency Preferably a
  • monochromatic laser source is used to provide the probe signal.
  • a Brillouin optoelectronic measurement method comprising the steps of,
  • the pump signal comprising at least a first and second monochromatic wave, such that the second monochromatic wave can generate a Brillouin loss spectrum and the first monochromatic wave can generate a first Brillouin gain spectrum, wherein the frequency of the first and second monochromatic waves are different, and wherein the frequencies of the first and second monochromatic waves are such that a frequency range of the first Brillouin gain spectrum generated by the first monochromatic wave only partly overlaps a frequency range of a Brillouin loss spectrum generated by the second monochromatic wave.
  • a system operable to perform a Brillouin optoelectronic measurement method comprising a coherent light source, a means for dividing light emitted from the coherent light source so as to define a pump signal and a probe signal, a sensing fiber which is arranged to receive the pump signal at one end there of and a probe signal at another end thereof, characterised in that the system further comprises a programmable frequency converter, which is programmed to modify the pump signal such that the pump signal comprises at least a first and second monochromatic wave, such that the first monochromatic wave can generate a Brillouin gain spectrum and the second monochromatic wave can generate a first Brillouin loss spectrum, wherein the frequency of the first and second monochromatic waves are different, and wherein the frequencies of the first and second monochromatic waves are such that a frequency range of the first Brillouin loss spectrum generated by the second monochromatic wave only partly overlaps a frequency range of a Brillouin gain spectrum generated by the first monochromatic wave.
  • the programmable frequency converter is programmed to modify the pump signal such that the pump signal comprises third monochromatic wave which generates a second Brillouin loss spectrum, wherein the frequency of the third monochromatic wave is different to the frequencies of the first and second monochromatic waves, wherein the frequency of the third monochromatic waves is such that a frequency range of the second Brillouin loss spectrum generated by the third monochromatic wave only partly overlaps a frequency range of a Brillouin gain spectrum generated by the first monochromatic wave.
  • the programmable frequency converter is programmed to modify the pump signal such that the pump signal comprises third monochromatic wave which generates a second Brillouin gain spectrum, wherein the frequency of the third monochromatic wave is different to the frequencies of the first and second monochromatic waves, wherein the frequency of the third monochromatic waves is such that a frequency range of the second Brillouin gain spectrum generated by the third monochromatic wave only partly overlaps a frequency range of a Brillouin loss spectrum generated by the second monochromatic wave.
  • the programmable frequency converter could be programmed to provide the pump signal with any of the features of the pump signals which have been mentioned above. Brief Description of the Drawings
  • FIGS. 1 -7 all illustrate aspects of the prior art
  • Fig. 1 shows a spectrum of the backscattered light generated at every point along an optical fibre when a laser light is launched in the optical fibre;
  • Fig. 2 shows illustrates how a distributing sensing method using Brillouin backscattering is performed to detect temperature and stain along a structure;
  • Fig. 3 is an exemplary graph of the intensity of simulated
  • Fig. 4 shows a typical pulse signal used when performing the method illustrated in Figure 2
  • Fig. 5 is a graph of measured simulated Brillouin backscattering, when the pulse signal of Figure 4 is used to perform the method illustrated in Figure 2;
  • Fig. 6 shows a pulse signal which can provide improve sensing resolution
  • Fig. 7 is a graph of measured simulated Brillouin backscattering, when the pulse signal of Figure 6 is used to perform the method illustrate in Figure 2;
  • Fig. 8 shows the frequency of the probe signal
  • Fig. 9(a) shows the Brillouin gain spectrum generated by the first monochromatic wave of the pump signal
  • Fig. 9(b) shows the Brillouin loss spectrums generated by the second and third monochromatic waves of the pump signal; [0069] Fig. 9(c) shows the effective Brillouin gain spectrum;
  • Fig. 10 shows the frequency of the probe signal and the frequencies of the first, second and third monochromatic waves which are in the pump signal, used when performing an method according to an embodiment of the present invention
  • Fig. 1 1 (a) shows the Brillouin loss spectrum generated by the first monochromatic wave of the pump signal
  • Fig. 1 1 (b) shows the Brillouin gain spectrums generated by the second and third monochromatic waves of the pump signal
  • Fig. 1 1 (c) shows an effective Brillouin loss spectrum
  • Fig. 12 shows a system which is configured to implement the distributing sensing technique using Brillouin backscattering, according to the present invention.
  • the method of distributing sensing technique using Brillouin backscattering is performed as it known in the art (as described above). That is, in summary, a sensing fibre (optical fibre) is arranged to cooperate with a structure the temperature of which, and strain within which, is to be monitored. A pump signal is provided in a first end of a sensing fibre, and a probe signal is provided in the opposite end of said sensing fibre, so that both the pump and probe signals can interact in the sensing fibre to generate simulated Brillouin backscattering. The stimulated Brillouin backscattering is then measured and used to detect shifts in the Brillouin frequency.
  • the pump signal used when performing the distributing sensing, comprises at least a first and second monochromatic wave, such that the first monochromatic wave can generate a Brillouin gain spectrum and the second monochromatic wave can generate a first Brillouin loss spectrum, wherein the frequency of the first and second monochromatic waves are different, and wherein the frequencies of the first and second monochromatic waves are such that a frequency range of the first Brillouin loss spectrum generated by the second monochromatic wave only partly overlaps a frequency range of a Brillouin gain spectrum generated by the first monochromatic wave.
  • Figure 8 shows the frequencies of the various signals used in an exemplary execution of a distributing sensing technique using Brillouin backscattering, according to the present invention.
  • the probe signal used comprises a monochromatic wave; in other words the probe signal has a single frequency V s .
  • the pump signal used comprises first, second and third monochromatic waves; the frequency of each of the first, second and third monochromatic wave are different.
  • the pump signal comprises three different frequencies.
  • Each of the first, second and third monochromatic waves will generate a Brillouin gain spectrum and Brillouin loss spectrum, when they are passed into the sensing fibre. However, for this example we will only focus on the Brillouin gain spectrum generated by the first monochromatic wave and the Brillouin loss spectrums generated by the second and third monochromatic waves.
  • the first monochromatic wave When the pump signal is passed into the sensing fibre, the first monochromatic wave will generate a Brillouin gain spectrum at the frequency V' p i, which is V p V B wherein V B is the Brillion frequency of the sensing fibre.
  • the Brillouin frequency V B of the sensing fibre is an intrinsic property of the optical fibre which defines the sensing fibre.
  • the frequency of the first monochromatic wave ⁇ ⁇ is chosen such that the frequency V' p i will be equal to the frequency V s of the probe signal.
  • the second monochromatic wave will generate a Brillouin loss spectrum at the frequency V' p2 , which is V p2 + V B
  • third monochromatic wave will generate a Brillouin loss spectrum at the frequency V' P 3, which is V P 3 + V B ; wherein in each case V B is the Brillouin frequency of the sensing fibre.
  • the Brillouin gain spectrum 90 generated by the first monochromatic wave is illustrated in Figure 9(a)
  • the Brillouin loss spectrums 91 ,92 generated by the second and third monochromatic waves respectively are illustrated in Figure 9(b).
  • the range of the Brillouin gain spectrum generated by the first monochromatic wave is from frequency Fp1 to frequency F'p1.
  • the range of the Brillouin loss spectrum generated by the second monochromatic wave is from frequency Fp2 to frequency F'p2
  • the range of the Brillouin loss spectrum generated by the third monochromatic wave is from frequency Fp3 to frequency F'p3.
  • the frequencies V p2 , V p3 of the second and third monochromatic waves are chosen such that the ranges of their respective Brillouin loss spectrums partially overlap the range of Brillouin gain spectrum generated by the first monochromatic wave.
  • the range (Fp2 to frequency F'p2) of the Brillouin loss spectrum generated by the second monochromatic wave overlaps a lower part 94 of the range of the Brillouin gain spectrum.
  • the range (Fp3 to frequency F'p3) of the Brillouin loss spectrum generated by the third monochromatic wave overlaps an upper part 95 of the range of the
  • the frequencies of the second and third monochromatic waves are chosen so that their resulting Brillouin loss spectrums overlap a lower part and upper part 94,95, respectively, of the Brillouin gain spectrum 90 generated by the first monochromatic wave.
  • the Brillouin gain spectrum 90 and Brillouin loss spectrums 91 ,92 overlap they will cancel each other i.e. the Brillouin backscattering will be zero.
  • the resulting Brillouin gain spectrum can be referred to as the effective Brillouin gain spectrum 96, and this is the spectral profile of Brillouin gain which will be experienced by the probe signal when the probe signal is passed through the sensing fibre and interacts with the pump signal.
  • the effective Brillouin gain spectrum 96 is given by the superposition of the Brillouin gain spectrum 90 and two Brillouin loss spectrums 91 ,92.
  • the effective Brillouin gain spectrum 96 is illustrated in Figure 9(c).
  • a shorter duration for the pump signal may be used since the problem of increasing Brillouin gain spectral width due to a decrease in the duration of the pump signal can be resolved by the partial overlapping of the Brillouin loss spectrums with the Brillouin gain spectrum. Accordingly a higher resolution measurement can be achieved using the method according to the present invention.
  • FIG 10 shows the frequencies of the various signals used in an exemplary execution of a distributing sensing technique using Brillouin backscattering, according to a further embodiment of the present invention.
  • the probe signal used comprises a monochromatic wave; in other words the probe signal has a single frequency V s i .
  • the pump signal used comprises first, second and third monochromatic waves; the frequency of each of the first, second and third monochromatic wave are different.
  • the frequency of the first monochromatic wave is V p n
  • the frequency of the second monochromatic wave is V p2 2, and the frequency of the third monochromatic wave V p33 .
  • the pump signal comprises three different frequencies.
  • Each of the first, second and third monochromatic waves will generate a Brillouin gain spectrum and Brillouin loss spectrum, when they are passed into the sensing fibre. However, for this example we will only focus on the Brillouin loss spectrum generated by the second
  • the second monochromatic wave will generate a Brillouin loss spectrum at the frequency V' p2 2, which is V p22 + V B i wherein V B i is the Brillion frequency of the sensing fibre.
  • the Brillouin frequency V B i of the sensing fibre is an intrinsic property of the optical fibre which defines the sensing fibre.
  • the frequency of the second monochromatic wave V p22 is chosen such that the frequency V' P 2 2 will be equal to the frequency V s i of the probe signal.
  • the frequency V p22 of the second monochromatic wave is chosen such that the resulting Brillouin loss spectrum will occur at a frequency which is equal to the frequency of the probe signal V s i .
  • this will maximise the amount of stimulated Brillouin backscattering which occurs when the pump signal interacts with the counter propagating probe signal within the sensing fibre.
  • the first monochromatic wave will generate a Brillouin gain spectrum at the frequency V' p n, which is V p n - V B i, and third
  • the monochromatic wave will generate a Brillouin gain spectrum at the frequency V' p3 3, which is V p33 - V B i; wherein in each case V B i is the Brillouin frequency of the sensing fibre.
  • the Brillouin loss spectrum 190 generated by the second monochromatic wave is illustrated in Figure 1 1 (a), the Brillouin gain spectrums 191,192 generated by the first and third
  • the range of the Brillouin loss spectrum 190 generated by the second monochromatic wave is from frequency Fp22 to frequency F'p22.
  • the range of the Brillouin gain spectrum 191 generated by the first monochromatic wave is from
  • the range of the Brillouin gain spectrum 192 generated by the third monochromatic wave is from frequency Fp33 to frequency F'p33.
  • the frequencies V p n, V p33 of the first and third monochromatic waves are chosen such that the ranges of their respective Brillouin gain spectrums 191 ,192 partially overlap the range of the Brillouin loss spectrum 190 generated by the second monochromatic wave. As illustrated by the dashed lines in Figures 1 1 a&b, the range (Fp1 1 to frequency F'p1 1 ) of the Brillouin gain spectrum 191 generated by the first monochromatic wave overlaps a lower part 194 of the range (Fp22 to frequency F'p22) of the Brillouin loss spectrum 190.
  • the range (Fp33 to frequency F'p33) of the Brillouin gain spectrum 192 generated by the third monochromatic wave overlaps an upper part 195 of the range (Fp22 to frequency F'p22) of the Brillouin loss spectrum 190.
  • the frequencies of the first and third monochromatic waves V p n, V p33 are chosen so that their resulting Brillouin gain spectrums 191,192 overlap a lower part and upper part 194,195, respectively, of the Brillouin loss spectrum 190 generated by the second monochromatic wave.
  • the Brillouin loss spectrum 190 and Brillouin gain spectrums 191 ,192 overlap they will cancel each other i.e. the Brillouin backscattering will be zero.
  • the width of the Brillouin loss spectrum is decreased due to the fact that the Brillouin gain spectrums 191 ,192 partially overlap the Brillouin loss spectrum 190.
  • the resulting Brillouin loss spectrum can be referred to as the effective Brillouin loss spectrum 196, and this is the spectral profile of Brillouin loss which will be experienced by the probe signal when the probe signal is passed through the sensing fibre and interacts with the pump signal.
  • the effective Brillouin loss spectrum 196 is given by the superposition of the Brillouin loss spectrum 190 and two
  • the invention is not limited to requiring three monochromatic waves; the invention could still be performed so long as there is at least two monochromatic waves with different frequencies, wherein the frequencies are such that the Brillouin loss spectrum generated by one of the monochromatic waves partially overlaps the Brillouin gain spectrum generated by the other
  • the effective Brillouin gain spectrum or effective Brillouin loss spectrum would be narrowed from either the upper or lower end of the Brillouin gain/loss spectrum and not symmetrically narrowed from both the upper and lower ends of the
  • Brillouin gain/loss spectrum as was the case in the previously described embodiments above. It should be noted that when it is said that a Brillouin loss spectrum or Brillouin gain spectrum is located 'at' a particular frequency it means that the Brillouin loss spectrum or Brillouin gain spectrum is centred at that frequency.
  • FIG. 12 shows a system 100 as Brillouin analyser according to an embodiment of the present invention, which is configured to implement the above mentioned distributing sensing technique using Brillouin backscattering.
  • the system 100 comprises a coherent light source 101 ; light 103 from the coherent light source is divided by a divider 1 19 to provide a pump signal 107 in a first optical branch 104 and a probe signal 108 and a second optical branch 105.
  • the divider may be defined simply by the provision of two optical paths for the light 103.
  • the pump signal 107 is sent to programmable frequency convertor 109a and the probe signal is sent to a programmable frequency convertor 109b; the programmable frequency convertors 109a,b will modify the pump signal 107 and probe signal 108 respectively so that each signal has the desired number of frequencies.
  • programmable frequency convertor 109b is programmed to modify the probe signal 108 so that the probe signal has single frequency; the programmable frequency convertor 109a is
  • the pump signal 107 is then sent to a pulse generator 1 1 1 , so optical pulses which define the pulse signal, each comprise multiple frequencies is output from pulse generator 1 1 1 .
  • the probe signal 108 will be configured to be a continuous wave (this will be the case for all embodiments).
  • Advantageously pump signal 107 can provide spectrally optimized Brillouin gain spectrum (i.e. a narrowed Brillouin gain spectrum) to enhance the spatial resolution in distributed sensing system 100.
  • the probe signal 108 is configured to have a frequency which is in the vicinity of the Brillouin gain spectrum. So, like the conventional BOTDA systems, simply by scanning the probe frequency the distribution of the peak frequency of Brillouin gain spectrum can be investigated along the entire sensing fiber.
  • the pump and probe signals 107,108 counter propagate in a sensing fiber 1 13 which is attached to a structure 1 1 5 being monitored.
  • the pump and probe signals 107,108 will interact in the sensing fiber 1 13 to generate stimulated Brillouin backscattering as previously described.
  • the generated stimulated Brillouin backscattering can be used, according to known methods, to monitor changes in temperature and strain which occur in the structure 1 1 5.
  • the generated stimulated Brillouin backscattering is detected and analysed to monitor changes in temperature and strain which occur in the structure 1 1 5, using a signal processor 1 18.
  • the programmable frequency convertor 109a could alternatively be programmed to modify the pump signal such that the pump signal comprises third monochromatic wave which generates a second Brillouin gain spectrum, wherein the frequency of the third monochromatic wave is different to the frequencies of the first and second monochromatic waves, wherein the frequency of the third monochromatic waves is such that a frequency range of the second
  • Brillouin gain spectrum generated by the third monochromatic wave only partly overlaps a frequency range of a Brillouin loss spectrum generated by the second monochromatic wave.
  • the Brillouin loss spectrum is narrowed and the resulting (effective) Brillouin loss is used to perform a Brillouin optoelectronic measurement method using step known in the art.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Optical Transform (AREA)

Abstract

L'invention concerne un procédé de mesure optoélectronique de Brillouin qui consiste à fournir un signal de pompe à une première extrémité d'une fibre optique et à fournir un signal de sonde à l'extrémité opposée de ladite fibre optique, pour que les signaux de pompe et de sonde puissent interagir dans la fibre optique afin de générer une rétrodiffusion simulée de Brillouin, à utiliser la rétrodiffusion simulée de Brillouin pour effectuer une mesure, caractérisé en ce que le signal de pompe comporte au moins une première et une deuxième onde monochromatique, de telle sorte que la première onde monochromatique peut générer un spectre de gain de Brillouin et la deuxième onde monochromatique peut générer un premier spectre de perte de Brillouin, les fréquences de la première et de la deuxième onde monochromatique étant différentes, et les fréquences de la première et de la deuxième monochromatique étant telles qu'une plage de fréquences du premier spectre de perte de Brillouin généré par la deuxième onde monochromatique ne chevauche que partiellement une plage de fréquences d'un spectre de gain de Brillouin généré par la première onde monochromatique.
PCT/EP2012/071389 2012-10-29 2012-10-29 Procédé de mesure optoélectronique de brillouin WO2014067548A1 (fr)

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CN113465676A (zh) * 2021-07-09 2021-10-01 浙江大学 一种双阶段布里渊频移提取方法

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