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WO2010058438A1 - Appareil de mesure optoélectronique d'une caractéristique physique distribuée - Google Patents

Appareil de mesure optoélectronique d'une caractéristique physique distribuée Download PDF

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Publication number
WO2010058438A1
WO2010058438A1 PCT/IT2009/000525 IT2009000525W WO2010058438A1 WO 2010058438 A1 WO2010058438 A1 WO 2010058438A1 IT 2009000525 W IT2009000525 W IT 2009000525W WO 2010058438 A1 WO2010058438 A1 WO 2010058438A1
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WIPO (PCT)
Prior art keywords
given
frequency
radiation
optical
electromagnetic
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PCT/IT2009/000525
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English (en)
Inventor
Gabriele Bolognini
Fabrizio Di Pasquale
Marcelo Alfonso Soto
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FIBERSENS Srl
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FIBERSENS Srl
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Priority claimed from IT000047A external-priority patent/ITRA20080047A1/it
Priority claimed from IT000014A external-priority patent/ITRA20090014A1/it
Application filed by FIBERSENS Srl filed Critical FIBERSENS Srl
Publication of WO2010058438A1 publication Critical patent/WO2010058438A1/fr
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    • 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
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00—Measuring force or stress, in general
    • G01L1/24—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet
    • G01L1/242—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00—Measuring force or stress, in general
    • G01L1/24—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet
    • G01L1/247—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet using distributed sensing elements, e.g. microcapsules
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M5/00—Investigating the elasticity of structures, e.g. deflection of bridges or air-craft wings
    • G01M5/0091—Investigating the elasticity of structures, e.g. deflection of bridges or air-craft wings by using electromagnetic excitation or detection

Definitions

  • the present invention relates to a measuring apparatus.
  • the present invention relates to a measuring apparatus of the optoelectronic type.
  • the present invention relates to an optoelectronic measuring apparatus, which can be used to monitor physical characteristics of an architectonic or engineering structure.
  • measuring apparatuses In the civil engineering sector, and in particular in the field of the construction of structures of great dimensions, such as for example bridges, dams or oil pipelines, the use is well known of measuring apparatuses to monitor continuously structural and/or functional parameters of these structures.
  • these measuring apparatuses are commonly used to control the trend over time of the temperature or of the strain, i.e. of the relative deformation or elongation, 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.
  • the optoelectronic devices based upon optical fibres have a great significance.
  • these apparatuses normally comprise an electronic measuring device, provided with an optical fibre probe presenting high extension, 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 engineering 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 in 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 elements.
  • measuring instruments based upon optical fibres can be subdivided in various types depending upon both the physical quantity/ies they are suitable to measure and the physical principle used to detect this quantity/these quantities.
  • SPBS spontaneous scattering or spontaneous Brillouin scattering
  • SBS stimulated scattering or stimulated Brillouin scattering
  • the measuring apparatuses based upon SPBS are provided with an optical source, for example a laser, suitable to emit, along the respective optical fibre, a plurality of light pulses presenting a duration in the order of a ten of nanoseconds.
  • an optical source for example a laser
  • these light pulses travel along the optical fibre of the probe for all the respective extension, and are subjected, at each segment of this optical fibre, to a process of partial anelastic backscattering, precisely known as Brillouin scattering.
  • both the optical power and the difference between its own frequency and the carrier frequency of the respective original light pulse depend upon both the temperature and the strain of the portion of optical fibre, wherein the anelastic interaction occurred, which originated this Brillouin component . It is therefore known that, by monitoring over time both the optical power and the frequency of at least a Brillouin component (Stoke's or Anti-Stokes) it is possible to obtain the local temperature and strain of each portion of optical fibre and thus of the respective structural element (pillar, tube, etc.)? with which it is coupled.
  • Stoke's or Anti-Stokes a Brillouin component
  • OTDR Optical Time Domain Reflectometry
  • the device described in the above mentioned article provides for the use of high-power laser pulses, which are necessary to perform measures presenting an acceptable signal-to-noise ratio and being thus reliable.
  • the optoelectronic measuring unit of the measuring device must be therefore provided with highly expensive elements such as, for example, a high peak power laser, for example in the order of hundreds of mW, and at least one pair of optical amplifiers. It is therefore clearly apparent that a so designed measuring device will present such a high cost to prevent the use thereof in the field of the ordinary civil engineering and to limit the use thereof almost exclusively to the field of the specialised research centres or for the safeguard of special engineering structures, such as for example undersea railway tunnels or space stations.
  • the use of high peak power lasers causes a greater wear of the optical components of the measuring apparatus, and it can entails problems as regards the reliability over time of these components, of the laser source and, therefore doubts about the reliability of the performed measures .
  • each respective optoelectronic measuring device comprises a pair of optical sources, for example lasers, suitable to emit a respective first and second electromagnetic radiations propagating in opposite directions along a common optical fibre.
  • a first source with reduced optical power emits continuously over time a first substantially monochromatic probe radiation
  • a second source emits a respective second electromagnetic radiation, which counter-propagates relative to the first radiation along the same optical fibre, in the form of high optical power pulses presenting a given duration, typically in the order of the nanosecond.
  • the frequency of the first radiation can be adjusted and it can be set so as to trigger phenomena of optical amplification based upon the stimulated Brillouin effect, wherein the first and the second electromagnetic radiation are suitable to exchange with each other fractions of their respective optical energy.
  • the first and the second electromagnetic radiation are suitable to exchange with each other fractions of their respective optical energy.
  • the gain curve describing, as a function of time, this effect of optical amplification is linked, as it is well known, both to the temperature and to the strain associated to the various portions of the optical fibre, which are responsible for the SBS of the second electromagnetic pumping radiation.
  • This technique which allows measuring the spatial distribution, along an optical fibre, of at least one given physical quantity by analysing at least one optical signal transmitted through this optical fibre, is called Optical Time Domain Analysis or OTDA.
  • the techniques based upon SBS even if more effective from the application point of view, present different characteristics and are in particular affected by some disadvantages.
  • the performances of measuring are limited by the presence of effects of depletion of the optical pump and by non-linear effects such as the instability in the modulation of the probe radiations (Modulation Instability) when the peak power of this latter achieves particularly high values.
  • These effects cause distortions in the acquired OTDA traces, limiting the signal-to-noise ratio of the acquired signals, and therefore the spatial resolution and the accuracy of the performed measurements of temperature and deformation.
  • the present invention relates to a measuring apparatus.
  • the present invention relates to a measuring apparatus of the optoelectronic type. More in particular, the present invention relates to an optoelectronic measuring apparatus, which can be used to monitor physical characteristics of an architectonic or engineering structure.
  • the object of the present invention is to provide a measuring apparatus, which allows the disadvantages described above to be solved, and which is suitable to satisfy a plurality of requirements that to date have still not been addressed, and therefore suitable to represent a new and original source of economic interest and capable of modifying the current market of measuring apparatuses.
  • a measuring apparatus is provided, whose main characteristics will be described in at least one of the appended claims.
  • a further object of the present invention is to provide a measuring method, which can be validly used to monitor continuously physical characteristics of a given body.
  • FIG. 1 - figure A illustrates a spectrum of backscattering of a monochromatic electromagnetic radiation transmitted along an optical fibre
  • FIG. 1 is a schematic block view of a first preferred embodiment of a measuring apparatus according to the present invention
  • FIG. 2 illustrates a second preferred embodiment of figure 1
  • figure 3 illustrates diagrams of the optical power associated with electromagnetic pulse trains generated, in use, inside the measuring apparatus according to the present invention
  • - figure 4 illustrates a frequency spectral decomposition of an electric signal associated with an electromagnetic radiation generated through spontaneous Brillouin effect, and a numerical reconstruction of the variation along an optical fibre of a frequency spectrum of this electromagnetic Brillouin radiation;
  • figure 5 illustrates a third preferred embodiment of figure 1
  • figure 6 illustrates, in enlarged scale, a detail extracted from figure 5;
  • figure 7 illustrates a third preferred embodiment of figure 1
  • - figure 8 illustrates a fourth preferred embodiment of figure 1; and - figure 9 illustrates diagrams of the optical power associated with electromagnetic pulse trains generated, in use, inside the measuring apparatus according to the present invention.
  • FIG. 1 indicates, in its entirety, a measuring apparatus of the optoelectronic type, which can be used to monitor continuously over time physical characteristics of a given body 1000.
  • the measuring apparatus 1 is preferably suitable to measure locally the temperature and the relative elongation, or strain, of the body 1000, which can comprise, for instance, an architectonical or engineering structure such as a bridge or a skyscraper, but also an oil platform or a pipeline.
  • the measuring apparatus 1 is provided with a probe 25 presenting a given extension L, which, in use, is arranged into contact with the body 1000 and allows to carry out local measurements of the physical properties of this body, i.e. measurements suitable to give information about the physical characteristics of limited portions of the body 1000. Therefore, by using a probe 25 presenting an extension L adequately commensurate with the dimensions of the body 1000, it will be possible to know a physical status of the entire body 1000 by monitoring, for instance, the local temperature and/or the local strain of a plurality of contiguous portions of the body 1000.
  • the measuring device 1 comprises an emitting group 10 associated with or provided with a first electromagnetic source 15, suitable to produce continuously a first substantially monochromatic electromagnetic radiation EMl and presenting a first carrier frequency F and a given optical power 10 constant over time.
  • a first electromagnetic source 15 comprising a laser 15 of the known type, for example an external cavity laser or a distributed feedback laser (DFB) , suitable to generate an electromagnetic radiation presenting preferably, but without limitation, a first given frequency F in the order of 193 THz.
  • a laser 15 of the known type for example an external cavity laser or a distributed feedback laser (DFB)
  • DFB distributed feedback laser
  • this design choice has a purely arbitrary character, and it does not represent a limit to the generality of the present invention, whose respective measuring apparatus 1 can be designed so as to use electromagnetic sources of different type.
  • the emitting group 10 comprises a first optical element 18, suitable to subdivide the first electromagnetic radiation EMl in a first fraction EMl ' and in a second fraction EMl ' ' , presenting respectively optical powers 10' e 10' ' .
  • this first optical element 18 can comprise an optocoupler, or optical coupler, suitable to serve as beam splitter.
  • the first fraction EMl' is sent to an amplifying device 19 suitable, in use, to amplify the optical power 10' of the first fraction EMl' until a given peak value, preferably in the order of the tens of mW.
  • the amplifying device 19 preferably comprises an optical amplifier 19', for example an EDFA amplifier based upon an erbium doped optical fibre, connected in series with a first band - pass optical filter 19'' presenting a given pass band, for example of 0.5 nm, to filter noise originated during the amplification process.
  • the laser 15 presents a peak power sufficiently high for all the required applications, the presence of the amplifying device 19 is not strictly necessary and, as illustrated in figure 2, it can be omitted so as to reduce production and maintenance costs of the measuring apparatus 1.
  • the emitting group 10 comprises a modulation device 11 designed so as to receive the first fraction EMl' and suitable, in use, to modulate the variation over time of the respective optical power of the fraction, so as to generate at least a train P of first electromagnetic pulses P'.
  • These first pulses P 1 are substantially monochromatic, present the same carrier frequency F as the first electromagnetic radiation EMl, present a first given duration D' and can be emitted in sequence by the emitting group 10 substantially continuously.
  • each train P of first electromagnetic pulses P' presents a second given duration Dl, during which up to a first given number N of first pulses P' can be emitted.
  • the trains P are generated periodically according to a given period T and, as it will be better described below, they all comprise preferably the same number of first pulses P 1 .
  • the modulation device 11 comprises an optoelectronic modulation unit 12 connected to a first control unit 14, which can be used to adjust the trend over time of the optical power I associated to each train P and therefore suitable to determine the first duration D', the optical power and the first given number N that are associated to the first electromagnetic pulses P 1 .
  • the first control unit 14 can preferably comprise a programmable wave-form generator or, alternatively, it can be interfaced with, or implemented through, a computer 50 suitable, in use, to send reference wave-forms to the modulation unit 12, according to which to modulate over time the optical power I associated to each train P.
  • the modulation unit 12 comprises preferably, but without limitation, a Mach-Zehnder interferometer 13, coupled in series to a polarisation controller 13' and connected to the first control unit 14 to modulate over time the optical power I associated with the pulses P according to respective reference wave forms.
  • each train P will be modulated according to a code C, preferably of the binary type, so as to associate to each train P a word, written according to this code C, that for the sake of simplicity will be indicated hereinafter with the same letter P.
  • a code C preferably of the binary type
  • P is composed exactly by a first given number N of characters.
  • the optical power I associated to each train P will present, according to the time, a stepped profile, wherein to each relative minimum and to each relative maximum one or more bit will be associated, presenting respectively the binary values 1 and 0.
  • the minima of the optical power I can be equal to zero, as illustrated in figure 3a, or they can present a non null value, as illustrated in figure 3b, however hereinafter the term "train P of electromagnetic pulses" will be used to indicate, in its entirety, only a single physical entity presenting a second duration Dl even if this, as illustrated in figure 3a, is divided into a plurality of distinct packets due to the presence of a bit with binary value 0, or, in other words, of first pulses P' with null optical power.
  • each first pulse P' can be of the NRZ, i.e. nonreturn to zero, type, as illustrated in figures 3a and 3b, or of the RZ, i.e. return to zero, type, as illustrated in figure 3c.
  • the pulses of the NRZ type present a constant optical power for all the respective first duration D 1
  • the pulses RZ present an optical power different than zero, or anyway different than a base value associated to a bit equal to zero, only for a given fraction of the first duration D', for example 20% or 50%, as illustrated in figure 3c.
  • each train P are associated, for example, 127 or 255 bits, and therefore respectively 127 or 255 first pulses P 1 , and that each word P has been coded according to the code C, known as Simplex code.
  • C the code C
  • this choice does not represent a limit to the present invention, which can be correctly implemented also by using binary codes C of different type, for example, although without limitation, Golay codes. Therefore, in view of the above description, it is clearly apparent that the emitting group 10 is suitable to emit in a periodic manner trains P of first electromagnetic pulses P', whose optical power I is modulated so as to associate to each train P a respective word P of the given code C containing a first given number N of bits.
  • the measuring apparatus 1 comprises an optical transmission group 20 provided with the probe 25 and designed so as to receive each train P and to transmit it along the probe 25.
  • the probe 25 comprises a wave guide 25 suitable to transmit each train P for all the respective linear extension L.
  • the wave guide 25 can comprise, without limitation, a first optical fibre 25.
  • this backscattering phenomenon can be interpreted as the superimposition of a phenomenon of elastic scattering, wherein a fraction of the electromagnetic radiation transmitted along the wave guide is backscattered without any variation in the respective carrier frequency, with a phenomenon of anelastic scattering, wherein the electromagnetic radiation is diffused after having been subjected to at least a variation in its carrier frequency.
  • sending a first train P of first electromagnetic pulses P 1 along the first optical fibre 25 generates a second electromagnetic radiation EM2, formed by the superimposition of all the fractions of this train P which are backscattered from respective portions of the first optical fibre 25 as the pulse travels along the linear extension L of this first optical fibre 25.
  • this second radiation will present a first component EM2 ' elastically scattered and therefore centred on the first carrier frequency F and at least a second component EM2 ' ' presenting a second given frequency FB.
  • the second components EM2 ' ' will be taken into consideration, generated from that phenomena of optical backscattering known as "spontaneous Brillouin effect", or SPBS, that, as illustrated in figure A, entail, at the first optical order, the creation of two second components EM2 ' ' of the second electromagnetic radiation EM2, presenting respective second frequencies FB arranged in a manner spectrally symmetric relative to the first carrier frequency F.
  • SPBS spontaneous Brillouin effect
  • spontaneous Brillouin effect reference will be made to any anelastic scattering phenomenon, which can be interpreted, according to the quantum physics, as an interaction between the photons that form the first electromagnetic pulses P and acoustic photons, which propagate along the wave guide 25.
  • the second components EM2 ' ' which are respectively indicated as Stokes and anti-Stokes components, present a greater spectral width and a significantly lower optical power than the first elastically backscattered first component EM2 ' , which is commonly indicated as Rayleigh component .
  • each second radiation EM2 through backscattering will present a third duration D2 substantially double than the time necessary for each train P to cross the first optical fibre 25 along all the respective given extension L. Therefore, with first electromagnetic pulses P' presenting a first given duration D 1 in the order of the ten of nanoseconds and with a first optical fibre 25 presenting a given extension L in the order of the tens of kilometres, the third duration D2 of emission of each second electromagnetic radiation EM2 will present a value in the order of hundreds of nanoseconds.
  • the third duration D2 presents a value which can be about ten thousand times greater than the value of the first duration D' and therefore the emission of each second electromagnetic radiation EM2 can be interpreted as an emission phenomenon substantially continuous over time relative to the periodic emission of the first pulses P'.
  • the period T of time, which separates the emission of each two consecutive trains P will present a value twice greater than the third duration D2, so that the second radiation EM2 generated by a given train P does not superimpose the second radiation EM2 generated by the subsequent train P.
  • each second frequency FB and of the optical power IB associated to a respective second Brillouin component EM2 ' ' depends upon the local temperature and the local strain of the portion of first optical fibre 25, which has generated this component during a respective backscattering phenomenon.
  • the measuring apparatus 1 is therefore suitable to monitor contemporaneously the local distribution of temperature and strain of a body 1000 through the measurement of the frequency and of the optical power associated to at least one electromagnetic component of a train P backscattered through Brillouin scattering.
  • the transmission group 20 comprises an optical device 22 suitable, in use, to superimpose the second fraction EMl' 1 of the first electromagnetic radiation EMl to at least a second component EM2 ' ' of the second electromagnetic radiation EM2.
  • the optical device 22 comprises preferably a second optical filter 23, suitable to select exclusively a second Brillouin component EM2 ' ' from the respective second radiation EM2 to mix it with the second fraction EMl''.
  • this second optical filter 23 can preferably comprise a band-reflection grating or FBG, designed so as to be passed by the second fraction EMl ' ' (coming from the left in figure 1) and to reflect only a second Brillouin component EM2 ' ' , i.e. to reflect a spectral portion of the second radiation EM2 (coming from the right in figure 1) centred around a second given frequency FB.
  • this optical device 22 can or cannot comprise one or more optical elements of the known type, for example optocouplers or optical circulators, to guide the propagation of the second fraction EMl'' and/or of the second radiation EM2 towards a first detecting group 30 of the measuring apparatus 1. As the function of these optical elements is well known, it has been deemed sufficient to indicate them generically in figures 1, 2, and 5, without assigning them a respective reference number.
  • each second frequency FB is spectrally near to the respective first carrier frequency F.
  • the second frequencies FB relating to the second Stokes and anti-Stokes components EM2 ' ' differ from the first carrier frequency F by about ⁇ 11 GHz.
  • the optical radiation resulting from the superimposition of a second component EM2 ' ' with the respective second fraction EMl' will present a respective electrical intensity, downstream of the photoelectric sensor 31, characterized by interference phenomena modulated according to a third frequency ⁇ FB substantially identical to the difference between the first frequency F and a second given frequency FB.
  • This third frequency ⁇ FB is commonly indicated as "frequency shift" of the respective Brillouin component and, as already noted, it is in this example in the order of 11 GHz.
  • the first detecting group 30 comprises a first photoelectric sensor 31, suitable to receive the superimposition of each second component EM2 ' ' with the respective second fraction EMl 1 , to convert it in a respective first electric signal Sl, for example an electric voltage signal.
  • This first sensor 31 comprises preferably, although without limitation, a photodiode 31, for example a photodiode of the PIN or APD type, which presents a pass-band greater than the third frequency ⁇ FB and which is therefore suitable to solve in frequency the modulations of the optical intensity of the radiation incident on it.
  • the first electric signal Sl generated by the photodiode 31 will present beating phenomena modulated according to the third frequency ⁇ FB and that the spectral characteristics of these beats substantially reflect, unless a scale factor, the spectral characteristics of the second selected Brillouin component EM2 " .
  • the first electric signal Sl can be interpreted, instant by instant, as a spectral image of a component EM2 ' ' centred to a third frequency ⁇ FB instead that to the respective more high second frequency FB.
  • the measurement of the optical power IB and of the second frequency FB of a second Brillouin component EM2 ' ', and of the third frequency ⁇ FB can be obtained through the analysis of the spectral characteristics of the beats presented by the first electric signal Sl.
  • the process of optical superimposition of a second component EM2 ' ' with the second fraction EMl'' of the first carrier radiation EMl and the subsequent conversion made by the first sensor 31 allows to translate the spectrum of the second selected Brillouin component EM2 ' ' from the range of the infrared radiation to the range of microwaves.
  • This method of analysis of a high-frequency signal is known with the name of "measurement” or "coherent detection” and presents the following advantages: first of all, the spectrum of the quantity to be analysed is translated to lower frequency, and this allows using more economical photoelectric sensors 31 presenting a lower pass-band.
  • each second Brillouin component EM2 ' ' presents a very reduced optical power relative to the optical power associated to the respective train P and therefore, if a direct optical measurement is performed, this component risks being below the detection limit of the firs sensor 31 or producing a measurement characterised by a very bad signal-to-noise ratio.
  • the superimposition between each second component EM2 ' ' and a respective second carrier fraction EMl' ' of greater optical power allows to make an electromagnetic radiation incident on the first sensor 31, the radiation being provided with an optical power sufficient to originate a measurement, and therefore a first electric signal Sl, presenting a high signal-to-noise ratio.
  • the set of the optical device 22 and of the first photoelectric sensor 31 can be therefore interpreted as a first opto-electronic coherent detection device 35, which can be used to detect spectral characteristics of a second given component EM2 ' ' of the second electromagnetic radiation EM2.
  • the first detection group 30 comprises a frequency selector 33 connected at the output to the first sensor 31 to take the first electric signal Sl, and suitable, in use, to emit a second electric signal S2, almost continuous and presenting, instant by instant, a value of voltage and/or electric current directly proportional to the average spectral amplitude of a respective given single spectral band BS of the first electric signal Sl presenting a respective given spectral width LS.
  • average spectral amplitude of a given band BS indicates the integral in frequency, along the respective given spectral width LS, of the amplitude of that spectral portion of the first electric signal Sl associated to this given band BS.
  • figure 4a illustrates a frequency spectrum of a first given signal Sl subdivided into a plurality of respective given bands BS.
  • the average spectral amplitude of each band BS is, instant by instant, substantially proportional to the respective area of its own frequency spectrum, an example of which relating to a generic instant is illustrated in figure 4.
  • the frequency selector 33 can be adjusted and it allows selecting each time the frequency on which is centred the given spectral band BS of the first signal Sl to be isolated. Therefore, the frequency selector 33 can be used to discretise each frequency spectrum of each first signal Sl by subdividing it in a second given number M of given spectral bands BS, of which it will be possible to analyse, independently one of the other, the changes over time of the respective average spectral amplitudes.
  • the frequency selector 33 is preferably of the programmable type and it allows varying the second given number M, the given spectral width LS of each spectral band BS and the spectral position of each given spectral band BS, so as to regulate the resolution and the speed of performing the spectral analysis which can be performed through the first detecting group 30, for example, to adapt it to the calculation speed of the computer 50.
  • the second given number M, the given spectral width LS of each spectral band BS and the spectral position of each given spectral band BS are mutually correlated parameters and the respective values are selected, in use, so that the frequency selector 33 is always suitable to scan a frequency interval sufficiently wide to contain completely each frequency spectrum of the first signal Sl.
  • each second Brillouin component EM2 ' ' and therefore each respective first signal Sl, presents an intrinsic spectral width, typically- in the order of tens of MHz, and this signal is suitable to vary, also by many tens of MHz, its frequency position depending upon the temperature and the strain of the various portions which compose the first optical fibre 25. Therefore, in use, the frequency selector 33 will be always suitable to scan a frequency interval presenting a width comprised, for example, between the tens of KHz and the hundreds of MHz.
  • the frequency selector 33 comprises preferably, although without limitation, an electric oscillator 34, whose carrier frequency can be tuned substantially at will about the third frequency ⁇ FB.
  • This electric oscillator 34 produces an harmonic third electric signal S3, which is mixed to the first signal Sl to generate a fourth electric signal S4 sent to a third band-pass filter 32 presenting a given pass-band substantially identical to the spectral width LS of the given spectral band BS which one desires to select.
  • the pass-band of the third filter 32 is spectrally fixed, constant over time and centred around a frequency value significantly lower than the third frequency ⁇ FB, for example in the order of some MHZ.
  • this filter 32 will select only that component of the fourth signal S4, and therefore of the first signal Sl, which presents beats modulated according respective frequencies falling within its own pass-band.
  • this filter 32 by varying each time the carrier frequency of the third signal S3 generated by the electric oscillator 34, it will be possible to select the fourth signal S4 so as to select through the third filter 32 a given number M of spectral bands of the fourth signal S4 to produce a complete, but discrete, spectral and time (or equivalently in distance) reconstruction of the first electric signal Sl and therefore of the second optical Brillouin component EM2 ' ' associated thereto .
  • the functioning principle of the frequency selector 33 is conceptually the same as that of the first opto-electronic coherent detection device 35 and therefore this frequency selector 33 can be interpreted as a second electric coherent detection device 36. It should be furthermore noted that, at reduced costs, the above illustrated configuration gives to the frequency selector 33 an operative speed greater than the electric spectrum analysers currently marketed, which are generally inadequate for the present application.
  • each signal exiting from the third filter 32 is sent to a second electric sensor 37 suitable, in use, to convert it into a second electric signal S2 almost continuous and proportional to the average spectral amplitude of the respective spectral band BS selected by the first signal Sl.
  • This second sensor 37 can preferably and economically comprise a current rectifier 37 connected in series to a fourth low- pass filter 38, suitable to remove from each second signal S2 electric noise generated from the conversion/rectification process performed by the second sensor 37.
  • the first detection group 30 can comprise, as the case may be, one or more electronic amplifiers of the known type, which therefore in figures 1, 2, and 5 have been generically illustrated without specific reference numbers; these amplifiers can be introduced or not, in the case the intensity of at least one of the electric signals S1-S4 is not sufficient to obtain measurements presenting an adequate signal-to-noise ratio. Therefore, in view of the above description, the frequency selector 33 can be also interpreted as an electric spectrum analyser 33 with high operative speed and therefore suitable to allow, substantially in real time, a full spectral analysis of each first electric signal Sl associated to a Brillouin optical component.
  • the first detection group -30 comprises preferably an analog-to-digital converter 39 suitable, in use, to receive and digitalise each second electric signal S2 exiting from the frequency selector/spectrum analyser 33.
  • each second signal S2 can be acquired by the computer 50 which, as it will be better explained hereunder, is suitable to process these data in order to reconstruct a spectrum of the second Brillouin component EM2 ' ' and to decode information coded in this spectrum, according to the given code C, during the phase of modulation of the first pulses P'. Therefore, the computer 50 is suitable both to control the/to act as, first control unit 14 for controlling the coding of the words P, and to act as decoder 60 of the information coded in each first signal Sl according to the given code C.
  • the emitting group 10 comprises a second electromagnetic source 16, for example a wide-band laser, suitable to produce continuously a third electromagnetic radiation EM3 preferably centred on the same carrier frequency F of the first electromagnetic radiation EMl.
  • this third radiation EM3 is sent to the modulation device 11 alternatively to the first electromagnetic radiation EMl so as to be modulated in the form of second pulses p' ' presenting an optical power and a duration given and constant over time.
  • Each second pulse p 11 is transmitted along the first optical fibre 25 so as to generate a respective fourth electromagnetic radiation EM4 backscattered by at least a portion of this optical fibre 25.
  • each fourth radiation EM4 will present at least one third Rayleigh component EM4 ' , generated by the elastic backscattering of the third electromagnetic radiation EM3.
  • the process of emission of each electromagnetic radiation EM4 presents a fourth duration D4, which is substantially identical to the third duration D2 of emission of each second ' radiation EM2.
  • each fourth radiation EM4 is sent to a second optical element 21 associated to the optical transmission group 20 and suitable, in use, to act as beam splitter and therefore suitable to send a given fraction of each fourth electromagnetic radiation EM4 to a second detecting group 30'.
  • This second detecting group 30' comprises a fourth pass-band optical filter 32' designed so as to select exclusively each third Rayleigh component EM4 ' and to send it to a third sensor 31' which comprises, for example, a photodiode 31' of the PIN type and is suitable to convert each third component EM4 ' into a respective fifth electric signal S5 representing the variation over time, and therefore the linear distribution along the first optical fibre 25, of the optical power associated to this third component EM4 ' .
  • a second analog-to-digital converter 39' so as to be acquired by the computer 50.
  • knowing at least one trace presenting a fourth duration D4 and illustrating the variation over time of the Rayleigh radiation backscattered along the first optical fibre 25 allows normalising the data relating to each second Brillouin component EM2 ' ' so as to take into account phenomena of optical attenuation which normally occur along any wave guide/first optical fibre 25.
  • This normalisation procedure is well known and can be performed, for instance, by the computer 50 by applying numerically the so-called Landau-Placzek ratio or "L-P ratio" between the signals acquired by the first detecting group 30, and subsequently processed, and at least a respective fifth signal S5 acquired by the second detecting group 30'.
  • figure 2 illustrates a second preferred embodiment of the measuring apparatus 1, wherein a second source 16 is not provided usable for measuring the Rayleigh component, but the first substantially monochromatic source 15 is used both to generate each train P of first pulses P' and each second pulse p' ' .
  • the transmission of substantially monochromatic pulses along an optical fibre can generate phenomena known as "coherent Rayleigh noise", due to phenomena of self-interference between the radiation sent along this optical fibre and the radiation (almost) elastically backscattered from distinct portions of the optical fibre.
  • the emitting group 10 is provided with a second control unit 17, controlled, as the case may be, by the computer 50 and suitable to adjust the power, and as the case may be the frequency, of emission of the first source/laser 15 to originate dithering phenomena which can be used to minimise this coherent Rayleigh noise until values lower than the percentage point of the emitted optical power and therefore substantially negligible.
  • the first source 15 in use, maintains an emission optical power constant and substantially identical during the emission of each train P, whilst the emission optical power will be modulated by the second control unit 17 every time second pulses p 1 ' must be generated, finalised to measure the intensity of the third Rayleigh component EM4 ' .
  • Figure 2 furthermore illustrates a second configuration of the transmission group 20 relative to that illustrated in the first preferred embodiment of figure 1.
  • the optical device 22 does not comprise a band-reflecting filter 22 to perform the mixing of the second fraction EMl' ' with a single second Brillouin component EM2 ' ' , but the second fraction EMl'' is optically superimposed together with a second electromagnetic radiation EM2 backscattered from the first optical fibre 25 by means of a third optical element 26 suitable to act as optical coupler.
  • this configuration is simpler and more economical than the configuration illustrated in figure 1, but it can be validly used only when the optical power associated to the first Rayleigh component EM2 ' is substantially negligible relative to the optical power associated to the second fraction EMl 11 of the first electromagnetic radiation EMl.
  • the presence of a second source 16 and the absence of the second filter 22 are mutually distinct characteristics which can be implemented or not, independently one of the other in each measuring apparatus 1 according to the present invention.
  • FIG 5 a third preferred embodiment is illustrated of the measuring apparatus 1, which presents a further variant of the optical transmission group 20, wherein the optical device 22 comprises an optical frequency changer 24, which is suitable, in use, to receive the second fraction EMl 1 ' and to emit a fifth electromagnetic radiation EM5 presenting a fourth carrier frequency F', spectrally arranged between the first carrier frequency F of the first radiation EMl and the second frequency FB of a second Brillouin component EM2 ' ' .
  • the optical device 22 comprises an optical frequency changer 24, which is suitable, in use, to receive the second fraction EMl 1 ' and to emit a fifth electromagnetic radiation EM5 presenting a fourth carrier frequency F', spectrally arranged between the first carrier frequency F of the first radiation EMl and the second frequency FB of a second Brillouin component EM2 ' ' .
  • the fourth frequency F' can present a frequency shift relative both to the first frequency F and to a second given frequency FB in the order, for example, of 5-6 GHZ.
  • the fifth radiation EM5 generated by the optical frequency changer 24 is optically superimposed to the second radiation EM2 coming from the first optical fibre 25, so that the first electric signal Sl associated to this optical superimposition presents, downstream of the first sensor 31, beats, whose respective third frequency ⁇ FB is substantially identical to the difference between the fourth frequency F' and the second frequency FB of a second given Brillouin component EM2 ' ' .
  • the optical frequency changer 24 allows measuring characteristics of a second Brillouin component EM2 ' ' using a more economical coherent detection group 30 designed for low frequencies and that can therefore present a lower operative speed relative to the detection group 30 implemented in the previous embodiments of the measuring apparatus 1.
  • the optical frequency changer 24 can comprise preferably, although without limitation, a high-speed phase modulator LN, or a high-speed acousto-optic modulator.
  • the optical frequency changer 24 can comprise a second optical fibre 24 ' , stably maintained at a given and constant temperature and subjected to a given and constant strain, and a fifth optical band-pass filter 24'' designed to select an optical band centred in the reference value of a given Brillouin component generated by the scattering of the second fraction EMl ' ' inside the second optical fibre 24'. Therefore, in use, the optical frequency changer will be suitable to generate a fifth radiation EM5, which presents a fourth carrier frequency F' substantially identical to a reference frequency for this given Brillouin component.
  • each first signal Sl associated to the superimposition of a second Brillouin component EM2 ' ' with the fifth radiation EM5 will present low- frequency beats, for example in the order of tens or hundreds of KHz, and therefore easily and economically detectable.
  • the second optical fibre 24 ' is preferably produced with a material at least partially different from that of the first optical fibre 25, so that the fourth frequency F' does not generate interference phenomena with the second frequency FB of each second Brillouin component EM2 ' ' .
  • each first pulse P' is emitted with a carrier frequency different relative to the first frequency F of the first radiation EMl emitted from the first laser source 15.
  • each first pulse P 1 will be emitted with such a frequency that the second frequency FB of each respective given second Bri'llouin component EM2 ' ' will be near the first frequency F of the first fraction EMl' and therefore the value of the difference thereof, i.e. the respective third frequency ⁇ FB, will be reduced and preferably in the order of the tens or hundreds MHz, similarly to what occurs in the above mentioned third preferred embodiment of the measuring apparatus 1.
  • the present invention also relates to a measuring method which can be performed through the apparatus 1 and is suitable to measure the linear distribution of at least one physical quantity, for example the temperature and/or the strain, along all the linear extension of the probe/first optical fibre 25.
  • this method comprises in particular the phase of generating a first electromagnetic radiation EMl through a first source 15, followed by a phase of modulating the optical power associated to at least one fraction of this first electromagnetic radiation EMl through an adequate modulation device 11, so as to generate a plurality of trains P of first pulses P 1 ; these trains P are emitted periodically and present a respective second duration Dl, which is substantially equivalent to the product of each first duration D' of each first pulse P' and the first given number N.
  • the phase of modulating the optical power associated to at least one fraction of the first radiation EMl comprises the phase of associating to each train P a respective word P coded according to a given code C, preferably of the binary type.
  • the phase follows of transmitting each train P along a probe 25 of given extension L and the phase of receiving, for each train P, a substantially continuous second electromagnetic radiation EM2 backscattered, both elastically and anelasticall'y due to spontaneous Brillouin effect, by at least one portion of the probe 25.
  • the measuring method according to the present invention comprises at least a phase of measuring the variation over time of the optical power IB and of the third frequency ⁇ FB associated to at least one second given Brillouin component EM2 ' ' of a given second radiation EM2 to obtain from this analysis a measurement of the temperature and/or of the strain associated to each portion of the probe 25.
  • This phase of measuring the variation over time of the optical power IB and of the third frequency ⁇ FB associated to at least one second given component EM2 ' ' comprises first of all a phase of mixing this second given Brillouin component EM2 ' ' with a further given electromagnetic radiation so that the resulting radiation presents, downstream of the first photoelectric sensor 31, a respective electric power modulated substantially according to a third frequency ⁇ FB substantially equivalent to the difference between the first frequency F and a second frequency FB.
  • this further given electromagnetic radiation can be a fraction of the first radiation EMl or a fifth radiation EM5 generated by an optical frequency changer 24 starting from a fraction of the first radiation EMl.
  • the phase of measuring the variation over time of the optical power IB and of the third frequency ⁇ FB associated to at least one second given component EM2 ' ' comprises a phase of converting the superimposition of this second given Brillouin component EM2 ' ' with a respective further electromagnetic radiation into a first electric signal Sl through a first photoelectric sensor 31, followed by a phase of analysing the variation over time of the frequency spectrum of this first signal Sl by means of the second electric coherent detection device 36.
  • this phase of analysing the variation over time of the frequency spectrum of the first signal Sl comprises the phase of discretising this frequency spectrum by subdividing it into a second given number M of given spectral bands BS, to each of which a trace OTDR will be associated, and the phase of converting each of these spectral bands into a respective second electric signal S2, preferably of the digital type, which represents the variation over time of the average spectral amplitude of the respective spectral band BS.
  • the phase of discretising the frequency spectrum of the first electric signal Sl comprises the cyclical repetition by a given number M of times of a phase of mixing this first signal Sl with a third harmonic carrier signal S3 generated by an oscillator 34 which can be adjusted in frequency to obtain a fourth electric signal S4, followed by a phase of tuning this fourth electric signal S4 relative to the pass-band of a second filter 37 to select a given spectral band of this fourth electric signal S4.
  • the phase of converting each of the spectral bands BS in a respective second electric signal S2 can be performed in succession for a given number K of times, at the end of which a phase can be performed of mediating statistically a third given number K of these electric signals S2 to obtain an average signal S2 presenting a signal-to-noise ratio significantly higher.
  • the third given number K can be in the order of the thousand or tens of thousand.
  • the phase of measuring the variation over time of the optical power IB and of the third frequency ⁇ FB associated to at least one second given component EM2 ' ' of a second given radiation EM2 comprises a phase of numerically reconstructing through a computer 50 the variation over time of the frequency spectrum of this second component EM2 ' ' starting from the digital electrical signals S2 relating to the spectral bands BS of the first signal Sl corresponding to this second component EM2 ' ' .
  • this phase of numerically reconstructing the variation over time of the frequency spectrum of this second component EM2 ' ' comprises the cyclical repetition for a given number of times of a phase of interpolating a given frequency spectrum of the second given component EM2 ' ' .
  • each phase of interpolating a given frequency spectrum is performed by interpolating with a continuous mathematic function, for example a Lorenz function, a second given number M of experimental points, each of which is associated to a respective spectral band BS.
  • a continuous mathematic function for example a Lorenz function
  • M of experimental points each of which is associated to a respective spectral band BS.
  • each spectrum reconstructed through interpolation is associated to a given time instant associated to the time of flight of the second radiation EM2 and, therefore, based upon the functioning principles of the OTDR, each spectrum reconstructed through interpolation can be associated to a given linear distance measured along the probe 25.
  • figure 4b illustrates an example of frequency spectrum of a second Brillouin component EM2 ' ' reconstructed through interpolation of the experimental points associated to the second electric signals S2.
  • the set of the phase of discretising the frequency spectrum of a first electric signal Sl in a plurality of spectral bands BS, of the phase of converting each of these spectral bands in a respective second electric signal S2 of the digital type, and of the phase of numerically reconstructing the variation over time of the frequency spectrum of a second given component EM2 ' ' can be interpreted as a phase of performing a digital sampling of the variation over time of this frequency spectrum of a second component EM2 ' ' to allow subsequent operations of numerical-statistical processing and/or operations of decoding information/OTDR traces coded inside this frequency spectrum.
  • the phase of measuring the variation over time of the optical power IB and of the third frequency ⁇ FB associated to at least one second given component EM2 ' ' comprises a phase of obtaining through the numerical reconstruction of the frequency spectrum of this second component EM2 ' ' , a first and a second numerical functions Fl and F2, which represent respectively the trend of the optical power IB and of the third frequency ⁇ FB as a function of time.
  • This phase can be performed by assigning to each instantaneous value of the optical power IB, and therefore to each point of the first numerical function Fl, the integral of the Lorenz function which better interpolates the frequency spectrum of the respective signal Sl in that given instant, as the case may be, by normalising through the use of the Landau-Placzek ratio.
  • the measuring method according to the present invention comprises a phase of decoding information contained in a fourth number Q of these first and second numerical functions Fl and F2 and originally associated to a given number Q of distinct words P transmitted preferably in succession through respective trains P of first electromagnetic pulses P'.
  • the result of the phase of decoding information contained in a fourth given number Q of first and second numerical functions relating respectively to the optical power IB and to the third frequency ⁇ FB is that of generating a third and a fourth numerical functions Fl 1 and F2 ' which represent respectively measurements of the variation over time of the optical power IB and of the third frequency ⁇ FB; these measurements associated to the third and to the fourth numerical functions Fl' and F2 ' are not influenced, as the first and second functions Fl and F2, by the presence of the original modulation of optical power of the trains P, and they are furthermore characterised by a signal-to-noise ratio, with equal spatial resolution, significantly better than the signal-to-noise ratio associated to the single measurements relating to the first and second functions Fl and F2.
  • the improvement in the signal-to-noise ratio in the measurement of the optical power IB due to the use of words P of simplex codes of 127 bits is in the order of 7.5dB.
  • the present measuring method also comprises a phase of normalising the third numerical function Fl ' relative to the variation over time of the optical power associated to a respective third Rayleigh component EM4 ' , so that the values of the third numerical function Fl 1 take into account phenomena of optical attenuation of the wave guide 25.
  • this phase of normalising the third numerical function Fl 1 is preceded by a phase of optically filtering at least one third component EM4 ' of a fourth given radiation EM4, and by a phase of converting, through a third photoelectric sensor 31', this first component EM2 ' in a fifth electric digital signal S5, whose intensity is substantially proportional, instant by instant, to the optical power associated to the respective third Rayleigh component EM4 1 .
  • the measuring method according to the present invention comprises the phase of calculating the linear distribution of the temperature and of the strain associated with the probe 25, and therefore with respective contiguous portions of the body 1000, applying to the third and fourth numerical functions Fl' and F2 ' empirical formulae, known and cited also in Docl.
  • the only way to improve the signal-to-noise ratio of the performed measurements seems to be the increase in the duration of each pulse/train of pulses to increase the respective optical energy thereof, but this choice clearly reduces the spatial accuracy of the measurements of temperature and strain.
  • the hypothesis of increasing the optical power supplied by the electromagnetic source entails high costs for producing and maintaining the measuring apparatus that, on the contrary, are to be minimised.
  • each train P will present an optical energy sufficient to allow measurements of the optical power IB and of the third frequency ⁇ FB characterised by a good signal-to-noise ratio. Furthermore, the use of the decoding techniques during the phase of analysing the spectrum associated to the Brillouin components allows decoupling the different optical contributions which form each second component EM2 ' ' and which are respectively related to the backscattering of each single first pulse P' associated to a respective train P.
  • the use of coding/decoding techniques allows to given to each measurement of temperature and/or strain performed by the apparatus 1 both the high spatial resolution associated to a single first pulse P' presenting a short first duration D' and, contemporaneously, the high signal-to-noise ratio associated to a train P presenting a respective optical energy substantially equivalent to the sum of the optical energies associated to all the first pulses P' /bits which forma each word P.
  • coding/decoding techniques which represents the characterising and most innovative aspect of the present invention, can be also applied in a optoelectronic measuring apparatus based upon the Stimulated Brillouin Scattering, or SBS, such as that illustrated in figures 7 and 8, wherein number 100 indicates in its entirety a measuring apparatus of the optoelectronic type, which can be used to monitor continuously over time the temperature and the elative elongation of the given body 1000.
  • the measuring apparatus 100 is provided with a probe 125 presenting a given extension L, which, in use, is arranged into contact with the body 1000 so as to perform a function substantially equivalent to that of the probe 25 previously described.
  • the measuring device 100 comprises an emitting group 110 associated with or provided with a given electromagnetic source 115, suitable to produce continuously a sixth given substantially monochromatic electromagnetic radiation EM6, presenting a first carrier frequency G and a given optical power JO constant over time.
  • a given electromagnetic source 115 comprising a laser 115 of the known type, for example an external cavity laser or a distributed feedback laser (DFB) , and suitable to generate an electromagnetic radiation presenting preferably, although without limitation, a given fifth frequency G in the order of 193 THz.
  • a given electromagnetic source 115 comprising a laser 115 of the known type, for example an external cavity laser or a distributed feedback laser (DFB) , and suitable to generate an electromagnetic radiation presenting preferably, although without limitation, a given fifth frequency G in the order of 193 THz.
  • DFB distributed feedback laser
  • the emitting group 110 comprises a first optical element 118, suitable to subdivide the sixth electromagnetic radiation EM6 in a third fraction EM6 ' and in a fourth fraction EM6' ', presenting respectively optical powers JO' and JO' ' .
  • this first optical element 118 can comprise an optocoupler, or optical coupler, suitable to serve as beam splitter.
  • the first optical element 118 is designed so as to subdivide the given optical power JO of the sixth radiation EM6 so that the third fraction EM6' presents a respective optical power JO' significantly higher than the optical power JO' 1 associated to the fourth fraction EM6 1 '.
  • the third fraction EM6 1 will present preferably an optical power JO' comprised between the 80% and the 90% of the optical power JO, whilst to the fourth fraction EM6' 1 a remaining optical power JO 1 ' will be associated, in the order of the 5-20% of the optical power JO of the sixth radiation EM6.
  • the third fraction EM6 1 is sent to an amplifying device 119 suitable, in use, to amplify the optical power JO' of the third fraction EM6' until a given peak value, preferably in the order of the tens of mW.
  • the amplifying device 119 preferably comprises an optical amplifier 119', for example an EDFA amplifier based upon an erbium doped optical fibre, connected in series with a first band-pass optical filter 119' ' presenting a given pass band, for example of 0.5 nm, to filter noise originated during the amplification process.
  • the laser 115 presents a peak power sufficiently high for all the required applications, the presence of the amplifying device 119 is not strictly necessary and, as illustrated in figure 8, it can be omitted so as to reduce production and maintenance costs of the measuring apparatus 1.
  • the emitting group 110 comprises a first modulation device 111 designed so as to receive the third fraction EM6' and suitable, in use, to modulate the variation over time of the respective optical power of the fraction, so as to generate at least a train P of first electromagnetic pulses P'.
  • These first pulses P' similarly to what described with reference to the measuring apparatus 1, are substantially monochromatic, present the same carrier frequency G as the sixth electromagnetic radiation EM6, present a first given duration D' and can be emitted in sequence by the emitting group 110 substantially continuously.
  • each train P of first electromagnetic pulses P' presents a second given duration Dl, during which up to a fifth given number N 1 of first pulses P' can be emitted.
  • the trains P are generated periodically according to a given period T and, as it will be better described below, they all comprise preferably the same number of first pulses P' .
  • the first modulation device 11 comprises an optoelectronic modulation unit 12 connected to a first control unit 114, which can be used to adjust the trend over time of the optical power Il associated to each train P and therefore suitable to determine the first duration D', the optical power and the fifth given number N 1 that are associated to the first electromagnetic pulses P'.
  • the first control unit 114 can preferably comprise a programmable wave-form generator or, alternatively, it can be interfaced with, or implemented through, a computer 150 suitable, in use, to send reference wave-forms to the modulation unit 112, according to which to modulate over time the optical power Il associated to each train P.
  • the modulation unit 112 comprises preferably, but without limitation, a Mach-Zehnder interferometer 113, coupled in series to a first polarisation controller 113' and connected to the first control unit 114 to modulate over time the optical power JI associated with the trains P according to respective reference wave forms.
  • the set of the given electromagnetic source 115, of the first optical element 118, of the first modulation device 111 and of the optical amplifying device 119, if any, can be interpreted as a third electromagnetic source 115' suitable, in use, to emit a seventh electromagnetic radiation EM7.
  • the technical solution of arranging the amplifying device 119 upstream of the first modulation device 111 is completely arbitrary and it must be considered substantially equivalent to the solution, not shown, of interposing this amplifying device 119 between the output of the first modulation device 111 and the probe 125.
  • the optical power associated to each train P will be modulated according to a code C, preferably of the binary type, so as to associate to each train P a word, written according to this code C, that for the sake of simplicity will be indicated hereinafter with the same letter P.
  • this binary code C to each electromagnetic pulse P' a bit is associated so that each word P is composed exactly by a fifth given number N' of characters.
  • the third source 115' is suitable, in use, to emit in a periodic manner trains P of first electromagnetic pulses P 1 , whose optical power Jl is modulated so as to associate to each train P a respective word P of the given code C containing a fifth given number N' of bits.
  • the emitting group 10 comprises a second modulation device 117, designed so as to receive the fourth fraction EM6' 1 and suitable, in use, to adjust at least one respective frequency of propagation along the probe 125.
  • the second modulation device 117 comprises a, electro-optical (EOM) modulator 117' connected to a wave-form generator 116 suitable, in use, to generate sixth electric sinusoidal signals S ⁇ oscillating with a frequency in the order of GigaHertz.
  • this wave-form generator 116 can preferably, although without limitation, comprise a microwave synthesizer which, for the sake of practicality, will be indicated with the same number.
  • the electro-optical modulator 117' is suitable to generate in the fourth fraction EM6' 1 two optical signals presenting respective seventh subcarrier frequencies G2 spectrally arranged in a symmetric manner relative to the fifth carrier frequency G.
  • each seventh subcarrier frequency G2 of the fourth fraction EM6 ' ' can be varied continuously by means of the electro-optical modulator 117' based upon the sixth sinusoidal signal S6 emitted by the microwave synthesizer 16 whose respective eighth frequency ⁇ G is substantially equivalent to the absolute value of the difference between the fifth carrier frequency G and a seventh subcarrier frequency G2. Therefore, the second modulation device 117 can be interpreted as a first optical frequency changer 117 of the fourth fraction EM6 ' ' .
  • the fourth fraction EM6' 1 exiting from the electro-optical modulator 177' and presenting a fifth carrier frequency G and two seventh subcarrier frequencies G2, is sent to an optical filtering device, indicated schematically in figure 7 as a single optical filter 117' ', which is suitable, in use, to filter the carrier signal and one of the two subcarriers so as to send to the probe 125 a seventh electromagnetic radiation EM7 presenting as carrier frequency a seventh given frequency G2.
  • the set of the electromagnetic source 115, of the first optical element 118 and of the second modulation device 117 can be interpreted as a fourth electromagnetic source 115' ' suitable, in use, to emit continuously over time a seventh electromagnetic radiation EM7 substantially monochromatic and presenting a reduced optical power, typically lower than the optical power JO' 1 , and a respective seventh carrier frequency G2.
  • the measuring apparatus 1 comprises an optical transmission group 20 provided with the probe 25 and designed so as to receive both each train P of the seventh radiation EM7, and transmit it along the probe 25 according to a first direction, and the seventh radiation EM7 and to transmit it along this probe 25 according to a second direction opposite to the first direction.
  • the probe 125 comprises a wave guide 125 suitable to transmit the seventh and the eighth electromagnetic radiation EM7 and EM8 along all the respective linear extension L.
  • the wave guide 125 can comprise, without limitation, a first optical fibre 125.
  • this optical amplification phenomenon will occur at each portion of the optical fibre 125 when it is crossed by a train P of the seventh electromagnetic radiation EM7 and when the eighth frequency ⁇ G is substantially identical to the Brillouin frequency shift ⁇ GB, i.e. the difference between the fifth frequency G and the ninth Brillouin frequency GB associated with this portion of optical fibre 125.
  • each ninth frequency GB generated by the Brillouin scattering of the seventh radiation EM7 by means of a given portion of the optical fibre 125 is specifically associated to this portion as function of the local temperature and strain of the portion, as illustrated in the article "Brillouin Gain spectrum Characterization in Single- Mode Optical Fibres" by Marc Nikles et al.
  • the measuring apparatus 100 is suitable to monitor contemporaneously the local distribution of temperature and strain of a body 1000 through the measurement of the dependence of the optical power J2 associated to the eight radiation EM8, and therefore of the respective optical gain curve, according to the eight frequency ⁇ G of the microwave synthesizer 116.
  • optical amplification due to stimulated Brillouin effect will refer to any phenomenon of optical amplification, which can be interpreted, according to the quantum physics, as a phenomenon of stimulated emission of photons due to an interaction between the photons forming the first electromagnetic pulses P' and the photons forming the eighth electromagnetic radiation EM8 mediated by at least one acoustic phonon which propagate along the wave guide 25.
  • the terms "amplification due through SBS” or "gain curve" of a given electromagnetic radiation will be used to indicate or describe in general phenomena of exchange of optical energy, wherein this given electromagnetic radiation can be indifferently subjected both to an increase and to a decrease in its own respective optical power. Therefore, in view of the above description and with particular reference to the terminology commonly used in quantum optics to describe the stimulated emission phenomena, the seventh electromagnetic radiation EM7 is suitable to act as a pumping radiation or "optical pump”, whilst the eighth radiation EM8 is suitable to act as probe radiation, or simply “probe", of the linear distribution of temperature and strain along the optical fibre 125.
  • the transmission group 20 comprises a depolariser 121 preferably interposed between the optical fibre 125 and the second modulation device 117 to depolarise the eighth probe radiation EM8 in order to maximise the effects of optical amplification due to SBS which occur inside the optical fibre 125.
  • the optical transmission group 120 can furthermore comprise a second optical filter 123 suitable, in use, to act as optoisolator, i.e. suitable to prevent the propagation of the seventh radiation EM7 outside the optical fibre 125 in the direction of the fourth electromagnetic source 115' ' .
  • the optical transmission group 120 comprises a second optical element 122, for example an optical circulator, interposed between the third source 115' and the optical fibre 125 and suitable, in use, to receive the eighth electromagnetic radiation EM8 exiting from the optical fibre 125, and to send it to an optoelectronic detection group 130 of the measuring apparatus 100.
  • the second optical element 122 may be suitable to send to the detection group 130 not only the eighth radiation EM8, but also the Rayleigh radiation composed by each fraction of the seventh radiation EM7 which has been backscattered by at least a portion of the optical fibre 125 in a manner non coherent with the eighth probe radiation EM8.
  • the first detection group 130 comprises a first photoelectric sensor 131 suitable to receive the eighth radiation EM8 to convert it into a respective seventh electric signal S7, for example a signal of electric voltage, substantially proportional to the optical power J2 associated with this probe eight radiation EM8.
  • This first sensor 131 comprises preferably, although without limitation, a photodiode 131, for example of the PIN type, suitable, in use, to measure the trend over time of the optical power J2 and therefore, based upon the OTDA principles, the spatial distribution of this optical power J2 along the extension L of the probe 125. Therefore, as it will be more apparent from the description below, each seventh electric signal S7, normalised as the case may be, can be interpreted as a gain curve of the optical amplification to which the probe radiation EM2 is subjected.
  • the detection group 130 comprises preferably a first analog-to-digital converter 139 suitable, in use, to receive and digitalise each seventh electric signal S7 exiting from the first sensor 131.
  • each seventh signal S7 can be acquired by the computer 150 which, as it will be better explained hereunder, is suitable to process these data in order to reconstruct the dependence of the trend over time of each gain curve of the eighth radiation EM8 as a function of the ninth frequency ⁇ G and to decode information coded in this gain curve, according to the given code C, following the interaction with first pulses P' .
  • the computer 150 is suitable both to control the/to act as, first control unit 114 for controlling the coding of the words P, and to act as decoder 160 of the information coded in each sixth signal S6 according to the given code C.
  • this computer 150 is also connected to the second waveform generator/microwave synthesizer 116 to regulate, in use, the value of the ninth freguency ⁇ G.
  • the first detection group 130 can preferably comprise one or more electronic amplifiers of the known type, for instance of the TIA type, and that therefore in figures 7 and 8 they have been generically illustrated without specific reference numbers; these amplifiers can be or cannot be introduced if the intensity of the seventh signal S7 is not sufficient to obtain a correct analog-to-digital conversion and therefore measurements presenting an adequate signal-to-noise ration.
  • figure 8 illustrates a second preferred embodiment of the measuring apparatus 100 wherein not a single electromagnetic source 115 and a beam splitter are provided, but a fifth and a sixth electromagnetic sources 115' ' ' and 115 IV distinct and suitable to generate respective ninth and tenth electromagnetic radiations EM9 and EMlO substantially identical in frequency and optical power to the first and second fractions EM6' and EM6' ' illustrated above.
  • the second preferred embodiment illustrated in figure 8 can comprise a second optical frequency changer 124 of the known type, arranged at the input, at the output or associated with the first modulation device 111 and suitable, in use, to vary the sixth carrier frequency Gl of the seventh radiation EM7 so that this sixth radiation Gl is slightly different from the fifth reference frequency G.
  • the two subcarrier frequencies of the eighth radiation EM8 generated by the second modulation device 117 will not be spectrally symmetric relative to the sixth frequency Gl of the seventh radiation EM7 and therefore the optical amplification phenomena due to SBS can be occur only between the seventh pumping radiation EM7 and a given fraction of the eighth radiation EM8 associated to one of the two subcarrier frequencies, it is clearly apparent that under these experimental conditions, the elimination of one of the two subcarriers by means of the optical filter device 117' ' is no necessary, as this subcarrier of the eighth probe radiation EM8 does not interact constructively or destructively with the seventh pumping radiation EM7.
  • the present invention also relates to a measuring method which can be performed through the apparatus 1 and is suitable to measure the linear distribution of at least one physical quantity, for example the temperature and/or the strain, along all the linear extension of the probe/first optical fibre 125.
  • this method comprises first of all the phase of generating a third optical pumping electromagnetic fraction EM6' and a fourth electromagnetic probe radiation EM6 1 ' through respectively a third and a fourth electromagnetic sources 115' and 115' ' or, alternatively, a fifth and a sixth electromagnetic sources 115' ' ' and 115 JV .
  • this phase of generating a seventh and a eighth radiation EM7 and EM8 will comprise a phase of generating a sixth given electromagnetic radiation EM6 through a given electromagnetic source 115, followed by a phase of subdividing this sixth radiation EM6 into a third and a fourth fractions EM6' and EM6' 1 .
  • a phase is provided of modulating the optical power JO' associated to the third fraction EM6' through a specific first modulation device 111 so as to generate a seventh electromagnetic pumping radiation EM7 presenting a plurality of trains P of first pulses P'; these trains P are emitted periodically and present a respective second duration Dl, which is substantially equivalent to the product of each first duration D 1 of each first pulse P 1 and the fifth given number N'.
  • the phase of modulating the optical power JO' associated to the third fraction EM6 comprises the phase of associating to each train P a respective word P coded according to a given code C, preferably of the binary type.
  • the present method provides for the execution of a phase of modulating at least a fifth frequency G associated with the fourth fraction EM6 ' ' to generate an eighth electromagnetic probe radiation EM8 presenting a respective optical power J2 substantially constant over time and a respective seventh carrier frequency G2, which is different from the sixth frequency Gl of the seventh pumping radiation EM7 by a value substantially equivalent to a ninth frequency ⁇ G.
  • the measuring method according to the present invention provides for a phase of transmitting each train P associated to the seventh radiation EM7 along a probe 125 of given extension L according to a first propagation direction and of transmitting the eighth radiation EM8 along the same probe 125 according to a second propagation direction opposite to the first propagation direction, so that phenomena can be generated of optical amplification due to SBS of the eighth electromagnetic radiation EM8.
  • This phase of transmitting the seventh and eighth radiations EM7 and EM8 along the probe 125 is followed by a phase of receiving through a detection group 130 an eighth electromagnetic radiation EM8 exiting from the probe 125, which has been potentially subjected to phenomena of optical amplification due to SBS.
  • the measuring method according to the present invention comprises at least a phase of measuring the variation s over time of the dependence of a gain curve associated to each respective eighth probe radiation EM8 as a function of the ninth frequency ⁇ G to obtain from the analysis of these measurements values of the temperature and/or of the strain associated to each portion of the probe 125.
  • this phase of measuring the variation over time of the dependence of a gain curve relative to the fourth frequency ⁇ G comprises first of all a phase of measuring at least a trend over time of the optical power J2 associated to the eighth radiation EM8 exiting from the probe 125 at the emission of a respective train P and for a period of time substantially equivalent to the double of the time necessary to an electromagnetic signal for travelling the given extension L of the probe 125.
  • the effect of optical amplification due to SBS can occur starting from the emission of a given train P for the time interval necessary to this train P for travelling the probe 125 for all the respective given extension L; in addition, the eight amplified radiation EM8 will need at the most the same time interval to propagate from an end of the probe 125 to the detection group 130. Therefore, with first electromagnetic pulses P' presenting a first given duration D' in the order of the ten of nanoseconds and with an optical fibre 125 presenting a given extension L in the order of the tens of kilometres, the third duration D2 of emission of each eighth electromagnetic radiation EM8 will present a value in the order of hundreds of nanoseconds.
  • the time period T which separates the emission of each two consecutive trains P will present a value double twice greater than the time necessary to an electromagnetic signal for travelling the probe 125 so that no fraction of the eighth electromagnetic radiation EM8, during the respective transmission along the probe 125, can be subjected to two distinct phenomena of optical amplification due to SBS.
  • each optical gain curve associated to a given word P of code it will be necessary to make a sixth given number M 1 of measurements of the trend over time of the optical power J2 exiting from the probe 125.
  • M 1 the value of the ninth frequency ⁇ G so that, in the range of the M measurements, this ninth frequency ⁇ G scans in a preferably uniform manner a frequency interval centred around a theoretical value of the Brillouin frequency shift ⁇ GB.
  • the Brillouin frequency shift ⁇ GB is about ⁇ 11 GHz and therefore, in use, the value of the ninth frequency ⁇ G will be preferably varied between 10.8 e 11.2 GHz with steps in the order of some MHz.
  • this ' phase can be performed in succession for a given number K' of times, at the end of which a phase can be performed of mediating statistically a seventh given number K' of measurements of the optical power J2 to obtain an average measurement of the trend over time of the optical power J2 presenting a signal-to-noise ration significantly greater than a single measurement.
  • the seventh given number K 1 can be in the order of the thousand or of the tens of thousand for each given word P and for each given value of the ninth frequency ⁇ G.
  • the phase of measuring the variation over time of the dependence of a gain curve associated with each respective eighth probe radiation EM8 as a function of the ninth frequency ⁇ G comprises a phase of reconstructing numerically, for each given train/word P, the respective optical gain curve associate with the eighth radiation EM8 through a computer 150.
  • this phase of numerically reconstructing an optical gain curve associated with the eighth radiation EM8 comprises the cyclical repetition for a given number of times of a phase of interpolating a sixth given number M 1 of experimental points with a continuous mathematical function, for example a Lorentz function, suitable to describe the trend of the optical gain curve relative to the ninth frequency ⁇ G.
  • each of these experimental points represent a respective value, adequately normalised, of the second optical power J2, which is associated to the respective value of the ninth frequency ⁇ G set by the microwave synthesizer 16 during the respective phase of transmission of the seventh and eighth electromagnetic radiations EM7 and EM8 along the probe 125.
  • each of these continuous mathematical functions used for interpolating the experimental points of the gain curve associated with the eighth radiation- EM8 is associated to a given time instant of the flight time of the eighth probe radiation EM8 and therefore, according to the functioning principle of the OTDA, each of these functions which reconstruct the dependence of the gain curve from the ninth frequency ⁇ G can be associated to a given linear distance measured along the probe 125.
  • each phase of numerically reconstructing, for each given train/word P, the respective optical gain curve associated with the eighth radiation EM8 can be interpreted as a phase of performing a digital sampling of the variation over time of this optical gain curve associated to a respective second radiation to allow subsequent operations of numerical-statistical processing and/or operations of decoding information/OTDA traces coded inside this gain curve.
  • each phase of measuring the variation over time of the dependence of a gain curve associated with each respective eighth probe radiation EM8 relative to the ninth radiation ⁇ G comprises a phase of obtaining from the numerical reconstruction of each of these gain curves associated with the eighth radiation EM8, a fifth and a sixth numerical functions Hl and H2, which represent respectively the trend, as a function of time, of the maximum gain for optical amplification due to SBS and of the respective value of the ninth frequency G for which this optical gain is maximum, i.e. for which the value is maximum of the mathematical function used for the respective interpolation.
  • the measuring method according to the present invention comprises a phase of decoding information contained in an eighth number Q 1 of these fifth and sixth numerical functions Hl and H2 and originally associated to a given eighth number Q' of distinct words P transmitted preferably in succession through respective trains P of first electromagnetic pulses P'.
  • the present phase of decoding information contained in an eighth number Q' of these fifth and sixth numerical functions Hl and H2 can comprise analysis and decoding methods both of the linear and of the non linear type, in this latter case preferably of the logarithmic type, which is necessary to linearise the effect on each second radiation EM2 by the trains P of the seventh radiation EM7 following the effect of optical amplification generated by the SBS.
  • the result of the phase of decoding information contained in a given eighth number Q' of fifth and sixth numerical functions Hl and H2 relating respectively to the optical power JB and to the ninth frequency ⁇ G is that of generating a seventh and an eighth numerical functions Hl' and H2 ' , which represent respectively measurements of the variation over time of the maximum of the gain curve and of the respective value of the ninth frequency ⁇ G; these measurements associated to the seventh and to the eighth numerical functions Hl 1 and H2 ' are not influenced, as the first and second functions Hl and H2, by the presence of the original modulation of optical power of the trains P, and they are furthermore characterised by a signal-to-noise ratio, with equal spatial resolution, significantly better than the signal-to-noise ratio associated to the single measurements relating to the fifth and sixth functions Hl and H2.
  • the measuring method according to the present invention comprises the phase of calculating the linear distribution of the temperature and of the strain associated with the probe 125, and therefore with respective contiguous portions of the "body 1000, applying to the seventh and eighth numerical functions Hl' and H2 ' empirical formulae, for the explanation of which reference shall be made to the already cited scientific documents Docl and Doc2.
  • the characteristics of the measuring method according to the present invention will be apparent from the above description and do not require further explanations; however it would be advisable further to highlight some advantages resulting from the use of the techniques for coding/decoding the optical power associated to the trains P of first electromagnetic pulses P 1 .
  • the use of these analysis technique based upon the decoding of information optically coded in the eighth probe radiation EM8 allows obtaining measurements of the gain curves associated with this second radiation EM2 presenting a high signal-to-noise ratio. Consequently, the computer 150 is suitable to calculate a linear distribution of the temperature and of the strain along the optical fibre 125 presenting greater accuracy and reliability than the measurements which can be performed with apparatuses according to the prior art.
  • a variant based upon the analysis of pairs of differential pulses will comprise both a further phase of generating in succession, for each given word P of the given code C, a pair of trains P of first electromagnetic signals P 1 presenting respective first and fifth durations D' and D 1 ' slightly different from each other, i.e. presenting the same order of magnitude, but not exactly the same value, and a phase of subtracting each from the other the OTDA traces obtained starting from these pairs of trains P of electromagnetic pulses before proceeding with the analysis and decoding the information present in this difference between OTDA traces following the numerical analysis method illustrated above.

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  • General Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Aviation & Aerospace Engineering (AREA)
  • Length Measuring Devices By Optical Means (AREA)

Abstract

L’invention concerne un appareil de mesure optoélectronique (1, 100) comprenant : un moyen émetteur (10, 110) permettant, lors de l’utilisation, de générer et de moduler la puissance optique d’une pluralité de trains (P) comprenant une pluralité de premières impulsions électromagnétiques ayant une première fréquence donnée et une première durée qui peuvent être déterminées essentiellement à volonté; un groupe de transmission optique (20, 120) pour les premières impulsions électromagnétiques, qui comprend un guide d’onde (25, 125); un groupe de détection (30, 30’, 130) permettant, lors de l’utilisation, d’échantillonner la variation dans le temps d’au moins une caractéristique optique d’au moins un rayonnement électromagnétique (EM2, EM8), chaque rayonnement (EM2, EM8) ayant interagi avec au moins un train (P) par le biais d’un phénomène de diffusion de Brillouin donné en au moins une partie duit guide d’onde (25, 125).
PCT/IT2009/000525 2008-11-21 2009-11-20 Appareil de mesure optoélectronique d'une caractéristique physique distribuée Ceased WO2010058438A1 (fr)

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Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2012084040A1 (fr) 2010-12-22 2012-06-28 Omnisens Sa Procédé et appareil de mesure optoélectroniques de brillouin
US20130265569A1 (en) * 2010-12-22 2013-10-10 Omnisens Sa Brillouin Optoelectronic Measurement Method and Apparatus
US9116119B2 (en) 2010-12-22 2015-08-25 Omnisens Sa Brillouin optoelectronic measurement method and apparatus
CN103680076A (zh) * 2012-08-31 2014-03-26 天津市誉航润铭科技发展有限公司 双主机分布式光纤预应力管线运行安全预警系统
CN103680076B (zh) * 2012-08-31 2016-07-06 天津市誉航润铭科技发展有限公司 双主机分布式光纤预应力管线运行安全预警系统
WO2014071997A1 (fr) * 2012-11-12 2014-05-15 Omnisens Sa Procédé de mesure optoélectronique de brillouin
US9500560B2 (en) 2012-11-12 2016-11-22 Omnisens Sa Brillouin optoelectronic measurement method
WO2014146676A1 (fr) * 2013-03-18 2014-09-25 Omnisens Sa Dispositif de détection distribué optique de brillouin et procédé ayant un niveau amélioré de tolérance aux défaillances d'un capteur
US9804001B2 (en) 2013-03-18 2017-10-31 Omnisens S.A. Brillouin optical distributed sensing device and method with improved tolerance to sensor failure
CN106525096A (zh) * 2016-11-28 2017-03-22 林文桥 一种布里渊分布式光纤传感器及减小增益谱线宽方法

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