WO2015067293A1

WO2015067293A1 - Optical distributed sensing device and method for simultaneous measurements of temperature and strain

Info

Publication number: WO2015067293A1
Application number: PCT/EP2013/072986
Authority: WO
Inventors: Etienne Rochat; Chin SANGHOON
Original assignee: Omnisens Sa
Priority date: 2013-11-05
Filing date: 2013-11-05
Publication date: 2015-05-14

Abstract

The present invention concerns an optical distributed sensing device, comprising (i) a distributed sensor with one or a plurality of polarization maintaining sensing optical fiber(s) (10) arranged along a same path and having respectively a first and a second birefringent axis, and (ii) transfer means arranged so as (i) to transfer into said sensing optical fiber(s) (10) at least a linearly polarized optical pulsed wave (17) aligned with a first birefringent axis, and a linearly polarized optical pulsed wave (17) aligned with a second birefringent axis (ii) and to transfer toward detection means (23, 50) measurement optical waves emerging from said sensing optical fiber(s) (10) and resulting respectively from scattering interactions along said first and second birefringent axes. The present invention concerns also a method implemented in the device.

Description

« Optical distributed sensing device and method for simultaneous measurements of temperature and strain »

Field of the invention

The invention relates to an optical distributed sensing device allowing simultaneous measurements of temperature and strain along a single sensing optical fiber. It relates also to a measurement method implemented in the device.

The field of the invention is, but not limited to, distributed optical temperature and strain measurements devices and methods.

Background of the invention

The optical distributed sensing techniques are widely used for collecting temperature and/or strain information along long-distance paths, or for monitoring large structures.

They use sensing elements which comprise optical fibers embedded in the structure or along the paths to monitor, so as to be submitted to the surrounding temperature and/or strain environmental conditions. These environmental conditions modify locally the condition of propagation of the light into the sensing fibers.

Several measurement techniques are known.

The Brillouin Optical Time Domain Analysers (BOTDA) are very efficient temperature and strain sensing devices based on stimulated Brillouin scattering.

Sensing is achieved by the interaction of a continuous wave probe counter-propagating with respect to a pulsed pump wave. In order to bring the probe to the far end of the sensing optical fiber, a second fiber, the lead fiber is used. The two fibers, namely the sensing fiber and the lead fiber, are connected at the far end, thus forming a loop.

In some applications however, such as for instance the monitoring of oil wells (under the generic term of downholes), building a loop is difficult because fiber bending radii are not compatible with the tight space available. In addition, locally high temperature may weaken fibers bent with small radii of curvature. Such environment could be measured using sing le-end technology, such as for instance Brillouin Optical Time Domain Reflecto meters ( BOTDR) . However, a BOTDR has a red uced optical budget, making it incompatible with the connectors and add itional losses encountered in a downhole environment for instance. In add ition, the accuracy and spatial resol ution of such systems are usually not sufficient.

Sing le-end BOTDA techniq ues are also known . A mirror is added at the end of the sensing fiber for reflecting the sig nals, creating thus a forth and back path which allows generating a continuous probe wave counter- propagating with respect to (and interacting with) a pulsed pump wave. The mirror may be for instance a simple pigtailed classical mirror or a Fiber Bragg Grating ( FBG) mirror.

This solution is efficient but suffers from major d rawbacks. As the probe wave is a continuous wave which propagates in both d irections along the sensing fiber, some interferometric noise is generated by the interaction of these forth and back waves. In add ition, the probe beam travelling towards the mirror acts as a pump which creates a Rayleig h scattering signal overlapping the probe returning from the mirror, thus further add ing to the noise. As a result, such a config uration can be five times worse than a loop configuration in terms of noise depending on the fiber length and the optical budget.

The document WO 201 1/022829 is known . It d iscloses a sing le-end BOTDA techniq ue in which the polarization of the pump and the probe waves are managed to limit the interferometric noise. The pump and probe waves are linearly polarized and orthogonal to each other. The probe wave is reflected at the end of the sensing fiber on a Faraday Rotating M irror ( FRM ) so that the incident probe wave and the reflected probe wave are rotated by 90 deg rees. As forward travel ling and backward travel ling probe are now orthogonal, results are improved .

However, as the polarization is not maintained along the sensing fiber, interference noise and Rayleigh scattering are still present and red uce the accuracy of the system . In add ition, the techniq ue req uires a Faraday Rotating M irror which is bul ky and not adapted for hig h temperature operation . In addition, the known single fiber - single end BOTDR or BOTDA techniques do not allow separating the effects of the temperature and the strain along the sensing fiber. For obtaining both measurements, at least two sensing fibers are usually required, one of which being protected from the strain by a hard casing. As a result, the distributed sensor may be too bulky for some applications.

Optical Time Domain Reflectometry (OTDR) techniques are also known. A pulsed wave is injected into the sensing fiber. A scattered wave is collected at the same end of that fiber, which results from Raleigh scattering of the incident wave on local inhomogeneities into the fiber. That scattered wave comprises terms which depend on the temperature and the strain along the sensing fiber, and which may be detected using a coherent detection scheme.

However, again, in the known single fiber - single end OTDR techniques, the effect of temperature and strain on the Raleigh scattering cannot be distinguished and separated.

It is an object of the invention to provide a single-end distributed sensor allowing simultaneous measurements of temperature and strain.

It is a further object of the invention to provide a single-end distributed sensor allowing temperature and strain measurements with a single sensing fiber.

It is a further object of the invention to provide a single-end distributed sensor adapted for measurements in downholes, possibly at high temperatures.

Summary of the invention

Such objects are accomplished by an optical distributed sensing device, characterized in that it comprises:

- a distributed sensor with one or a plurality of birefringent polarization maintaining sensing optical fiber(s) arranged along a same path and having respectively a first and a second birefringent axis;

- transfer means arranged so as (i) to transfer into said sensing optical fiber(s) at least a linearly polarized optical pulsed wave aligned with a first birefringent axis, and a linearly polarized optical pulsed wave aligned with a second birefringent axis (ii) and to transfer toward detection means measurement optical waves emerging from said sensing optical fiber(s) and resulting respectively from scattering interactions along said first and second birefringent axes;

- detection means and processing means able to deduce temperature and strain information from said measurement optical waves using the indices of refraction along respectively the first and the second birefringent axes.

The birefringent axes of a Polarization Maintaining (PM) fiber are the orthogonal polarization axes having respectively the highest and the lowest refractive index. A linearly polarized wave having its polarization aligned with one of these birefringent axes propagates into the fiber along this axis, regardless of the curvatures of the fiber. So, linearly polarized waves aligned with the birefringent axes of the fiber remains orthogonal all along the path and so do not interfere. So, advantageously, the interference noise is limited .

In addition, if linearly polarized waves are injected into the fiber with their polarization axis aligned with one of the birefringent axes of the PM fiber, they emerge from that fiber also in a well-known state of polarization, as they are still aligned with the same birefringent axis.

According to an advantageous aspect of the invention, the use of PM fibers allows doing fairly independent scattering measurements along the first and the second birefringent axis of the fiber. The indices of refraction being different, two different measurements are obtained, which allow separating the effect of the temperature and the strain, as it will be explained later.

It should be noted that the invention is not limited to a specific kind of scattering interactions. It may be easily implemented, for instance:

- in Raleigh scattering detection schemes such as Optical Time Domain Reflectometer schemes (OTDR) or Coherent Optical Time Domain

Reflectometer schemes (COTDR);

- in Brillouin scattering detection schemes such as Brillouin Optical Time Domain Reflectometer schemes (spontaneous Brillouin scattering, BOTDR), Brillouin Optical Time Domain Analyser schemes (stimulated Brillouin scattering, BOTDA), Brillouin Optical Frequency Domain Reflectrometry /

Analyser (BOFDR / BOFDA), Brillouin Optical Correlation Domain Reflectometry / Analyser (BOCDR / BOCDA) ;

- in detection schemes using Bragg gratings.

According to some modes of realization, the transfer means may comprise a polarization switching means for transferring sequentially into a sensing optical fiber a linearly polarized optical pulsed wave aligned with a first birefringent axis, and a linearly polarized optical pulsed wave aligned with a second birefringent axis.

In that case, the invention may be implemented for instance in OTDR or BOTDR detection schemes by doing sequential measurements with the probe optical waves aligned respectively with the first birefringent axis of the sensing fiber, and then with the second birefringent axis of the sensing fiber. The switching of the polarization may be done with a classical polarization rotation device, using for instance a waveplate between polarizers.

The invention may also be implemented in the same way in a BOTDA detection scheme, with the polarization of the probe wave and of the pump wave propagating in opposite direction in the sensing fiber aligned respectively with the first birefringent axis of the sensing fiber, and then with the second birefringent axis of the sensing fiber.

According to some modes of realization, the device of the invention may comprise a distributed sensor with a first and a second polarization maintaining sensing optical fiber arranged along a same path, and transfer means comprising :

- an optical connection means connecting the first sensing optical fiber according to a second end to the second sensing optical fiber so that the first birefringent axis of said first sensing optical fiber is aligned with the second birefringent axis of said second sensing optical fiber;

- optical coupling means connected to a first end of said first sensing optical fiber, and arranged so as (i) to transfer into said first sensing optical fiber at least a linearly polarized optical pulsed wave aligned with a first birefringent axis, and (ii) to transfer toward detection means measurement optical waves emerging from said first sensing optical fiber and resulting respectively from scattering interactions along the first birefringent axis of the first sensing fiber and along the second birefringent axis of the second sensing optical fiber.

According to some modes of realization, the device of the invention may comprise a distributed sensor with a polarization maintaining sensing optical fiber, and transfer means comprising :

- a light reflecting means for reflecting optical waves, connected to a second end of the sensing optical fiber and able to shift the direction of polarization of reflected optical waves relative to incident optical waves from one birefringent axis of said fiber to the other;

- optical coupling means connected to a first end of said sensing optical fiber, and arranged so as (i) to transfer into said sensing optical fiber at least a linearly polarized optical pulsed pump wave aligned with a first birefringent axis, and (ii) to transfer toward detection means measurement optical waves emerging from said sensing optical fiber and resulting, along forth and back paths into said optical fiber, from scattering interactions respectively along the first and the second birefringent axis;

In the distributed sensing devices of the prior art, such as the single fiber BOTDA systems disclosed in WO 2011/022829, in which single-mode, non polarization maintaining fibers are used, the orientation of the polarization of the waves emerging from the fiber is not known . In order to rotate the polarization axis of the reflected waves of 90 degrees relative to the polarization axis of the incident wave, it is necessary to use a light reflecting means which is able to rotate in the same way polarizations aligned with any direction . This requires using devices such as Faraday Rotating Mirrors (FRM) which are based on a magneto-optical effect. But such devices are not well adapted for being used in a place such as deep wells, because, notably, they are sensitive to temperature and cannot withstand high temperatures.

With PM fibers, as the state of polarization of the waves emerging from the fiber is well-known, it is sufficient to use a light reflecting means which is just able to rotate the polarization from one birefringent axis of the fiber to the orthogonal axis. It does not matter if the device used is not able to rotate properly the others polarizations. This allows using light reflecting means which are much more robust to the environment and much more compact than Faraday Rotating Mirrors.

In addition, thanks to the use of the light reflecting means which switch the polarization of the reflected wave from one birefringent axis of the fiber to the other one, the two measurements may be done by accessing only one end of the sensing fiber. This is possible because the forth path of the sensing fiber (towards the light reflecting means) and the back path (towards the entrance side of the fiber) may be used in a fairly independent way. According to some modes of realization, the light reflecting means may comprise a birefringent element and a reflecting surface.

The birefringent element may comprise a quarter wave plate made of a birefringent crystal .

The quarter wave plate may be positioned relative to the PM sensing fiber so that its birefringent axes are rotated of 45 degrees relative to the birefringent axes of the PM sensing fiber. It may be for instance glued or attached in any way to the PM sensing fiber.

The birefringent element may comprise a section of polarization maintaining fiber connected to the sensing optical fiber.

The section of polarization maintaining fiber may be connected to the sensing optical fiber so that the orientation of its birefringent axes is shifted of about ±45 degrees or an odd multiple of ±45 degrees relative to the orientation of the birefringent axes of the sensing optical fiber.

The section of polarization maintaining fiber may have a length which corresponds to one or an odd multiple of a quarter of the beat length of said polarization maintaining fiber at the wavelengths of interest.

The beat length of such a fiber for a particular wavelength is the distance over which the wave in one polarization mode corresponding to one birefringent axis experiences an additional delay of one wavelength compared to the wave in the other polarization mode corresponding to the other birefringent axis. That beat length is typically a few millimeters.

The section of polarization maintaining fiber may be connected to the sensing optical fiber by means of any suitable technique, such as for instance splicing or optical fibers connectors.

The section of PM fiber behaves then as a quarter wave plate with the advantage of not being larger than the sensing fiber.

The birefringent element may comprise a reflective coating .

The reflective coating may be applied on the side of the birefringent element opposite to its side facing the sensing fiber. It may comprise for instance a metal or a dielectric coating deposited on a birefringent crystal or on the extremity of a section of PM fiber.

According to some modes of realization, the light reflecting means may comprise a polarization maintaining coupler having an output connected to an input with a loop of polarization maintaining fiber connected on one side with a 90 degrees angular shift so as to swap the polarization axes.

So, a linearly polarized wave incident on a birefringent axis of the PM sensing fiber is re-injected on that fiber with its polarization aligned with the other birefringent axis.

According to some modes of realization, the light reflecting means may comprise a Faraday Rotating Mirror (FRM) based on a magneto-optical effect.

According to some modes of realization, the device of the invention may implement a stimulated Brillouin scattering detection scheme.

The optical coupling means may be arranged so as (i) to transfer into the sensing optical fiber a linearly polarized optical pulsed pump wave aligned with the first birefringent axis, and a linearly polarized optical probe wave aligned with the second birefringent axis, and (ii) to transfer toward the detection means measurement optical waves resulting from stimulated Brillouin scattering interactions between said optical pulsed pump wave and said optical probe wave.

The optical coupling means may comprise :

- a polarization maintaining sensing coupler for combining at least the optical pulsed pump wave and the optical probe wave;

- a circulator between the polarization maintaining sensing coupler and the detection means;

- a polarizer between the circulator and the detection means and arranged so as to block the pulsed pump wave reflected by the light reflecting means.

According to some modes of realization, the device of the invention may further comprise a laser source for generating at least the optical pulsed pump wave and the optical probe wave.

According to some other modes of realization, the device of the invention may further comprise a first laser source for generating at least the optical pulsed pump wave and a second laser source for generating at least the optical probe wave.

The device of the invention may further comprise tuning means for varying the optical frequency of at least one spectral component of at least one of the following optical signals : the optical pulsed pump wave, the optical probe wave. The tuning means may comprise an electro-optic modulator for varying the optical frequency of at least one spectral component of the first optical probe wave.

According to some modes of realization, the device of the invention may implement a coherent optical time-domain reflectometer scheme.

The optical coupling means may be arranged so as (i) to transfer into the sensing optical fiber a linearly polarized optical pulsed wave aligned with the first birefringent axis, and (ii) to transfer toward the detection means measurement optical waves resulting from Raleigh scattering of said optical pulsed wave into said sensing fiber.

The optical coupling means may comprise :

- a circulator for directing the measurement optical waves towards the detection means;

- a detection coupler for combining on the detection means (23) the measurement optical waves with a reference wave issued from the same laser source.

The device of the invention may further comprise a continuous wave laser source, tuning means for varying the optical frequency of said laser source, and a pulse generator for generating the pulsed optical wave.

According to another aspect, it is proposed an optical distributed sensing method, using a distributed sensor with one or a plurality of birefringent polarization maintaining sensing optical fiber(s) arranged along a same path and having respectively a first and a second birefringent axis, and comprising steps of:

- transferring into said sensing optical fiber(s) at least a linearly polarized optical pulsed wave aligned with a first birefringent axis, and a linearly polarized optical pulsed wave aligned with a second birefringent axis (ii) and transferring toward detection means measurement optical waves emerging from said sensing optical fiber(s) and resulting respectively from scattering interactions along said first and second birefringent axes;

- using detection means and processing means, deducing temperature and strain information from said measurement optical waves using the indices of refraction along respectively the first and the second birefringent axes. Accord ing to some modes of implementation, the method of the invention may use a d istributed sensor with a polarization maintaining sensing optical fiber, and further comprise steps of:

- using a l ight reflecting means connected to a second end of the sensing optical fiber, reflecting the optical waves and shifting the d irection of polarization of the reflected optical waves relative to the incident optical waves from one birefringent axis to the other;

- using optical coupl ing means connected to a first end of said sensing optical fiber, (i) transferring into said sensing optical fiber at least a linearly polarized optical pulsed wave alig ned with a first birefringent axis, and (ii) transferring toward detection means measurement optical waves emerging from said sensing optical fiber and resulting, along forth and back paths into said optical fiber, from scattering interactions respectively along the first and the second birefringent axis;

Accord ing to some modes of implementation, the method of the invention may further comprise steps of:

- transferring into the sensing optical fiber a linearly polarized optical pulsed pump wave aligned with the first birefringent axis, and a l inearly polarized optical probe wave aligned with the second birefringent axis;

- relatively positioning in freq uency the first optical probe wave and the optical pulsed pump wave so as to generate into the sensing optical fiber a first stimulated Brillouin scattering interaction between the optical probe wave reflected in the light reflecting means and the incident optical pulsed pump wave, and a second stimulated Bril louin scattering interaction between the incident optical probe wave and the reflected optical pulsed pump wave;

- ded ucing simultaneously d istributed temperature and d istributed strain information along the sensing optical fiber using said first stimulated Brillouin scattering interaction and said second stimulated Bril louin scattering interaction .

Accord ing to some other modes of implementation, the method of the invention may further comprise steps of:

- for several optical waves freq uency, measuring a first Raleig h scattering information resulting from the scattering of the optical pulsed wave into the sensing fiber along a forth path, and measuring a second Raleigh scattering information resulting from the scattering of the optical pulsed wave into said sensing fiber along a back path after reflection on the light reflecting means;

- deducing distributed temperature and distributed strain information along the sensing optical fiber using said first Raleigh scattering information and said second Raleigh scattering information.

Description of the drawings

The methods according to embodiments of the present invention may be better understood with reference to the drawings, which are given for illustrative purposes only and are not meant to be limiting. Other aspects, goals and advantages of the invention shall be apparent from the descriptions given hereunder.

- Fig. 1 shows a first mode of realization of the invention implementing a stimulated Brillouin scattering detection scheme (BOTDA),

- Fig . 2 shows a light reflecting means of the invention using a section of polarization maintaining fiber, with on Fig 2(a) a side view and on Fig. 2(b) a cut view showing the birefringent axes.

- Fig. 3 shows a second mode of realization of the invention implementing a coherent time domain reflectometer scheme based on Raleigh scattering (OTDR).

- Fig . 4 shows a mode of realization of single-end distributed sensor using two birefringent fibers, with on Fig 2(a) a side view and on Fig. 2(b) a cut view showing the birefringent axes at the connection point.

Detailed description of the invention

With reference to figure 1, we will now describe a mode of realization of a distributed sensing device of the invention implementing a stimulated Brillouin scattering detection scheme (BOTDA).

For the sake of clarity, only the components which are relevant for the description of the invention are shown on Fig. 1. Of course, the device of the invention may include any other necessary components, such as amplifiers, isolators, polarizers...

In the mode of realization presented on Fig . 1, the sensing device of the invention comprises a light source 11 which is used for generating all necessary optical signals. This light source 11 comprises a distributed feedback laser diode (DFB-LD) with a wavelength around 1.5 prn, which generates a continuous wave. A source coupler 12 d irects a part of the l ig ht issued from the source 1 1 towards a pulse generator 13 for generating an optical pulsed pump wave 17.

The pulse generator 13 comprises a semicond uctor optical ampl ifier (SOA) d riven by an electrical pulsed signal . It is used as an optical gating device for generating the optical pulsed pump wave 17.

The source coupler 12 d irects also a part of the light issued from the source 11 towards a probe mod ulator 14 for generating a probe wave 18.

The probe modulator 14 comprises an electro-optic mod ulator 14 configu red so as to mod ulate the intensity of the incoming sig nal accord ing to a Dual Side Band with Suppressed Carrier ( DBS-SC) mod ulation scheme. So, the generated optical probe sig nal comprises two spectral components located symmetrically relative to the freq uency of the laser source 11. The freq uency of these spectral components may be varied by varying the control sig nal applied to the electro-optic mod ulator 14.

The electro-optic mod ulator 14 is preferably a l ithium niobate electro- optic modulator based on a Mach-Zehnder architecture. In order to generate the Dual Side Band with Suppressed Carrier ( DSB-SC) modulation, a control sig nal is applied , which comprises :

- a bias voltage establishing a destructive interferences cond ition between the optical waves in both arms of the interferometer (extinction cond ition), and ,

- a mod ulation frequency correspond ing to the desired freq uency sh ift of the spectral components of the optical probe sig nal relative to the optical frequency of the incoming optical sig nal .

The device of the invention further comprises a polarization-maintaining sensing coupler 19 which al lows combining the pulsed pu mp wave 17 and the probe wave 18, and transferring them into the sensing optical fiber 10.

The device of the invention further comprises an optical circulator 21. Such optical circulator 21 is a well-known optical component which allows :

- d irecting an optical sig nal incident on a first branch (label " 1") of the circulator to a second branch (label "2"), and

- d irecting an optical signal incident on the second branch of the circulator (label "2") to a third branch (label "3") .

The optical circulator 21 is arranged so as to : - d irect the probe wave 18 which is incident on its first branch towards the sensing coupler 19, and

- d irect the sig nals emerg ing from the sensing coupler 19 which are incident on its second branch towards a photodetector 23, for detection and further processing .

As previously explained, the optical set-up is arranged so as to be able to manage the polarization of the optical waves. So it uses mostly polarization maintaining optical fibers and components, at least for the parts where the polarization needs to be managed .

The pulsed pump wave 17 and the probe wave 18 which are incident on the sensing coupler 19 are linearly polarized waves, each aligned with a d ifferent birefringent axis of the polarization maintaining fibers of the sensing coupler 19. So both waves have l inear polarization orthogonal to each other.

The polarization of the pulsed pump wave 17 and of the probe wave 18 are respectively managed by the pump polarization control ler 15 and the probe polarization controller 16.

Depending on the state of polarization of the respective incident waves, the pump polarization control ler 15 and the probe polarization control ler 16 may comprise any necessary well-known components, such as for instance :

- polarizers for filtering out wave components with unwanted polarization modes,

- polarization control lers or waveplates such as q uarter wave plates or half wave plates for rotating linear polarizations or transforming el liptical polarizations into linear polarizations.

The sensing fiber 10 is a polarization maintaining fiber, or more generally speaking a birefringent fiber with a refractive index asymmetry which allows maintaining the polarization of linearly polarized waves injected along its principal birefringent axes.

The pulsed pump wave 17 and the probe wave 18 with l inear orthogonal polarization are transferred into that sensing fiber 10 throug h the sensing coupler 19, with their respective polarization aligned with the birefringent axes of the sensing fiber 10.

The pulsed pump wave 17 and the probe wave 18 are reflected at the end of the sensing fiber 10 by a lig ht reflecting means 20, in such a way that an incident wave linearly polarized along one birefringent axis of the sensing fiber 10 is reflected with a linear polarization aligned along the other birefringent axis of that sensing fiber 10.

The detailed description of that light reflecting means 20 will be done later.

As a result, the reflected probe wave 18 is linearly polarized along the same birefringent axis as the incident pulsed pump wave 17 and both waves propagates in opposite directions. Provided that their respective frequencies are correctly set as explained later, the conditions are met to generate a stimulated Brillouin scattering signal resulting from the interactions in the sensing fiber 10 of the reflected optical probe wave 18 with the incident optical pulsed pump wave 17.

The stimulated Brillouin scattering signal is directed through the sensing coupler 19 and the circulator 21 towards the detector 23 for further processing .

A polarizer 22 is inserted between the circulator 21 and the detector 23 for rejecting the pulsed pump wave 17 reflected by the light reflecting means 20, which emerge from the circulator with a polarization perpendicular to the direction of polarization of the stimulated Brillouin scattering signal .

The polarizer 22 is not mandatory if an optical bandpass filter is inserted before the detector 23 for rejecting the unwanted optical frequencies, as it will be explained later.

With reference to Fig . 2(a) and Fig . 2(b), the light reflecting means 20 comprises a section 30 of Polarization Maintaining (PM) fiber.

The length of that section 30 of PM fiber is adjusted so as to introduce a phase shift of about 90 degrees (at the wavelengths of interest) between the polarization modes of an incident wave propagating respectively on the two birefringent axes. This can be achieved by adjusting that length so that it corresponds to about one or an odd multiple of a quarter of the beat length of the PM fiber at the wavelengths of interest.

The section 30 of PM fiber is attached to the sensing fiber 10 with a splicing 31. The cores of the two fibers are locally fused .

As shown on Fig . 2(b), the splicing 31 is done so that the sensing fiber 10 and the section 30 of PM fiber are angularly positioned relative to each other so that the birefringent axes 33 of the sensing fiber are shifted of about 45 degrees (in absolute value) relative to the birefringent axes 31 of the section 30 of PM fiber.

So, the section 30 of PM fiber behaves as a quarter wave plate for the waves emerging from the sensing fiber 10.

The terminal end 32 of the section 30 of PM fiber is coated with a high- reflectivity dielectric coating or a metal coating, so as to reflect efficiently the incident waves. The coating may be done by a vacuum deposition process.

So, the light reflecting means 20 behaves like a quarter wave plate positioned at 45 degrees from the polarization axis of the incident waves and a mirror. It has the advantages of being as compact as a pigtail mirror, and robust to severe and high-temperature environments.

The device of the invention allows performing distributed strain and temperature measurements along the sensing fiber 10 using a classical stimulated Brillouin scattering detection scheme.

The optical pulsed pump wave 17 comprises an optical frequency v_PU which corresponds to the optical frequency of the laser source 11.

The optical probe wave 18 comprises two spectral components of optical frequencies v_PR+ and v_PR-. These spectral components are located symmetrically relative to the optical frequency v_PU of the pulsed signal.

The propagation in the forward direction of the optical pulsed wave in the sensing fiber 10 generates Brillouin scattering . The spectrum of that Brillouin scattering comprises two spectral components, including a Stokes component around a center frequency v_sBs lower than the pulsed signal optical frequency v_PU and an anti-Stokes component around a center frequency v_sBas higher than the pulsed signal optical frequency v_PU. As the spontaneous Brillouin scattering depends on the local conditions along the sensing fiber 10, the Brillouin spectrum may also vary along the fiber depending on the local conditions of temperature and strain.

As previously explained, the backward-propagating optical probe wave 18 (after reflection by the light reflecting means 20) propagates in the sensing fiber 10 with a state of polarization parallel to that of the forward- propagating optical pulsed wave 17. So both waves may interact.

For performing measurements, the frequency of the optical probe wave 18 is varied using the probe modulator 14 so as to scan the frequency ranges where Brillouin scattering may appear. When the optical frequency of the backward-propagating optical probe wave 18 is scanned over the spectral range of the spontaneous Brillouin scattering generated by the optical pulsed wave in the sensing fiber 10, an energy transfer occurs between both signals, which modifies the amplitude of the backward-propagating optical probe wave. The optical frequency of the probe wave at which the maximum modification of the probe wave amplitude occurs is defined as the Brillouin frequency. The energy transfer induces a gain in the Stokes region of the Brillouin spectrum and a loss in the anti- Stokes region.

The Stokes region of the Brillouin spectrum is then filtered out by an optical bandpass filter, not shown, which is inserted between the circulator 21 and the photodetector 23. So, only the anti-Stokes region is transmitted towards the photodetector 23, which measures its intensity.

That optical bandpass filter comprises an optical circulator with a Fiber Bragg Grating (FBG) on one branch. The optical waves which are incident on a first branch of the circulator are transmitted to a second branch with the FBG. Only the optical waves whose wavelength matches the Bragg reflection condition of the FBG are reflected towards the circulator, and transmitted via the third branch towards the photodetector 23.

So, for a given probe frequency v_PR+, we obtain at the output of the photodetector 23 an electrical signal whose time profile is representative of the Brillouin scattering along the sensing fiber 10 at that probe frequency Vp_R+ . Knowing the speed of light in the fiber, the time profile may be converted in distance profile. The resolution in distance or time of the measurements depends on the pulse duration of the optical pulsed signal .

Then, by scanning the probe frequency v_PR+ over the frequency ranges where Brillouin spectrum may appear, the Brillouin scattering spectrum may be sampled in frequency for any location along the sensing fiber 10.

In the same way, the forward propagating optical probe wave 18 propagates in the sensing fiber 10 with a state of polarization parallel to that of the backward-propagating optical pulsed wave 17 (after reflection by the light reflecting means 20). So both waves may also interact and generate a secondary stimulated Brillouin scattering signal. That secondary stimulated Brillouin scattering signal propagates towards the light reflecting means 20 in which it is reflected back with a l inear polarization aligned along the other birefringent axis of the sensing fiber 10.

As a result, both stimulated Brillouin scattering sig nals propagate towards the detector 23 with the same state of polarization relative to the birefringent axis of the sensing fiber 10, so both may be detected .

Actually, the two stimu lated Brillouin scattering sig nals are separated in time on the photodetector 23 :

- the stimu lated Brillouin scattering sig nal resulting from an interaction of the forward-propagating optical pulsed wave 17 with the backward- propagating optical probe wave 18 travels only along a single forth and back path along the sensing fiber 10, up to the interaction zone;

- the secondary stimulated Bril louin scattering sig nal resulting from an interaction of the backward-propagating optical pulsed wave 17 with the forward-propagating optical probe wave 18 travels forth along the whole sensing fiber 10 up to the light reflecting means 20, then back to the interaction zone, forth to the light reflecting means 20 and back along the whole sensing fiber 10.

So, these two stimulated Brillouin scattering sig nals may be detected separately, of at least d istingu ished from each other.

Advantageously, these two stimulated Brillouin scattering sig nals have a d ifferent Brillouin freq uency, because they result respectively from Brillouin interactions along the two d ifferent birefringent axes of the sensing fiber 10, which have d ifferent indices of refraction .

As each of these Brillouin frequencies depends on the temperature and the strain, by solving an eq uation system with two eq uations and two unknowns, the temperature and the strain may be calculated separately along the fiber.

More precisely, the Stokes Brillouin freq uencies along the x and y birefringent axes of the sensing fiber 10 may be expressed as, respectively :

VsBSx = 2 Πχ V_ax Vpu / c

VsBSy = 2 n_y V_ay V_PU / C

where v_PU is the pulsed pump optical freq uency, c the velocity of l ight in vacuum, n_x and n_y the respective ind ices of refraction along the birefringent axes, and V_ax (or V_ay) the velocity of the acoustic waves in the fiber for each birefringence axis, respectively. For instance, the variation of the Brillouin frequency along the x axis with the temperature T may be expressed as:

dv_SBsx/dT = 2 vpu/c V_ax dn_x/dT + 2 v_PU/c n_x dV_ax/dT

or, keeping the most sensitive term :

dv_SBsx/dT = 2 vpu/c n_x dV_ax/dT

The same applies for the strain ε.

So, we obtain an equation system which may be solved to obtain the temperature and the strain :

SBS_X ⁼ CT_X T + C_EX ε

VsBSy = C_Ty T + C_Ey £

It should be noted that this simultaneous measurement of the strain and the temperature is made possible by the use of the birefringent sensing fiber 10. If the fiber was not birefringent, we would just have two times the same Brillouin frequency measurement, on which the effects of the temperature and the strain may not be separated.

The mode of realization of Fig . 1 allows thus measuring temperature and longitudinal strain along the sensing fiber 10.

With reference to Fig. 3, we will now describe a second mode of realization of the invention which implements a coherent optical time domain reflectometer scheme (OTDR). This mode of realization allows measuring the strain and the temperature along the sensing fiber 10 using Raleigh scattering .

For the sake of clarity, only the components which are relevant for the description of the invention are shown on Fig. 3. Of course, the device of the invention may include any other necessary components, such as amplifiers, isolators, polarizers...

The optical set-up is arranged so as to be able to manage the polarization of the optical waves. So it uses mostly polarization maintaining optical fibers and components, at least for the parts where the polarization needs to be managed.

In the mode of realization presented on Fig. 3, the sensing device of the invention comprises a light source 41 which is used for generating all necessary optical signals. This light source 41 comprises a distributed feedback laser diode (DFB-LD) with a wavelength around 1.5 prn, which generates a continuous wave. The light issued from the source 41 is d irected towards a source modulator 42 for generating a monochromatic light wave whose freq uency may be varied .

The source mod ulator 42 comprises an electro-optic mod ulator 42 configu red so as to mod ulate the intensity of the incoming signal accord ing to a Dual Side Band with Suppressed Carrier ( DBS-SC) mod ulation scheme. So, the generated optical probe sig nal comprises two spectral components located symmetrically relative to the freq uency of the laser source 41. The freq uency of these spectral components may be varied by varying the control sig nal applied to the electro-optic mod ulator 42.

The electro-optic mod ulator 42 is preferably a l ithium niobate electro- optic modulator based on a Mach-Zehnder architecture. In order to generate the Dual Side Band with Suppressed Carrier ( DSB-SC) modulation, a control sig nal is applied , which comprises :

- a mod ulation frequency correspond ing to the desired freq uency sh ift of the spectral components of the optical probe sig nal relative to the optical frequency of the incoming optical signal .

As only one optical freq uency or only one sideband is necessary for the measurements, the other generated sideband is filtered out using an optical filter (for instance based on a Bragg grating or dielectric layers), so as to generate a sing le-sideband modulation scheme (SSB) .

The polarization of the source wave is managed by a source polarization controller 43.

Depending on the state of polarization of the incident wave, the source polarization control ler 43 may comprise any necessary well-known components, such as for instance :

- polarization control lers or waveplates such as q uarter wave plates or half wave plates for rotating linear polarizations or transforming el liptical polarizations into linear polarizations. Of course, the source polarization control ler 43 may be omitted if the light issued from the source 41 and the sou rce mod u lator 42 is al ready in the req uired state of polarization .

The device further comprises a polarization-maintaining source coupler 44 which d irects a part of the light issued from the source modulator 42 towards the detection for generating a continuous reference wave.

The source coupler 44 d irects also another part of the light issued from the source modulator 42 towards a pulse generator 45 for generating an optical pulsed wave 17.

The pulse generator 45 comprises a semicond uctor optical ampl ifier

(SOA) d riven by an electrical pulsed signal . It is used as an optical gating device for generating the optical pulsed wave 17.

The device of the invention further comprises an optical circulator 46 which d irects the pulsed wave 17 issued from the pu lse generator 45 towards the sensing fiber 10, and which d irects the waves emerg ing from the sensing fiber 10 towards the detection .

The sensing fiber 10 is a polarization maintaining fiber, or more general ly speaking a birefringent fiber with a refractive index asymmetry which allows maintaining the polarization of linearly polarized waves injected along its principal birefringent axes.

It is terminated by a light reflecting means 20, whose detailed description has been done in relation with Fig . 2.

The pulsed wave 17, which is linearly polarized , is transferred into that sensing fiber 10 by the optical circu lator 46 with its polarization alig ned with one birefringent axis of the fiber 10.

The forward-propagating pulsed wave 17 is then reflected at the end of the sensing fiber 10 by the l ig ht reflecting means 20, in such a way to generate a backward-propagating pulsed wave 17 with a l inear polarization al ig ned along the other birefringent axis of that sensing fiber 10.

While propagating in the sensing fiber 10, the pulsed wave 17 is partial ly scattered by inhomogeneities or other scatterers, and a reverse propagating Raleig h scattering wave is generated .

The Raleig h scattering wave, or the measurement optical wave, emerging from the sensing fiber 10 comprises two contributions which appear seq uential ly on the measurement wave : - the Raleigh scattering of the forward-propagating pulsed pump wave 17, which is generated along a first birefringent axis of the fiber 10;

- the Raleigh scattering of the backward propagating pulsed pump wave after reflection on the light reflecting means 20, which is generated along the other birefringent axis of the fiber 10. In that case, the Raleigh scattering wave propagates towards the light reflecting means 20 in which it is reflected (with the polarization shifted) towards the entrance of the fiber 10.

So, the measurement optical wave emerging from the sensing fiber 10 comprises sequentially the Raleigh scattering generated on both birefringent axes of that sensing fiber 10. Thanks to the light reflecting means 20, that measurement optical wave is mostly aligned with the same birefringent axis as the forward-propagating pulsed wave 17.

The measurement optical wave is directed by the optical circulator 46 towards the detection .

A polarizer 48 is used to reject the unwanted waves, such as the backward propagating pulsed wave 17. So, only the measurement optical wave is incident on the detection .

The measurement optical wave is then fed in a polarization-maintaining detection coupler 49 in which it is mixed with the CW reference wave issued from the source coupler 44. The mixed waves emerging from the detection coupler 49 are fed to a balanced detector 50 for detecting the intensity of the measurement optical wave, while minimizing the intensity noise of the laser source 41 and the source modulator 42.

The intensity of the measurement optical wave comprises essentially two terms :

- a first term corresponding to a sum of time-delayed intensities of the optical waves generated by the scatterers all along the fiber (along the forth and back paths as previously explained) and propagating back to the entrance side of the sensing fiber 10 without being further scattered . This term, which corresponds to an echo term does not bring much information ;

- a second interference term corresponding to the interferences of optical waves which have been scattered several times on several consecutive scatterers along the fiber 10.

The interference term depends on the phase of the interferences, or in other words on the optical path along the sensing fiber 10 between the scatterers. So, it depends on the respective ind ices of refraction of the fiber 10 along the two birefringent axis, while the optical pulsed wave 17 travels along the forth and the back path, respectively.

As the ind ices of refraction of the fiber are sensitive to the temperature and the strain, the interference term depends also of these parameters.

For measuring the variations of index of refraction along the sensing fiber 10 between two acq uisition times, the freq uency of the pulsed wave is varied, and several measurements are made for several frequencies for each acq uisition time.

As the scatterers are due to physical inhomogeneities in the sensing fiber

10, their d istribution pattern along the fiber is not supposed to change much between consecutive acquisition times, at least for small variations of environmental conditions.

In order to determine the local variations of ind ices of refraction along the sensing fiber 10 between two acq uisition times, the interference terms acqu ired at d ifferent frequencies d uring these two acq uisition times respectively are correlated, and the freq uency d ifferences which allow the best local matching or correlation of these interference terms are determined .

The effect of a freq uency variation on an interference term being fairly eq uivalent to the effect of a variation of index of refraction, the corresponding local variations of index of refraction are determined .

As the invention allows doing two independent measurements of the two indices of refraction along the two birefringent axes of the fiber 10, the effects of the temperature and the strain may be separated by solving an eq uation system with two eq uations and two unknown .

With reference to Fig . 4, the invention may be implemented using a d istributed sensor with two birefringent fibers.

In that case a first sensing fiber 10 is connected at a second end with a second sensing fiber 60. The connection may be done with a spl icing 61. The fibers (which are preferably of the same type) are spl iced so that the slow axis 62 of the first fiber 10 is facing the fast axis of the second fiber 60, and the fast axis of the first fiber 10 is facing the slow axis 63 of the second fiber 60.

The second fiber 60 is terminated by a mirror 64 which reflects back the waves with the same state of polarization . The first fiber 10 and the second fiber 60 are arranged in the d istributed sensor so that they are exposed to the same environment variations.

A probe wave propagating in the forward d irection along a first birefringence axis in the first sensing fiber 10 propagates further in the second sensing fiber 60 in the backward d irection along the second birefringent axis.

So, the d istributed sensor of Fig . 4 may be used in the modes of real ization of Fig . 1 and Fig . 3 in the same way as the d istributed sensor shown in Fig . 2. From the point of view of the measurement system it is functionally eq uivalent, except that the forward and backward paths are in d ifferent fibers.

Accord ing to some modes of real ization, the first mode of realization of the invention correspond ing to Fig . 1 may be implemented using :

- several laser sources,

- one laser source for generating each pump or probe wave,

- tunable laser sources for shifting the frequency of the optical waves without using mod ulators.

Accord ing to some modes of real ization, the second mode of realization of the invention corresponding to Fig . 3 may be implemented using a tunable laser source 11. The source modulator 42 may then be omitted .

While this invention has been described in conjunction with a number of embodiments, it is evident that many alternatives, modifications and variations wou ld be or are apparent to those of ord inary skil l in the appl icable arts. Accord ing ly, it is intended to embrace al l such alternatives, modifications, eq uivalents and variations that are within the spirit and scope of this invention .

Claims

CLAI MS

1. Optical d istributed sensing device, characterized in that it comprises : - a d istributed sensor with one or a plurality of birefringent polarization maintaining sensing optical fiber(s) ( 10) arranged along a same path and having respectively a first and a second birefringent axis;

- transfer means arranged so as (i) to transfer into said sensing optical fiber(s) ( 10) at least a l inearly polarized optical pulsed wave ( 17) al igned with a first birefringent axis, and a linearly polarized optical pulsed wave ( 17) aligned with a second birefringent axis (ii) and to transfer toward detection means (23, 50) measurement optical waves emerg ing from said sensing optical fiber(s) ( 10) and resulting respectively from scattering interactions along said first and second birefringent axes;

- detection means (23, 50) and processing means able to ded uce temperature and strain information from said measurement optical waves using the ind ices of refraction along respectively the first and the second birefringent axes.

2. The device of claim 1 , wherein the transfer means comprises a polarization switching means for transferring seq uentially into a sensing optical fiber ( 10) a linearly polarized optical pulsed wave ( 17) aligned with a first birefringent axis, and a linearly polarized optical pulsed wave ( 17) aligned with a second birefringent axis.

3. The device of claim 1 , which comprises a d istributed sensor with a first and a second polarization maintaining sensing optical fiber ( 10) arranged along a same path, and transfer means comprising :

- an optical connection means connecting the first sensing optical fiber ( 10) accord ing to a second end to the second sensing optical fiber so that the first birefringent axis of said first sensing optical fiber ( 10) is al ig ned with the second birefringent axis of said second sensing optical fiber;

- optical coupling means ( 19, 21 , 22, 46, 48, 49) connected to a first end of said first sensing optical fiber ( 10), and arranged so as (i) to transfer into said first sensing optical fiber ( 10) at least a linearly polarized optical pulsed wave ( 17) aligned with a first birefringent axis, and (ii) to transfer toward detection means (23, 50) measurement optical waves emerging from said first sensing optical fiber ( 10) and resulting respectively from scattering interactions along the first birefringent axis of the first sensing fiber ( 10) and along the second birefringent axis of the second sensing optical fiber.

4. The device of claim 1 , which comprises a d istributed sensor with a polarization maintaining sensing optical fiber ( 10), and transfer means comprising :

- a l ig ht reflecting means (20) for reflecting optical waves, connected to a second end of the sensing optical fiber ( 10) and able to sh ift the d irection of polarization of reflected optical waves relative to incident optical waves from one birefringent axis of said fiber ( 10) to the other;

- optical coupling means ( 19, 21 , 22, 46, 48, 49) connected to a first end of said sensing optical fiber ( 10), and arranged so as (i) to transfer into said sensing optical fiber ( 10) at least a l inearly polarized optical pulsed wave ( 17) aligned with a first birefringent axis, and (ii) to transfer toward detection means (23, 50) measurement optical waves emerg ing from said sensing optical fiber ( 10) and resulting , along forth and back paths into said optical fiber ( 10), from scattering interactions respectively along the first and the second birefringent axis.

5. The device of claim 4, wherein the light reflecting means (20) comprises a birefringent element (30) and a reflecting surface (32) .

6. The device of claim 5, wherein the birefringent element comprises a q uarter wave plate made of a birefringent crystal .

7. The device of claim 5, wherein the birefringent element comprises a section of polarization maintaining fiber (30) connected to the sensing optical fiber ( 10) so that the orientation of its birefringent axes (31 ) is shifted of about ±45 deg rees or an odd multiple of ±45 deg rees relative to the orientation of the birefringent axes (33) of said sensing optical fiber ( 10), said section of polarization maintaining fiber (30) having a length which corresponds to one or an odd multiple of a q uarter of the beat length of said polarization maintaining fiber at the wavelengths of interest.

8. The device of any of claims 5 to 7, wherein the birefringent element comprises a reflective coating (32) .

9. The device of claim 4, wherein the light reflecting means (20) comprises a polarization maintaining coupler having an output connected to an input with a loop of polarization maintaining fiber connected in one side with a 90 deg rees angular shift so as to swap the polarization axes.

10. The device of any of claims 3 to 9, implementing a stimu lated Brillouin scattering detection scheme, wherein the optical coupl ing means ( 19, 21 , 22) are arranged so as (i) to transfer into the sensing optical fiber ( 10) a linearly polarized optical pulsed pump wave ( 17) aligned with the first birefringent axis, and a linearly polarized optical probe wave ( 18) alig ned with the second birefringent axis, and (ii) to transfer toward the detection means (23) measurement optical waves resulting from stimulated Bril louin scattering interactions between said optical pulsed pump wave ( 17) and said optical probe wave ( 18) .

11. The device of claim 10, wherein the optical coupl ing means comprises :

- a polarization maintaining sensing coupler ( 19) for combining at least the optical pulsed pump wave ( 17) and the optical probe wave ( 18) ;

- a circulator (21 ) between the polarization maintaining sensing coupler ( 19) and the detection means (23) .

12. The device of claim 10 or 11 , which further comprises :

- a laser source ( 11 ) for generating at least the optical pulsed pump wave ( 17) and the optical probe wave ( 18), or

- a first laser source for generating at least the optical pulsed pump wave ( 17) and a second laser source for generating at least the optical probe wave ( 18) .

13. The device of any of claims 9 to 12, which further comprises tuning means ( 14) for varying the optical frequency of at least one spectral component of at least one of the fol lowing optical sig nals : the optical pulsed pump wave, the optical probe wave.

14. The device of any of claims 3 to 9 implementing a coherent optical time-domain reflectometer scheme, wherein the optical coupl ing means (46, 48, 49) are arranged so as (i) to transfer into the sensing optical fiber ( 10) a linearly polarized optical pulsed wave aligned with the first birefringent axis, and (ii) to transfer toward the detection means (50) measurement optical waves resulting from Raleigh scattering of said optical pulsed wave into said sensing fiber ( 10) .

15. The device of claim 14, wherein the optical coupl ing means comprises :

- a circulator (46) for d irecting the measurement optical waves towards the detection means (50) ;

- a detection coupler (49) for combining on the detection means (50) the measurement optical waves with a reference wave issued from the same laser source (41 ) .

16. The device of claim 14 or 15, which fu rther comprises a continuous wave laser source (41 ), tuning means (42) for varying the optical freq uency of said laser source (41 ), and a pulse generator (45) for generating the pulsed optical wave ( 17) .

17. Optical d istributed sensing method , using a d istributed sensor with one or a plurality of birefringent polarization maintaining sensing optical fiber(s) ( 10) arranged along a same path and having respectively a first and a second birefringent axis, characterized in that it comprises steps of:

- transferring into said sensing optical fiber(s) ( 10) at least a linearly polarized optical pulsed wave ( 17) al ig ned with a first birefringent axis, and a linearly polarized optical pulsed wave ( 17) aligned with a second birefringent axis (ii) and transferring toward detection means (23, 50) measurement optical waves emerg ing from said sensing optical fiber(s) ( 10) and resulting respectively from scattering interactions along said first and second birefringent axes;

- using detection means (23, 50) and processing means, ded ucing temperature and strain information from said measurement optical waves using the ind ices of refraction along respectively the first and the second birefringent axes.

18. The method of claim 17, using a d istributed sensor with a polarization maintaining sensing optical fiber ( 10), which further comprises steps of:

- using a light reflecting means (20) connected to a second end of the sensing optical fiber ( 10), reflecting the optical waves and shifting the d irection of polarization of the reflected optical waves relative to the incident optical waves from one birefringent axis to the other;

- using optical coupl ing means ( 19, 21 , 22, 46, 48, 49) connected to a first end of said sensing optical fiber ( 10), (i) transferring into said sensing optical fiber ( 10) at least a l inearly polarized optical pulsed wave ( 17) aligned with a first birefringent axis, and (ii) transferring toward detection means (23, 50) measurement optical waves emerg ing from said sensing optical fiber and resulting , along forth and back paths into said optical fiber ( 10), from scattering interactions respectively along the first and the second birefringent axis.

19. The method of claim 18, which further comprises steps of:

- transferring into the sensing optical fiber ( 10) a linearly polarized optical pulsed pump wave ( 17) aligned with the first birefringent axis, and a l inearly polarized optical probe wave ( 18) alig ned with the second birefringent axis;

- relatively positioning in freq uency the first optical probe wave ( 18) and the optical pulsed pump wave ( 17) so as to generate into the sensing optical fiber ( 10) a first stimulated Bril lou in scattering interaction between the optical probe wave reflected in the l ig ht reflecting means (20) and the incident optical pulsed pump wave, and a second stimulated Brillouin scattering interaction between the incident optical probe wave and the reflected optical pulsed pump wave; - deducing distributed temperature and distributed strain information along the sensing optical fiber ( 10) using said first stimulated Brillouin scattering interaction and said second stimulated Brillouin scattering interaction .

20. the method of claim 18, which further comprises steps of:

- for several optical waves frequency, measuring a first Raleigh scattering information resulting from the scattering of the optical pulsed wave into the sensing fiber ( 10) along a forth path, and measuring a second Raleigh scattering information resulting from the scattering of the optical pulsed wave into said sensing fiber ( 10) along a back path after reflection on the light reflecting means (20);

- deducing distributed temperature and distributed strain information along the sensing optical fiber ( 10) using said first Raleigh scattering information and said second Raleigh scattering information .