Patent Description:
The invention also relates to a device for implementing this method.

<CIT> discloses a method of calibrating a distributed vibration sensing system. <NPL>, discloses property measurement utilizing atomic/molecular filter-based diagnostics.

Raman or Brillouin measurement system are known for temperature and/or strain measurement in an optical fiber.

One can measure a temperature of the fiber with Distributed Temperature Sensing (DTS) using a Raman system or a Brillouin system and/or a strain of the fiber with Distributed Strain Sensing (DSS) using a Brillouin system.

The use of a Rayleigh system is not obvious for measuring an absolute value of temperature or strain.

Rayleigh backscattering (RBS) is commonly used to measure propagation loss in fiber. This is the so-called Optical Time Domain Reflectometry (OTDR). This is done by sensing a powerful pulse of broadband light (in other words, incoherent light) into a fiber and looking at the averaged RBS.

When the laser has a narrow linewidth (hence, high coherence), then there is enhanced interferometric noise (coherent noise) on the RBS. When looking at a single RBS (almost no averaging), the comparison of the coherent noise patterns of successive RBS measurements for any given position along the fiber provides information on the local variation of phase at the corresponding position. Since coherent noise is being used for detection, the technology became known as COTDR (Coherent OTDR) in the industry.

The technology has been further developed and one can identify four families in the time domain (OTDR).

Nevertheless, the use of a Rayleigh system has limited interests.

For instance, Becker et al (<NPL>) report:.

"<NPL>et al. , discloses the use of swept wavelength interferometry for distributed fiber optic sensing in single and multimode optical fiber using intrinsic Rayleigh backscatter.

"<NPL>et al. discloses a technique for measuring distributed strain or temperature.

discloses the ability to analyze Rayleigh scatter in single and multi mode fused silica fibers to deduce strain and temperature shifts.

<CIT> discloses enhanced spatial detection of optical backscatter for sensor applications.

<CIT> discloses a method of calibrating a distributed vibration sensing system.

NIST special publication <NUM>, "<NPL>, discloses a full measurement of spatial distribution of PMD using backscatter.

The goal of the invention is to present a method or device, based on Rayleigh backscattering signal, allowing to measure a temperature and/or strain in an optical fiber:.

An aspect of the invention concerns a method for measuring a temperature and/or a strain in an optical fiber, comprising the steps of:.

determining, by technical means, the absolute value of the unknown state of temperature and/or the unknown state of strain, for at least one longitudinal position inside the optical fiber.

The measurement of the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain can be done for more frequencies or wavelengths than the measurement of the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain.

The measurement of the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain can be done for a frequency interval of at least <NUM>, preferably at least <NUM>.

The method according to the invention can comprise, before and/or during the calibration step, a measurement or setting of the known state of temperature and/or of the known state of strain.

The measurement or setting of the known state of temperature and/or of the known state of strain can be done less than one hour before the calibration step.

The method according to the invention can simultaneously comprise:.

The calibration step can be done using one of the four following sensing techniques:.

The calibration step and the measurement of the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain can be done using the same sensing technique.

The Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain and/or the Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain can be obtained by injecting a laser beam in the optical fiber.

The Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain and/or the Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain can be obtained by injecting a laser beam in the optical fiber while the frequency or wavelength or wavenumber of the laser beam is measured and/or stabilized and/or locked.

The method according to the invention can comprise a compensation of a drift of the laser between:.

this compensation comprising preferably:.

The optical fiber can be in a protective sheath protecting it from strain variation, the determination step thus comprising determining, by technical means, the absolute value of the unknown state of temperature, for at least one longitudinal position inside the optical fiber.

The method according to the invention can be implemented in two different optical fibers:.

The first optical fiber can be in a protective sheath protecting it from strain variation.

An other aspect of the invention concerns a device for measuring a temperature and/or a strain in an optical fiber , comprising:.

the absolute value of the unknown state of temperature and/or the unknown state of strain, for at least one longitudinal position inside the optical fiber.

The means for measuring the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain can be arranged for measuring the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain for more frequencies or wavelengths than the measurement of the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain.

The means for measuring the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain can be arranged for measuring the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain for a frequency interval of at least <NUM>, preferably at least <NUM>.

The device according to the invention can comprise means for measuring or setting the known state of temperature and/or of the known state of strain; preferably:.

The device according to the invention can be arranged for simultaneously:.

The means for calibrating the fiber can comprise one of the four following reflectometers:.

The means for calibrating the fiber and the means for measuring the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain can comprise the same reflectometer or the same category of reflectometer among Wavelength scanning Optical Time Domain Reflectometer, Chirped pulse Optical Time Domain Reflectometer, Phase Optical Time Domain Reflectometer and Optical Frequency Domain Reflectometer.

The device according to the invention can comprise means for injecting a laser beam in the optical fiber arranged to generate the Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain and/or arranged to generate the Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain.

The device according to the invention can comprise means for measuring and/or stabilizing and/or locking the frequency or wavelength or wavenumber of the laser beam while the laser beam is injected in the fiber in order to generate the Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain or in order to generate the Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain is obtained by injecting a laser beam in the optical fiber.

The device according to the invention can comprise means for compensating a drift of the laser between:.

the compensation means comprising preferably:.

The device according to the invention can comprise a protective sheath arranged for protecting the fiber from strain variation.

An other aspect of the invention concerns a system comprising:.

Other advantages and characteristics of the invention will appear upon examination of the detailed description of embodiments which are in no way limitative, and of the appended drawings in which:.

These embodiments being in no way limitative, we can consider variants of the invention including only a selection of characteristics subsequently described or illustrated, isolated from other described or illustrated characteristics (even if this selection is taken from a sentence containing these other characteristics), if this selection of characteristics is sufficient to give a technical advantage or to distinguish the invention over the state of the art. This selection includes at least one characteristic, preferably a functional characteristic without structural details, or with only a part of the structural details if that part is sufficient to give a technical advantage or to distinguish the invention over the state of the art.

We are now going to describe, in reference to <FIG>, embodiments of a device <NUM> according to the invention implementing embodiments of a method <NUM> according to the invention.

Method <NUM> is a method for measuring a temperature and/or a strain in an optical fiber <NUM>.

Method <NUM> comprises, before and/or preferably during the calibration step <NUM>, a measurement of a known state of temperature in the optical fiber <NUM> for at least one longitudinal position inside the fiber <NUM> and/or of a known state of strain in the optical fiber <NUM> for at least one longitudinal position <NUM> inside the fiber <NUM>. The known state of temperature and/or of the known state of strain is done less than one hour before the calibration step <NUM>.

The measurement of the known state of temperature is done:.

The measurement of the known state of strain is done:.

Indeed, the future calibration step is possible if stable and measurable conditions is applied to the fiber <NUM> during the initial measurement of a known state. This can be done for instance by:.

It must be understood that the calibration error will be defined by the accuracy of the DTS/DSS measurement, which is way above the sensitivity of a Distributed acoustic sensing (DAS) system.

For example, if one is using a <NUM> calibrated thermometer, there will be a <NUM> absolute error on the temperature along the fiber <NUM>, although it will be possible to measure changes which are in the order of <NUM> mK.

By cable is meant in the present description any cable equipped with at least one optical fiber <NUM>.

Example of known and measurable condition for strain are:.

Example of a dynamic known measurable condition to build the database (strain) are:.

Preferably, method <NUM> simultaneously comprises:.

The known states of temperature and/or strain are then memorized in unit <NUM>.

Method <NUM> comprises the step <NUM> of calibrating the optical fiber <NUM> (i. e a Rayleigh measuring step at the known state of temperature and the known state of strain) by measuring an electronic (analogic or digital) signal <NUM>, <NUM>, <NUM>, <NUM>, <NUM> (from a detector <NUM>) sensitive to (and from) an intensity I and a phase ϕ of an optical Rayleigh backscattering signal of the optical fiber <NUM> at the known state of temperature and the known state of strain, for various frequencies (noted f or v) or wavelengths or wavenumbers of the backscattered signal (preferably equal (or almost equal in case of CP-DAS) to frequency or wavelength or wavenumber of an excitation laser <NUM> ) and for the at least one longitudinal position <NUM> inside the fiber <NUM>.

Unit <NUM> memorizes a database of such electronic (analogic or digital) signal <NUM>, <NUM>, <NUM>, <NUM>, <NUM> (from a detector <NUM>) sensitive to (and from) an intensity I and a phase ϕ of an optical Rayleigh backscattering signal of the optical fiber <NUM> at the known state of temperature and the known state of strain, for various frequencies or wavelengths or wavenumbers of the backscattered signal and for at the least one longitudinal position <NUM> inside the fiber <NUM>.

In the present specification, each signal <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> can have one component or several components, preferably two components (one intensity component and one phase component) for phase DAS or preferably one mixed component (intensity and phase component) for CP DAS, lambda DAS or OFDR.

The measurement of the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber <NUM> at a known state of temperature and a known state of strain is done for a frequency interval of at least <NUM>, preferably at least <NUM>, more preferably at least <NUM>.

The calibration step <NUM> is done using one of the four following Distributed acoustic sensing (DAS) techniques:.

The Rayleigh backscattering signal of the optical fiber <NUM> at a known state of temperature and a known state of strain is obtained by injecting a laser beam in the optical fiber <NUM>.

For a chirp system (CP-DAS), this is about moving the central frequency of the chirp over a wide bandwidth corresponding to the maximum calibration range, typically the following sensitivity (at <NUM>): -<NUM>/K and -<NUM>/µε. A frequency range sufficient to have "calibration" data over the expected range is scanned:.

As the frequency shift is the same for all DAS families, this give the order of magnitude for all techniques λ DAS, CP DAS, Φ DAS and OFDR.

One can consider an OFDR instead of a OTDR (a wide scan in the frequency domain with a long pulse instead of a short pulse, with different processing). The principle is the same; it is about moving the laser central frequency for each of the measurement (i.e. each scan in the frequency domain) in order to build the database.

In other words, calibrating <NUM> the optical fiber is done by measuring, for at least one longitudinal position inside the fiber, a signal <NUM>, <NUM>, <NUM>, <NUM>, <NUM> sensitive to an intensity and a phase of a Rayleigh backscattering signal of the optical fiber <NUM> generated by injecting the laser beam in the optical fiber at a known state of temperature and a known state of strain, for various conditions of frequencies or wavelengths or wavenumbers of the backscattered signal by:.

generates (during calibration at a known state of temperature and a known state of strain but also during a measurement at an unknown state of temperature and/or an unknown state of strain for determining this temperature and/or strain), a trace corresponding to a measured backscattered signal as a function of several frequencies of the laser, these several frequencies corresponding:.

The frequency sweep during calibration step <NUM> is the global frequency range covered by:.

same definition can be given by replacing frequency by wavelength or wavenumber.

In all cases except OFDR, the sweep is preferably covering a frequency interval greater than <NUM> and/or less than <NUM>.

In the OFDR case, the sweep is preferably covering a frequency interval greater than <NUM> and/or less than <NUM> THz.

Each respectively trace or chirp or scan or sweep generated during the calibration step <NUM> is called:.

In case of OFDR or CP DAS, several calibration traces are thus obtained during calibration step <NUM>.

Each trace or chirp or scan or sweep generated during the step of measuring at an unknown state of temperature and/or an unknown state of strain is called a determination trace or chirp or scan or sweep.

For a λ-DAS, the database is obtained by scanning over a wide range with respect to the later measurement.

For a phase DAS, the database is obtained by scanning over a wide frequency range with respect to the measurement.

It is possible to change the laser frequency, and/or it is also possible to change the reference condition. Assuming is it possible to change the temperature of the fiber <NUM> being calibrated in a well-controlled way, then the effect is equivalent to changing the frequency of laser <NUM> whilst keeping the temperature of fiber <NUM> constant (the phase is a function of the index of refraction, which is temperature and strain sensitive and of the wavelength/frequency, so that changing an either the frequency whilst keeping temperature / strain constant or changing temperature / strain whilst keeping frequency constant is equivalent in terms of phase variation).

Thus, and regardless of the DAS family, it is possible to link changes in phase with changes and temperature/strain and frequency, by varying one or the other.

Thus, all the embodiments and variants of the invention described with reference to <FIG> can be generalized by replacing.

Method <NUM> recovers the phase shift at a given conditions with respect to the calibrated data:.

Method <NUM> comprises then the following steps:.

a step of determining, by technical means (by unit <NUM>), the absolute value of the unknown state of temperature and/or the unknown state of strain, for the at least one longitudinal position <NUM> inside the optical fiber <NUM>.

The term absolute value of temperature is understood to mean a value (typically in ° C, or Kelvin or ° F) which is not relative i.e. is not a simple deviation from an unknown temperature reference.

The term absolute value of strain is understood to mean a value (typically in µε) which is not relative i.e. is not a simple deviation from an unknown strain reference.

The measurement of the signal <NUM>, <NUM>, <NUM>, <NUM>, <NUM> sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber <NUM> at a known state of temperature and a known state of strain is done for more frequencies or wavelengths than the measurement of the signal <NUM> sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber <NUM> at an unknown state of temperature and/or an unknown state of strain.

The calibration step <NUM> and the measurement of the signal <NUM> sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain are done using the same sensing technique among λ DAS, CP DAS, Φ DAS and OFDR.

The Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain is obtained by injecting a laser beam in the optical fiber <NUM>.

Thus, once the system has been calibrated (available database), then, a change in temperature or strain in fiber <NUM> is measured as the difference between the current laser frequency and the laser frequency in the database that provides the same Intensity I, Phase ϕ measurement pattern locally. In other words, if the local I, ϕ is found in the database at the same laser frequency, there was no change. If it is found at a different frequency, then the strain change Δε or temperature change ΔT can be related through the relation: <MAT> where υ is the central frequency of the references array, kε is a constant relating strain and frequency shift and kT is a constant relating temperature and frequency shift.

To reduce computation time, one can assume that changes are small with respect to the stored data. Thus, for a given laser frequency / measurement, there is no need to check the complete data base of signals <NUM>, <NUM>, <NUM>, <NUM>, <NUM> for every point. It is likely that looking at the <NUM>-<NUM> adjacent calibration traces in the database is sufficient. If the event is larger, then gradually increase the search in the database.

This trace recovery principle according to the invention can be done in the time domain or in the frequency domain.

In the time domain, applying the trace recovery principle can involve using a gauge length for computing the similarity of the reference and measured determination traces which is much longer than the pulse width, in order to have sufficient features for obtaining a reasonably high confidence in the similarity estimation. This can imply losing spatial resolution, which is generally unwanted, hence the operation is more usually done in the frequency domain.

In the frequency domain, the trace recovery principle involves the acquisition of two frequency sweeps (a reference sweep and a measured determination sweep) and computing the similarity in the spectra recorded at each position.

To use this calibration principle, the spatial resolution is preferably sufficiently small so that the spatial thermal/strain gradients appearing along the fiber <NUM> are not causing a deformation of the acquired I, ϕ patterns with respect to those stored in the database. As a first estimation, differential thermal and/or strain spatial gradients across the spatial resolution is preferably be kept below the temperature or strain measurement resolution of the method divided by <NUM>.

The Rayleigh backscattering signal of the optical fiber <NUM> at a known state of temperature and a known state of strain and the Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain is obtained by injecting a laser beam in the optical fiber <NUM> while the wavelength of the laser beam is measured and/or stabilized and/or locked.

Method <NUM> comprises a compensation of a drift of the laser between:.

Generally speaking, the better the phase measurement, the more accurate the calibration. If the laser <NUM> is stable in frequency (and in phase, frequency noise and phase noise are related), results will be better.

To enhance the quality of the calibration, the laser absolute frequency is stabilized. This is done for instance by locking the laser <NUM> on an absorption line (gas cell) or on a temperature stabilized optical comb (for instance a Fabry-Perot resonator, a ring resonator etc). The cavity is a function of temperature but has much narrower resonance, thus allowing a finer frequency control. Classical locking methods with sideband Pound-Drever-Hall (PDH) can be performed with direct laser modulation, preferably with some synchronization with the measurement. Else PDH is done using a modulated version of the laser (for instance using the <NUM>% output of a <NUM>/<NUM>% coupler, the remaining <NUM>% not being modulated and being used for the measurement).

As for Brillouin based distributed strain sensing, a DAS is temperature and strain cross-sensitive. DTS is used to correct slow thermal variations. If the DAS is measuring a highly dynamic signal, then a high pass filter will provide the strain variation without the thermal component and there is no temperature coupling. For very low frequency measurements, the DTS will allow to subtract the thermal component up to its repeatability (if the DTS measures 100mK variations accurately, then any thermal information below 100mK will be interpreted as strain).

The step of determining the absolute value of the unknown state of temperature and/or the unknown state of strain, for at least one longitudinal position inside the optical fiber, is not performed in a purely abstract or purely intellectual manner but involves use of a technical means, typically by the unit <NUM>.

The step of determining the absolute value of the unknown state of temperature and/or the unknown state of strain, for at least one longitudinal position inside the optical fiber is implemented by technical means (typically by unit <NUM>), comprising at least one computer, and/or one central processing or computing unit, and/or one analogue electronic circuit (preferably dedicated), and/or one digital electronic circuit (preferably dedicated) and/or one microprocessor (preferably dedicated) and/or software means and/or a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the step of determining the absolute value of the unknown state of temperature and/or the unknown state of strain and/or a computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to carry out the step of determining the absolute value of the unknown state of temperature and/or the unknown state of strain.

Device <NUM> does not comprise only the determination means <NUM>.

The other means, in particular the optical means, are detailed in the below embodiments, in particular in reference to <FIG> and <FIG>.

We are now going to describe, in references to <FIG>, an example embodiment for CP-DAS.

An example embodiment of device <NUM> implementing method <NUM> for CP-DAS is illustrated in <FIG>.

<NUM> is the fiber under test
Device <NUM> of <FIG> comprises the following means <NUM> to <NUM> and <NUM> to <NUM>:.

In this CP-DAS example, for the calibration step <NUM>, as illustrated in <FIG>:.

The scan <NUM> (i.e. the successive acquisition of signals <NUM>, <NUM>, <NUM>, <NUM>) corresponds to the sweep of laser <NUM> on the axis of the frequency f of laser <NUM>. Usually, there is a central wavelength λC/ frequency fc for the laser <NUM> and the scan is done at two values fC+fstep and fC-fstep , fC+2fstep and fC-2fstep, etc The aim is to get typically at least <NUM> of such calibration traces covering <NUM>, preferably each covering <NUM> chirp and each separated by <NUM> from the previous frequency/wavelength (λ<NUM> and λ<NUM> are <NUM> apart and so on).

Note that temperature axis T is in the opposite direction from the frequency f scan: <MAT> with:.

For each backscattering signal <NUM>, <NUM>, <NUM>, <NUM> (and <NUM> described below) in <FIG>:.

During this calibration step <NUM>, the temperature and/or the strain applied to the fiber <NUM> are known and controlled.

During the calibration step <NUM>, the settings are the following:.

The corresponding λstep is computed by: <MAT>.

Thus going from <NUM> to <NUM> for a total variation of Δ λ =200pm.

Each laser position is averaged <NUM> times to get a good and stable measurement.

Then the Rayleigh measuring step <NUM> at the unknown state of temperature and/or unknown state of strain and the determination step are implemented as follows.

The setup is exactly the same than for the calibration step <NUM>.

For step <NUM>, the laser <NUM> is set at a wavelength λ, typically the central one λC from the previous scan and does not move. The backscattering signal <NUM> is acquired along the fiber <NUM>; again we concentrate on the backscattering <NUM> for the <NUM> window at position <NUM>, now in an unknown state (temperature and / or strain).

For the determination step, determination trace <NUM> is compared to each of the calibration traces <NUM> to <NUM> (cross-correlation) in order to find which one is the closes one (in the <FIG>, <NUM> is close to <NUM> on <NUM> side).

The cross-correlation results in a figure with a maximum corresponding to the best match between <NUM> and all the reference curves <NUM>, <NUM>, <NUM>, <NUM>, etc..

<FIG> is an auto-correlation result of the determination step following steps <NUM> and <NUM> of <FIG>, where the bottom axis is the wavelength position with respect to λC= λ<NUM> (in the center). Thus, for each cross-correlation 25x21, 25x22 etc, there is one point in <FIG>, and when the cross-correlation is high, we get the peak.

The peak is fitted and the central position is found and can be between two measured calibration traces (in this case somewhere between position <NUM> and position <NUM>).

The distance between the center of the peak and λC = λ<NUM> corresponds to a frequency shift Δv related to a change in T or strain thanks to equation <MAT>.

For instance, the correlation peak is offset by +<NUM>. 6pm from the central position, which corresponds to <NUM>. Using this equation and assuming that we only have temperature variation (see below example of fiber <NUM> of <FIG>), then: <MAT> <MAT> we have a temperature shift of -60mK.

Knowing the temperature / strain at λ<NUM>, which was constant for all calibration, one can recover the absolute temperature / strain over time by applying successively the <NUM> x <NUM> to <NUM> auto-correlation function at every position along the fiber <NUM>. The same method applies for strain.

<FIG> illustrates the determined temperature over time following steps <NUM> and <NUM> of <FIG> (horizontal axis: time in hour; left vertical axis: frequency offset in Hz; right vertical axis: temperature in °C of fiber <NUM>; greyscale level: correlation amplitude from low (dark) to high (white)).

Looking over <NUM>, one sees that the position of the correlation peak (seen from <FIG>) slightly moves over time.

As illustrated in <FIG>, looking over time only at the maximum value correlation curve <NUM> and comparing with a reference temperature measurement <NUM> (thermocouple) , one sees a good match.

If the measurement is interrupted, then, when the instrument is turned on again, cross-correlation are done with the original reference and measurement resumes accordingly.

When the system was calibrated for strain (typically at zero strain), then measurement over time provide the local strain variation. For instance, using a piezo actuator, one sees a small variation of a couple of µε, as illustrated in <FIG>.

We are now going to describe, in references to <FIG>, an example embodiment for phase DAS or OFDR.

An example embodiment of device <NUM> implementing method <NUM> for Phase DAS and OFDR is illustrated in <FIG>. Device <NUM> of <FIG> comprises references <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> already described for <FIG>.

Phase DAS and OFDR rely on a coherent detection to acquire the backscattering.

The local oscillator is superposed to the detection signal on the detector <NUM>. The measured intensity on the photodiode <NUM> is the square of the sum of the two contributions<MAT> The intensity of the signal <NUM> and of the local oscillator (E<NUM>sig and E<NUM>LO) corresponds to the continuous contribution, stable over time, which does not bear any information to the measurement. They can be removed using a high pass filter. Then, the -<NUM>ELOEsig cos(ωsigt + φ) cos(ωLOt) contribution that oscillate at the beat signal (ωLOt) only is retrieved. It contains the phase information of the Rayleigh backscattering φ and its amplitude Esig. Thus, the measured trace is exactly similar to the one formed for CP-DAS.

Coherent detection is achieved by deriving typically <NUM>% of the laser flow (thanks to coupler <NUM>) to form the local oscillator (LO). The LO is combined with the backscattering signal on the photodiode <NUM> using the <NUM>/<NUM> coupler <NUM>. The photodiode <NUM>, shown schematically as a single device is usually a balanced detector so that both arms of the <NUM>/<NUM> couplers are used. This directly filters out the (E<NUM>sig and E<NUM>LO) signal. Laser <NUM> is driven by λ<NUM> for the central wavelength (this corresponds to the laser driver with stable current and stable temperature control).

For OFDR only: Δλ is applied to continuously scan the laser over a wide range. In the case of the CP-DAS , it is achieved externally with an EOM. But it can also be directly achieved by driving a current ramp on the laser <NUM>, which is what is done usually for OFDR (very large chirp). In other words, compared to the previously described embodiment for CP-DAS, in case of OFDR several scans of the laser frequency are done, each scan in the frequency domain being centered on a different central frequency respectively fC+fstep , fC-fstep , fC+2fstep , fC-2fstep, etc. fc is the central frequency of the "sweep" i.e. of the sum of all the frequency scans.

In the Phase DAS case or OFDR case, for steps <NUM>, <NUM> and determination step, reference measurement is made by scanning the laser <NUM>, similarly to the CP-DAS and cross-correlation between reference and determination traces is done afterwards for the measurement. Measurement step <NUM> and determination step are done as per CP-DAS.

Laser is set to λ<NUM>, measurement is performed with averaging. Then the laser is moved and acquisition is performed again.

The settings are the same than in the CP-DAS case.

We are now going to describe, in references to <FIG>, an example embodiment for lambda DAS.

The example embodiment of device <NUM> implementing method <NUM> for lambda DAS is the same than for Phase DAS and OFDR and is thus illustrated in <FIG>.

A lambda-DAS (or "lambda-scan") system is also retrieving phase like a phase-OTDR. In fact, once can consider that lambda-DAS is a phase-DAS that is scanning, contrary to a standard phase-DAS that does not scan (unless we need to calibrate).

For calibration step <NUM>, reference measurement is made by scanning the laser <NUM>, similarly to the CP-DAS. The Rayleigh backscattering is acquired over the distance for each wavelength.

Then, for each distance (corresponding to the pulse duration), the intensity on the photodiode <NUM> (from the coherent detection, e. g, made of the full intensity/phase interaction of the backscattering with the LO) is extracted over distance. Each of this wavelength slices built the references <NUM> as illustrated in <FIG>.

Each laser position is averaged <NUM> times (typically) to get a good and stable measurement.

From a processing point of view, the correlation is not done in the time/distance domain, but in the frequency domain. For this reason, the spatial resolution must be smaller.

For step <NUM>, using the same settings, the laser <NUM> is scanned over <NUM> only around the central wavelength λ<NUM>. Thus, only a subset of <FIG> is measured, e.g. only λ-<NUM>, λ<NUM> and λ<NUM>, corresponding to measurement <NUM> in <FIG>. Measurement <NUM> is made at position <NUM> and is compared with reference <NUM>.

When doing the auto-correlation, the max may not be centered around λ<NUM> but at λ<NUM>. This corresponds to a frequency shift of <NUM> and using <MAT>.

As illustrated in <FIG>, in any embodiment or example previously described of the method <NUM>, the invention can be implemented in two different optical fibers <NUM> preferably at the same time (or in a delayed or slightly delayed manner) :.

This can be implemented in a system according to the invention comprising:.

The first device and the second device can have means in common, for example:.

The first optical fiber <NUM> is in a protective sheath <NUM> protecting it from strain variation.

Thus, this variant of method <NUM> uses a cable with loose tube fiber <NUM> design (strain free) for temperature measurement and one tight buffer <NUM> sensitive to strain and temperature. Both fibers <NUM>, <NUM> are calibrated.

During measurement, it is possible to measure temperature variation dT on loose tube <NUM> and to compensate (subtraction) on the tight buffer <NUM>.

Of course, the invention is not limited to the examples which have just been described and numerous amendments can be made to these examples without exceeding the scope of the invention.

Claim 1:
Method for measuring a temperature and/or a strain in an optical fiber (<NUM>), comprising the steps of:
- Calibrating (<NUM>) the optical fiber by measuring, for at least one longitudinal position inside the fiber, a signal (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) sensitive to an intensity and a phase of a Rayleigh backscattering signal of the optical fiber (<NUM>) generated by injecting a laser beam in the optical fiber at a known state of temperature and a known state of strain, for various conditions of frequencies or wavelengths or wavenumbers of the backscattered signal by:
∘ moving the laser central frequency for each scan in the frequency domain among several scans in the frequency domain, or
∘ changing the laser frequency,
- Measuring (<NUM>) the signal (<NUM>) sensitive to an intensity and a phase of a Rayleigh backscattering signal of the optical fiber (<NUM>) at an unknown state of temperature and/or an unknown state of strain, for various frequencies or wavelengths or wavenumbers of the backscattered signal and for at least one longitudinal position inside the fiber,
- Based on a shift of frequency or temporal period or wavelength or wavenumber or a correlation between:
∘ the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain, for various conditions of frequencies or wavelengths or wavenumbers of the backscattered signal and/or of known states of temperature and/or known states of strain, and
∘ the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain,
determining, by technical means, the absolute value of the unknown state of temperature and/or the unknown state of strain, for at least one longitudinal position (<NUM>) inside the optical fiber
the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at a known state of temperature and a known state of strain being done for more frequencies or wavelengths than the measurement of the signal sensitive to the intensity and the phase of a Rayleigh backscattering signal of the optical fiber at an unknown state of temperature and/or an unknown state of strain.