WO2011072927A1

WO2011072927A1 - Device and method for rapidly measuring elongation

Info

Publication number: WO2011072927A1
Application number: PCT/EP2010/066087
Authority: WO
Inventors: Sascha Liehr
Original assignee: Bundesanstalt für Materialforschung und -Prüfung (BAM)
Priority date: 2009-12-16
Filing date: 2010-10-25
Publication date: 2011-06-23
Also published as: DE102009058520B4; DE102009058520A1

Abstract

The invention relates to a method for measuring elongation on a sensor fiber, comprising the steps: (a) providing at least one sensor fiber having a reflection point each at n predetermined positions (z_r), where n ≥ 1; (b) providing a reference transmission function (H₀, H_r) of the sensor fiber, wherein the reference transmission function (H₀, H_r) was determined in a predetermined initial state of the sensor fiber; (c) performing of m ≥ n optical frequency range reflectometry measurements at m measurement frequencies (f_k) which differ from each other to determine m measurement transmission functions (M_k); (d) determining a local shift (Δz_r) of at least one reflection point on the basis of the reference transmission function (H₀, H_r) and the measurement transmission functions (M_k); and (e) determining the elongation of a section of the sensor fiber adjacent to the at least one reflection point on the basis of the determined local shift (Δz_r) of the reflection point.

Description

Apparatus and method for rapid strain measurement

The present invention relates to a device for rapid strain measurement and an associated method for performing a rapid strain measurement.

Many components and structures that are exposed to dynamic loads can not be monitored satisfactorily by conventional sensors. While selective sensors, such as strain gauges (DMS) or fiber Bragg gratings (FBG), provide rapid and accurate strain values for particular points, they lack the measurement basis to provide integrative length changes, i. Length changes between two points, to capture. Therefore, it is possible in such systems that damage that lies between two measuring points are not recognized. This information gap represents a major risk in the operation of plants, components and buildings.

Although known in the prior art fiber Bragg gratings (FBG) can measure high-frequency and exactly the elongation, but only selectively at certain points of a fiber. Therefore, FBG systems are not suitable for the applications described above. In addition, FBG systems are also relatively expensive.

Furthermore, fiber optic sensors based on direct phase measurement of a

intensity modulated signal in transmission known. These systems are also capable of high-frequency and accurate measure length changes. However, these sensors can only determine the total length change along the fiber, but not the change in length of defined fiber sections.

Furthermore, the so-called optical time domain reflectometry (OTDR technique) and the optical frequency domain reflectometry (OFDR technique) are known as strain measurement. With the aid of the OTDR technique or the OFDR technique reflection points can be measured relatively accurately. However, the OTDR technique and the OFDR technique require a comparatively long measurement time, which precludes the application of these two techniques for dynamic applications. The well-known electrical sensors, such as strain gauges, similar to FBGs measure the strain only selectively and have beyond the usual

Disadvantages of electrical sensors compared to fiber optic sensors.

Furthermore, a device for Brillouin frequency domain analysis from WO

2007/131794 known. It describes a monitoring system that uses Brillouin frequency domain analysis to monitor the elongation of an optical fiber to detect, for example, changes in a dike or similar safety-related structure.

Also, a device for Brillouin frequency domain analysis is disclosed in the article "Brillouin optical-fiber frequency-domain analysis for distributed temperature and strain measurements" by Garus et al., Journal of Lightwave Technology, Vol. 15, No. 4, pp. 654-662 , 1997. Furthermore, a device for Brillouin frequency domain analysis from the article "Distributed fiber-optic frequency domain Brillouin sensing" by Bernini et al., Sensors and Actuators A: Physical, Vol 123-124, Eurosensors XVIII 2004 - The 18th European Conference on Solid-State Transducers, pages 337-342, 2005.

In view of this, it is therefore an object of the present invention to provide a method and an apparatus for strain measurement which at least partially overcomes the disadvantages mentioned.

This object is achieved by a method of strain measurement according to claim 1 and by a system according to claim 8. Further aspects, details, advantages and features of the present invention will become apparent from the subclaims, which

Description and attached drawings.

According to an embodiment of the present invention, a method of strain measurement on a sensor fiber comprises the steps of providing at least one sensor fiber having a reflection location at n predetermined positions, where n>1; the provision of a reference transmission function of the sensor fiber, wherein the reference transmission function has been determined in a predetermined initial state of the sensor fiber; performing m> n optical frequency domain reflectometry Measurements at m different measuring frequencies for the determination of m

Meßübertragungsfunktionen; the determination of a local displacement of at least one reflection point based on the reference transfer function and the

Meßübertragungsfunktionen; and the determination of the elongation of an adjacent to the at least one reflection point portion of the sensor fiber based on the determined local displacement of the reflection point.

With the method described above very fast strain measurements are possible, since only n frequencies for n reflections must be measured, that is, it comes with the repetition rate in the order of 100 Hz or more. In this way, in particular dynamic processes, such as the deflection of rotor blades of

Wind turbines in operation, very well detected. At the same time, the method provides very high accuracy down to the micrometer range. The accuracy of the measurement can be increased if more measurements are made than reflection points are present in the sensor fiber (m> n).

The method also allows the simultaneous measurement of n reflections, i. n individual fiber lengths, in reflection. Therefore, it is possible to measure from one side of the fiber and also only one sensor fiber has to be installed in the structure to be monitored.

In contrast to conventional phase evaluation methods, the present

Process on a greatly extended range at the same time great accuracy. However, the precision of the measurement is not, as in conventional

Phase evaluation method, depending on the maximum modulation frequency, but only on the accuracy of the device used to measure the

Transfer function. Therefore, the present method makes relatively lower demands on the frequency range of the components used, thereby

less expensive components can be used.

Typically, the sensor fiber reflection sites comprise one end of the sensor fiber, a connector interconnecting two adjacent portions of the sensor fiber, a reflection spot created in the manufacture of the sensor fiber, or one after the sensor fiber

Production of the sensor fiber introduced reflection point. According to a development, the provision of the reference transfer function comprises performing a frequency domain reflectometry measurement on the sensor fiber in

Initial state for determining the transfer function of the sensor fiber in

Frequency range; the determination of the impulse response in the time domain of the sensor fiber by means of inverse Fourier transformation of the transfer function; decomposing the impulse response into n single-pulse functions and a time-domain background function obtained by subtracting the single-pulse functions from the impulse response; and performing a Fourier transform on the n single-pulse functions and the time-domain background function to obtain the reference transfer function in the frequency domain as n

Single-reflection digit functions and a frequency-domain background function

provide.

According to one embodiment, the modulation frequencies for the optical

Frequency domain reflectometry measurements are in the range of 1 Hz to 10 GHz.

As stated above, the accuracy of the measurement depends on the device used to measure the transfer function. For example, network analyzers can be used in the frequency range between 1 Hz and 10 GHz. If a high accuracy is desired, a small filter bandwidth should be used for the devices. When measuring very short sections, devices in the frequency range above 10 GHz must be used.

According to a further development, determining the local displacement can

Subtracting the frequency domain background function for a respective measurement frequency from the measurement transfer function at that measurement frequency; determining a

Phase shift for a respective reflection point; and determining the local displacement by means of the phase shift. Typically, these steps can be repeated at least once, and as a new input for the positions of the reflections, the positions of the reflections corrected by the previously determined local displacement are used.

The measurement frequencies are freely selectable and each combination of measurement frequencies provides a solution, so that the method described is very flexible. According to yet another embodiment, the method may further comprise an optical frequency domain reflectometry measurement on a reference sensor fiber or a

Include reference portion of the sensor fiber to determine a temperature dependence of the optical frequency domain reflectometry measurements on the sensor fiber. Subsequently, the compensation of the determined temperature dependence in the determined elongation of the sensor fiber and / or the determination of the temperature of at least one section of the sensor fiber can then take place.

In this way, the measuring method can also be used in areas that are exposed to strong temperature fluctuations.

According to another aspect of the present invention, a system for the

Strain measurement on a sensor fiber, a sensor fiber having at n predetermined positions each have a reflection point, where n> 1, connected to the sensor fiber means for performing an optical frequency domain reflectometry measurement, and connected to the device evaluation unit for determining an elongation of the sensor fiber wherein the system is set up, a method as described above

perform.

Such a system may have the above-described advantages of the present invention

Implement strain measurement in a suitable manner.

According to a further development, the system comprises a plurality of sensor fibers which are connected to the means for performing an optical frequency domain reflectometry measurement via at least one optical splitter, wherein the reflection points in the plurality of sensor fibers are arranged so that their respective positions are so different from each other at maximum strain of the sensor fibers the positions are distinguishable.

In this way, a single system can simultaneously monitor multiple structures or different portions of a structure. The use of one or more other systems is thereby superfluous.

According to a further development, the system further comprises a reference sensor fiber or a reference section of the sensor fiber, wherein the reference sensor fiber or the Reference portion of the sensor fiber extend substantially parallel to at least one sensor fiber, wherein the reference sensor fiber or the reference portion of the sensor fiber are adapted to determine a temperature dependence of the optical frequency domain reflectometry measurement.

In this way, the system can be used while maintaining its accuracy in areas where severe temperature fluctuations occur.

Reference to the accompanying drawings, embodiments of the present invention will now be explained. 1 shows a schematic representation of a system according to an exemplary embodiment of the present invention.

Fig. 2 is a schematic representation of a system according to another

Embodiment of the present invention.

Fig. 3 shows an example of a transfer function of the sensor fiber in the frequency domain, which was determined in the initial state of the sensor fiber.

FIG. 4 shows the impulse response determined in FIG. 3 from the transfer function according to FIG

The time domain. the decomposition of the impulse response into single impulse functions.

FIG. 6 shows the subtraction of the individual impulse functions from the impulse response by one

To obtain time domain background function. from the single-pulse functions by Fourier transformation g

Single reflection point functions in the frequency domain. the frequency domain subfunction derived from the time domain subsurface function by Fourier transformation. 9 shows a strain measurement on the fiber shown in FIG.

10 shows a further strain measurement on a vibrating plate.

11 shows a schematic illustration of a system with a plurality of sensor fibers according to a further exemplary embodiment of the present invention.

12 is a schematic representation of a system with temperature compensation according to yet another embodiment of the present invention.

Fig. 1 shows a schematic representation of a system 100 according to a

Embodiment of the present invention. Therein, a sensor fiber 110 is connected to a device 120 for carrying out an optical frequency domain reflectometry measurement (OFDR measurement). The device 120 comprises a device for measuring the complex transfer function 122, for example a network analyzer,

typically a vector network analyzer connected to a laser source 124,

typically a continuous wave laser source. The network analyzer 122 generates a sinusoidal output signal of frequency f. By means of this signal, an amplitude modulation of the laser source 124 is performed, wherein the

Network analyzer 122 can tune the frequency f in a predetermined frequency interval with predetermined frequency steps. For example, a

Frequency interval from 1 MHz to 500 MHz in 1 MHz increments. The light emitted by the laser source 124 is coupled into the sensor fiber 110 via an optical circulator 126. The circulator 126 further decouples the signal reflected in the sensor fiber 110 and passes it to a photodiode 128. The photodiode 128 converts the optical signal from the circulator 126 into an electrical signal and outputs this signal S to the network analyzer 122. The network analyzer 122 now determines the transfer function of the sensor fiber 110 on the basis of the signal S. An evaluation unit 130 for determining an expansion of the sensor fiber 110 is connected to the device 120, and typically to the network analyzer 122. The evaluation unit 130 is set up to carry out a method according to the exemplary embodiments of the present invention. In particular, the evaluation unit 130 may be a computer that is set up by the program to execute such a method. The evaluation unit 130 may be provided with an output unit (not shown), for example a screen or a printer, be connected. The strain determined by the evaluation unit 130 can then be displayed or output on the output unit.

In Fig. 2 is a schematic representation of a system according to another

Embodiment of the present invention shown. Therein, the sensor fiber 110 at predetermined positions z _1; z ₂ each have a reflection point Rl, R2. The

Reflection Rl, R2 of the sensor fiber can be formed for example by connectors, in particular those with 0 ° -schliff, the connectors connect two adjacent portions of the sensor fiber with each other. In the context of the present

Revelation are therefore also understood to mean a plurality of interconnected fibers as a sensor fiber, since, as in the embodiment of FIG. 2, they can form a common fiber measurement path. However, the reflections R1, R2 can also be formed differently. For example, the reflection points R 1, R 2 can be formed by reflection positions produced during or after the production of the sensor fiber. Furthermore, the open end of the sensor fiber 110 itself can serve as a reflection point, so it is designed in such a way that the light is substantially reflected. By way of example, the fiber end can have a 0 ° grating and / or be mirrored. Of course, the number and the position of the reflection points in the sensor fiber is essentially freely selectable. Thus, instead of the two reflection points shown in FIG. 2, n> 1 reflection points in the sensor fiber 110 can be provided quite generally.

In the following, a method for measuring the strain on the sensor fiber 110 according to FIG. 2 will now be explained by way of example. For this purpose, first a reference transfer function of the sensor fiber is provided. The reference transfer function was determined in a predetermined initial state of the sensor fiber. This may, for example, be an unstretched condition of the sensor fiber if subsequently strains are to be observed. Conversely, the sensor fiber can also be installed in a stretched initial state, if subsequent upsetting is to be observed. To determine the

Reference transfer function is first made an OFDR measurement on the sensor fiber 110 in the initial state and after calibration of the measuring system. Based on this measurement, the transfer function H (f) of the sensor fiber is determined in the frequency domain. The result of this measurement is shown in FIG. 3, where both amount and phase of the transfer function are shown. In a next step, an impulse response in the time domain is now determined from the transfer function H (f). For this purpose, the transfer function H (f) becomes an inverse

Subjected to Fourier transformation. Typically, the inverse Fourier transform is performed as an inverse fast Fourier transform. The obtained in this way

Impulse response h (z) in the time domain is shown in FIG. The two reflection parts Rl, R2 are clearly recognizable in the impulse response h (z). It should be mentioned that the

Signal propagation times were converted directly into distances by means of the speed of light in the sensor fiber. Nevertheless, the representation in FIG. 4 is a

Time domain representation.

In a next step, the impulse response h (z) is now decomposed into two single-impulse functions hi (z) and h ₂ (z). This is shown in FIG. For this purpose, a region is defined around each individual pulse (hatched box in FIG. 5) and the impulse response outside of these regions is set to zero. In this way, the respective impulse responses are obtained separately for hi (z) and h ₂ (z). For example, overshoots typically occur adjacent the respective major peak of a single pulse. In this case, for example, the minimum between the main peak and the first side peak can be determined on each side of the single pulse. The range of the single pulse would then be located between these two minima. However, it has been found that the results of the present method are robust to the off-cutting criterion, so that other than the just-described criterion for selecting the range for a particular single pulse can be used. The decomposition into individual pulses is analog, if the sensor fiber has a number other than two reflections.

Once the individual impulse functions h _r (z) have been isolated, then a time domain subsurface function ho (z) is obtained by subtracting the single impulse functions h _r (z) from the impulse response h (z).

This process as well as the result are illustrated in FIG. Subsequently, for the individual impulse functions h _r (z), that is to say i and h ₂ in the present exemplary embodiment, they are returned to the mean by means of a Fourier transformation

Frequency domain to obtain single reflection site functions H _r (f). The single-reflection site functions H ^ f) and H ₂ (f) for the first and the second

Reflection Rl, R2 are shown in Fig. 7, wherein each amount and phase are shown. Similarly, the time domain background function ho (z) is used by

Fourier transform to the frequency domain to obtain a frequency domain background function H ₀ (f). The frequency domain background function H ₀ (f) for the specific embodiment is shown in FIG. The amount IH _r (f) l and phase cp _r (f) of all reflections for each measured frequency f and Ho (f) are now known and form the

Reference transfer function for the further process steps. The creation of the reference transfer function described above is a one-time process. Is the

Reference transfer function determined so it can be held and retrieved as needed. In this respect, the steps described so far are merely to be regarded as preparatory to the actual strain measurement procedure.

To perform the actual strain measurement, OFDR measurements are performed on the sensor fiber. In this case, a total of m measurements at m different measuring frequencies f _{k are performed} to determine m Meßübertragungsfunktionen M _k . In this case, m> n, ie the number of frequency points is at least equal to the number of reflection points in the sensor fiber. If one chooses m> n, one obtains an overdetermined system of equations in the evaluation and can thus the accuracy of the

Increase evaluation. This will be explained in more detail below.

In the next step, the local displacement Δζ _Γ a respective reflection point is determined, as can be determined from the local displacement, the strain. In general, one obtains the local displacement Δζ _Γ by first determining the frequency domain subsurface function H ₀ (f _k ) for a respective measurement frequency f _k of the

Measurement transfer function M _k subtracted at this measurement frequency. In other words, the background is first removed from the Meßübertragungsfunktionen M _k . Furthermore, a phase shift Δφ _Γ for a respective reflection point is determined and from this in turn the local displacement Δζ _Γ . The results of the measurements M _k at the arbitrary frequency points f _{k of} the frequencies f _k , k C {1,..., M}, correspond (within the measuring error) to the sum of the

Single Reflection Position Functions H _rk and the Frequency _Domain Underground Function Ho _k - A change in the transit time, ie a change in position, one of the reflection points compared to the output measurement causes a change in the phase position Δφ _{Γ of} the respective one

Reflection point R _r . The change of the phase angle Δφ _Γ thus serves as an indicator for the change in position and thus also the strain. Furthermore, the assumption \ H \ = const, ie no loss change occurs along the fiber. This assumption is, for example, for singlemode and multimode glass fibers throughout the permissible

Stretching area fulfilled. For polymer fibers, if necessary, the range of expansion would have to be determined, within which this assumption is fulfilled. Because of the assumption IH _rl = const. By calculating a complex system of equations with m = n rows and n columns, the respective phase φ _Γ can be calculated. Thus, for the determination of the phase position of all n reflection sites in the fiber, only n measurements M _k = 1... N are necessary. For the specific embodiment of two reflections, which were measured at the frequencies f _k = 1 and f _k = 2, with f _k > f _k -i for k = 2..m., the following equation system results:

Therein, the individual equations are linked to one another via the auxiliary quantities A _rk . The A _rk represent the relative phase shift of the measurement with the frequency f _k = 1 for the measurement with the frequency f _k and depend on the position of the reflex z _r and the

Frequency difference Af = f _k - fi. More precisely, the _{rules are} as follows:

Here, c _{0 is} the vacuum speed of light and n _{gr is} the refractive index of the fiber.

As already mentioned, the accuracy can be increased by overdetermination of the system of equations to m> n frequency points. For the overdetermined case, the following general equation system results for n reflections and m frequency points:

The equation systems obtained in the manner described above now become

resolved and the phase shift Δφ _Γ is calculated. The change of the calculated cpr compared to the phase position determined at the beginning results in the reference measurement

the phase shift Δφ _Γ . This can be determined by the following expression:

Where the function atan2 is the arctangent with two arguments, defined as follows:

The first argument is the cross product and the second argument is the scalar product between

From the thus obtained Δφ _Γ can the position changes Δζ _Γ with very high

Calculate accuracy as follows:

From these changes in position Δζ _{Γ of} the reflection Rl, R2 can then be determined on the basis of the original lengths z _\ and z _{2 of} the fiber sections, the strain values for one or more sections of the sensor fiber.

According to a development, the above steps can also be repeated. In this case, then, as new input values for the positions z _{r of} the reflection sites, the positions z _r + Δζ _{Γ of} the reflection sites corrected by the previously determined local displacement Δζ _Γ are used. Namely, since the calculation of A _rk the positions of the reflection points z _\ and z ₂ of the reference measurement adopted, the result by means of an iteration with the newly calculated reflection positions can, for _r = z _r + _Δζ Γ be clarified. The value z _r converges quickly and stably so that only a few iterations

are to be carried out.

In contrast to conventional phase evaluation methods, the present

Transfer function. Therefore, the present method is comparatively less Demands on the frequency range of the components used, whereby less expensive components can be used.

FIG. 9 shows a time-resolved strain measurement on the fiber shown in FIG. In this case, the second fiber path (z = z _\ z ₂ , green line) was stretched several times by hand, wherein the fiber distance has remained unchanged up to the first reflection point (z = zo ... z _1, blue line).

FIG. 10 shows another time-resolved measurement according to an embodiment of the present invention. To demonstrate the dynamic measurement with multiple reflectors, the demonstrator described below was built. Two single-mode fibers were glued in a prestressed state to an 8 mm thick chipboard on both sides, so that in each case a bending of the chipboard two fibers are stretched and two fibers are compressed. The individual fiber sections are connected by means of fiber connectors with strong reflections. The measurement in FIG. 10 shows the chipboard at rest (t <ls), the deflection to one side (t = (ls... 3,2s)) and a free, damped one

Oscillation after deflection (t> 3,2s). The measurement shows that all

Fiber sections show excellent agreement in magnitude and sign with the expected behavior. The measurement also demonstrates the ability of the system to monitor multiple sensor fibers simultaneously and independently.

Referring now to Figure 11, there is shown a schematic representation of a multiple sensor fiber system 200 in accordance with another embodiment of the present invention. In this case, the system comprises a first sensor fiber 210 and a second sensor fiber 212. The first sensor fiber 210 has a first reflection point Rl at a distance z _\ and a second

Reflection R2 at a distance z ₂ on. The second sensor fiber 212 has a third one

Reflection point R3 at a distance z ₃ and a fourth reflection point R4 at a distance z ₄ . The reflection points R1, R2, R3, R4 in the first and second sensor fibers 210, 212 are arranged so that their respective positions z _1; z ₂ , z, z ₄ are so different from one another that even at maximum elongation of the sensor fibers 210, 212 the positions z _1; z ₂ , z, z _{4 are} distinguishable. For example, z _\ = 100 m, z ₂ = 200 m, z ₃ = 50 m and z ₄ = 150 m. In this way, the individual pulses can be clearly assigned to the respective reflection points, even if the first and the second sensor fibers are stretched maximally and in opposite directions. The first and second sensor fibers 210, 212 are Connected via an optical coupler or an optical splitter 240 to the device 120 for OFDR measurement. By means of the coupler / splitter 240 multiplexing of the signals in the first fiber 210 and the second fiber 212 is possible. Due to the sufficient distance between the reflection, however, these can be unambiguously assigned.

FIG. 12 shows a schematic representation of a system 300 with

Temperature compensation according to yet another embodiment of the present invention. For long measuring distances or in applications where strong

Temperature changes may occur, the accuracy of the change in length measurement is basically limited by temperature influences. The thermal expansion of the fiber or sheath material or of the cable and the temperature dependence of the refractive index of the fiber have an effect on the absolutely measured positions of the reflection parts. A temperature compensation for the separation of temperature and strain is achieved by the substantially parallel installation of a fiber of the same type with length reserve. Due to the length reserve, no stretching of the sensor fiber occurs, so that only the temperature-induced effect is measured in these fibers or fiber sections.

Therefore, the system 300 according to FIG. 12 has reference sections 312, 314 of the sensor fiber 310, which run essentially parallel to a sensor fiber 310 and are laid with a length reserve. Here, the reference section 312 serves between a third and a fourth reflection point R3, R4 of the sensor fiber 310 as a temperature reference to

Strain gauge between zo and z _1; ie the fiber section to the first

Reflection point Rl. Similarly, the reference portion 314 between the second and the third reflection point R2, R3 of the sensor fiber 310 serves as a temperature reference to the strain gauge between z _\ and z ₂ , ie the fiber section between the first

Reflection point Rl and the second reflection point R2. Due to the thus determined temperature dependence, the measured strain of the sensor fiber can be compensated accordingly. In this case, the measured change in length of the reference section can be normalized to the respective installed lengths of the associated strain gauges of the sensor fiber. The thus determined temperature-dependent effect is then subtracted from the measured change in length of the strain gauges. In this way one obtains a temperature-compensated strain signal. Additionally or alternatively On the basis of the reference sections, the temperature of a respective section of the sensor fiber 310 can also be determined.

The present invention has been explained with reference to exemplary embodiments. These embodiments should by no means be construed as limiting the present invention.

Claims

Claims 1. A method of strain measurement on a sensor fiber comprising the steps of:

(a) providing at least one sensor fiber which is predetermined at n

Positions (z _r ) each having a reflection point, where n>1;

(b) providing a reference transmission function (H ₀ , H _r ) of the sensor fiber, wherein the reference transfer function (H ₀ , H _r ) has been determined in a predetermined initial state of the sensor fiber;

(c) performing m> n optical frequency domain reflectometry measurements at m different measuring frequencies (fjj for determining m Meßübertrag sfunktionen (M _k );

(d) determining a local displacement (Δζ _Γ ) of at least one reflection point from the reference transfer function (Ho, H _r ) and the measurement transfer functions (MjJ;

(E) determining the strain of a portion of the sensor fiber adjacent to the at least one reflection point on the basis of the determined local displacement (Δζ _Γ ) of the reflection point.

2. The method of claim 1, wherein in step (a) the reflection points of

Sensor fibers include: one end of the sensor fiber, a connector interconnecting two adjacent portions of the sensor fiber, a reflection spot created in the manufacture of the sensor fiber, a reflection spot introduced after fabrication of the sensor fiber.

The method of claim 1 or 2, wherein step (b) further comprises:

(bl) performing a frequency domain reflectometry measurement on the

Sensor fiber in the initial state for determining the transfer function (H (f)) of the sensor fiber in the frequency domain; (b2) determining the impulse response (h (z)) in the time domain of the sensor fiber by means of inverse Fourier transformation of the transfer function (H (f));

(b3) decomposing the impulse response (h (z)) into n single-pulse functions (h _k (z _r )) and a time-domain background function (ho (z)) obtained by subtracting the single-pulse functions (h _k (z _r )) from the Impulse response (h (z)) is obtained; and

(b4) performing a Fourier transform for the n single-pulse functions (h _k (z _r )) and the time-domain background function (ho (z)) to obtain the

Frequency-domain reference transfer function as n single-reflection digit functions (H _k (f)) and a frequency-domain subsurface function (Ho (f)).

4. The method according to any one of the preceding claims, wherein in step (c) the

Modulation frequencies (f _k ) for the optical frequency domain reflectometry measurements in the range of 1 Hz to 10 GHz.

5. The method according to any one of the preceding claims, wherein step (d) comprises:

(dl) subtracting the frequency domain background function (Ho (f _k )) for a respective measurement frequency (f _k ) from the measurement transfer function (M _k ) at that measurement frequency;

(d2) determining a phase shift (Δφ _Γ ) for a respective reflection location; and

(d3) determining the local displacement (Δζ _Γ ) by means of

Phase shift (Δφ _Γ ).

6. The method of claim 5, wherein the steps (dl) to (d3) are repeated at least once, and wherein as a new input value for the positions (z _r ) of Reflection points, which is used to the previously determined local displacement (Δζ _Γ ) corrected positions (z _r + Δζ _Γ ) of the reflection points.

A method according to any one of the preceding claims, further comprising:

(f) an optical frequency domain reflectometry measurement on a

Reference sensor fiber or a reference portion of the sensor fiber to a temperature dependence of the optical

Determine frequency domain reflectometry measurements on the sensor fiber; and

(fl) compensation of the determined temperature dependence in the determined strain of the sensor fiber; and or

(f2) determining the temperature of at least a portion of the sensor fiber.

8. A system for strain measurement on a sensor fiber, comprising a sensor fiber (110) having at n predetermined positions (z _r ) each having a reflection point (Rl, R2), where n> 1, one with the sensor fiber (110) connected A device (120) for performing an optical frequency domain reflectometry measurement, and an evaluation unit (130) connected to the device (120) for determining an elongation of the sensor fiber (110), the system being set up, a method according to one of

previous claims.

The system of claim 8, wherein the system comprises a plurality of sensor fibers (210, 212) connected to the means (120) for performing an optical frequency domain reflectometry measurement via at least one optical splitter (240), and wherein the reflection locations (Rl , R2, R3, R4) in the a plurality of sensor fibers are arranged so that their respective positions (z _r ) of each other are so different that even at maximum strain of the sensor fibers, the positions (z _r ) are distinguishable.

10. System according to claim 8 or 9, further comprising a reference sensor fiber or a reference portion (312, 314) of the sensor fiber (310), wherein the reference sensor fiber or the reference portion (312, 314) of the sensor fiber substantially parallel to the at least one sensor fiber (310). run, wherein the reference sensor fiber or the reference portion (312, 314) of the sensor fiber are arranged, a temperature dependence of the optical

Determine frequency domain reflectometry measurement.