Optical fiber system for determining at least one physical parameter and method for determining at least one physical parameter using optical fiber system

Abstract

To provide a more compact optical fiber sensor system with higher performance.SOLUTION: A sensor system of the present invention includes: at least two sensing reflectors; at least one reference reflector; at least one optical fiber including a distal end and a proximal end; a modulable light source arranged at the proximal end of the optical fiber; and a photodetector. A measurement segment formed between the two sensing reflectors is arranged between the distal end and the proximal end. The reference reflector is arranged between the proximal end and the distal end, and is separated from the distal end by at least 1 mm.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an optical sensor system and a method for determining a physical parameter using an optical sensor system. [Background technology]

[0002] Optical fibers are well known in communication technology to transfer information over long distances in the form of light. However, optical fibers can also be used to probe the physical parameters of the environment, as these affect the optical properties of the fiber. As a result, the light emitted from the fiber is affected by the physical parameters of the environment through which the fiber passes. Physical parameters are, for example, temperature, strain, magnetic fields, electric fields, etc.

[0003] One way to probe a particular physical parameter is to introduce an optical element into the fiber that is particularly sensitive to that physical parameter. One well-known approach is fiber Bragg grating-based fiber sensing, which usually refers to a technique that directly uses the physical deformation of a Bragg grating (FBG for short) embedded within the core of an optical fiber. These gratings consist of a uniform series of partially reflective / refractive defects, spatially separated by a defined period (Λ). When the reflected light resonates with the grating structure, the reflected wave is enhanced (constructive interference). The reflected wavelength λ B is also called the Bragg wavelength, λ B =2n e Λ, where n eis the refractive index of the fiber core. For a non-ideal grating of finite length, the Bragg wavelength is the central wavelength of the characteristic bandwidth. In general, the more elements in the grating, the higher the reflectivity of the grating and the narrower the bandwidth of the reflected wavelengths. When the FBG is deformed by strain, the Bragg wavelength of the grating shifts: it shifts red when the grating is stretched and blue when the grating is compressed. In a typical FBG sensor system, each grating in the fiber sensor has a different Bragg wavelength, and they can be easily distinguished spectroscopically, usually using a diffraction grating and a linear array detector, or a wavelength-modulated light source and a single detector. If the bandwidth is narrow enough, many sensors can be tracked in a single fiber (wavelength division multiplexing, WDM). However, there is usually a trade-off: the reflectivity of the grating must be high to achieve a sufficiently narrow bandwidth, and the attenuation of the light passing through the fiber reduces the signal-to-noise ratio.

[0004] Another quite different method of determining the physical properties of an environment using optical fibers and reflectors is Fiber Segment Interferometry (FSI for short). FSI is a range-resolved interferometry technique originally developed at Cranfield University in the UK and disclosed in US Pat. Nos. 5,999,313 and 5,999,322. FSI and other fiber techniques are described in detail in the paper "Fiber Segment Interferometry: A Novel Approach to Optical Fiber Spectroscopy," by John Wiley & Sons, Inc., and published in the journal Nature Communications.

[0005] Fiber optic sensors based on FSI technology can be used to measure very small changes in the length of an object (i.e., parts per billion), which can be achieved by utilizing range-resolved interferometric peak tracking algorithms, as disclosed in the prior art and in [2].

[0006] FSI is a measurement technique that uses a coherent light source (e.g., a laser). Light is directed through an optical fiber. The light is wavelength modulated at a desired modulation frequency (e.g., up to 200 kHz). The modulated light is preferably launched into the optical fiber through a coupling device (e.g., a three-port optical circulator). The light reflects from the optical fiber and returns through the coupling device to the photodetector, as shown in prior art FIG. 1. Of course, one skilled in the art can also implement other means (e.g., fiber coupler and / or splitter arrangements) for launching light into the optical fiber and directing the reflected light from the optical fiber to the photodetector.

[0007] The optical sensor system includes a fiber, a reference reflector at the distal end of the fiber, and multiple sensing reflectors disposed along the length of the fiber between the distal and proximal ends of the fiber. The sensing reflectors, together with the reference reflector, form a series of Fabry-Perot optical resonators, also called Fabry-Perot (FP) or resonators. Because the distance from the reference reflector to each sensing reflector is unique, each resonator exhibits a different resonant wavelength.

[0008] In-fiber reflectors can be made by creating a localized periodic variation in the refractive index within the core of an optical fiber. The variation in refractive index causes light to be reflected according to the resulting Fresnel coefficient. If the period of the structure is half the wavelength of the light passing through it, the reflected waves will be in phase or resonant, satisfying the Bragg condition. When illuminated by a light source, these fiber Bragg gratings generate reflected light waves with a reflection profile as a function of wavelength and bandwidth, the characteristics of which depend on the design parameters of the grating, e.g., number of periods, refractive index contrast, apodization function, chirp, etc.

[0009] Of course, other reflective elements can also be used as the sensing reflector.

[0010] Thus, the modulated light of the desired modulation frequency launched into the fiber is influenced by the reflection profile of the reference and sensing reflectors, and the length between both reflectors is influenced by physical parameters. In the case of a fiber Bragg grating sensing reflector, the period of the fiber Bragg grating depends on the thermal expansion coefficient and the temperature, or the physical strain induced in the fiber Bragg grating. The light reflects from the optical fiber and returns to the photodetector through a coupling device, as shown in prior art Figure 1. The detected signal is digitized, for example at 200 MHz, providing about 1000 samples per measurement / modulation cycle.

[0011] Tracking the relative resonance phase over time for each modulation cycle provides the relative distance (also known as the optical path length) between each sensing reflector and the reference reflector multiplied by the refractive index of the fiber. In the following, the distance is always the optical distance. The spacing between each pair of adjacent sensing reflectors, called the measurement segment, is then known. Due to the relatively high reflectivity of the reference reflector, the mutual interference between the sensing reflectors is relatively weaker than their interference with the reference reflector.

[0012] The optical path length of each measurement segment changes with strain. This is determined by the difference between the path length from the first sensing reflector to the reference reflector of the measurement segment and the path length from the second sensing reflector to the reference reflector of the measurement segment. Thus, optical fibers can be used for strain measurement applications if they are rigidly attached to another element and this element is subjected to strain. The optical path length of each segment can also change as a function of temperature due to both the thermo-optic coefficient (temperature-dependent change in refractive index) and the thermal expansion of the fiber. For this reason, fibers are typically placed inside a suitable smooth, perforated sheathing material, such as a polymer tube or a glass capillary tube, which allows the optical fiber to expand freely with temperature. Of course, since the measurement segments are provided along the length of the fiber, only the measurement segments used for temperature measurement need be placed in such a sheathing material. Temperature measurements can also be used if the fiber is rigidly attached to another material and is subject to strain caused by the thermal expansion and changes in the thermo-optic coefficient of the other material. Of course, optical fibers can also be used to measure pressure if they are bonded to a flexible membrane such that pressure causes bending of the membrane, resulting in strain on the optical fiber.

[0013] When the wavelength of the incident light is modulated, the reflected optical signal forms a time-resolved interference fringe containing characteristic resonance peaks whose amplitudes correspond to the lengths of each Fabry-Perot cavity, which consists of a reference reflector and each sensing reflector.

[0014] The known state-of-the-art reference reflector is implemented as a cleavage at the distal end of the fiber, as disclosed in [2]. Mechanically cleaving the end of the optical fiber creates a glass-air interface. At such an interface, the Fresnel reflection is about 3.5%.

[0015] However, the reflectors disclosed above are very sensitive to contamination. Moreover, the maximum reflectance of the reference reflector is limited by Fresnel reflections unless expensive coatings are applied to the cut surfaces. Even with coatings, the reference reflector may still be susceptible to contamination or mechanical damage.

[0016] Cutting also adds additional length to the end of the fiber optic sensor that is not useful for the measurement. Particularly in applications where space is limited, dead space can be created by the reference reflector at the distal end of the fiber. The dead space is a portion of the fiber that is needed for the reference reflector, but does not provide temperature or strain data by itself. Thus, the fiber becomes very long with no added value. The additional fiber length, including the reference reflector, must be accommodated within the test object.

[0017] Another drawback is that state-of-the-art truncated reference reflectors can be contaminated with fluid media other than air, which affects the Fresnel reflection at the interface and changes the reflectivity of the reference reflector. In the case of particulate contamination, scattering losses reduce the signal-to-noise ratio of the measurement. [Prior art documents] [Patent documents]

[0018] [Patent Document 1] GB Patent Application Publication No. 2568305 [Patent Document 2] GB 2571575 [Non-patent literature]

[0019] [Non-Patent Document 1] "Range-resolved optical interferometric signal processing", Thomas Kissinger (paper, Cranfield University) [Non-Patent Document 2] Thomas Kissinger, Ricardo Correia, Thomas OHCharrett, Stephen, W. James, and Ralph P. Tatam, "Fiber Segment Interferometry for Dynamic Strain Measurements," J. Lightwave Technol. 34, 4620-4626 (2016) Summary of the Invention [Problem to be solved by the invention]

[0020] An object of the present invention is to alleviate the above-mentioned drawbacks.A further object of the present invention is to make a more compact optical fiber. [Means for solving the problem]

[0021] At least one of the problems is solved by the features of the independent claims.

[0022] The sensor system of the present invention comprises at least one optical fiber, a modulatable light source, and a light detector. The at least one optical fiber comprises at least two sensing reflectors. The at least one optical fiber comprises at least one reference reflector. Each of the at least one optical fiber further comprises a distal end and a proximal end. A measurement segment is formed between the two sensing reflectors. The measurement segment is disposed between the distal end and the proximal end. A light source is disposed at the proximal end of the optical fiber. The light source is configured to emit light having a coherence length longer than the maximum distance between any of the sensing reflectors and the closest one of the at least one reference reflector. Otherwise, the reference reflector and the sensing reflector cannot be used as a Fabry-Perot cavity. Of course, when incorporating multiple reference reflectors, the coherence length only needs to be longer than the maximum optical distance between any of the sensing reflectors and the one reference reflector to which they are referenced. Typically, each sensing reflector is referenced by the reference reflector that is closest to it in terms of optical distance.

[0023] In what follows, distances are always optical distances.

[0024] It is clear that the segment formed between the reference reflector and the sensing reflector may also be influenced by a physical parameter and therefore may also be used to obtain information about the physical parameter. Also, in the case of multiple reference reflectors, the segment between two reference reflectors may also be influenced by a physical parameter and therefore may also be used to obtain information about the physical parameter. Thus, the term "measurement segment" is not intended to limit the measurement function only to the mutual interference of a reference reflector and two associated sensing reflectors, but is simply intended to indicate the segment that is most often used to measure a physical parameter.

[0025] According to the invention, the reference reflector is placed between the proximal and distal ends. The reference reflector is at least 1 mm away from the distal end. Replacing the reference reflector cut according to the state of the art with an internally placed reference reflector about 1 mm (millimeter) away from the distal end of the fiber removes the risk of contamination or mechanical damage of the reference reflector. This has the advantage that the optical fiber based sensor of the invention is more robust. Also, the optical fiber based sensor of the invention can be easily enclosed and / or immersed in a fluid medium.

[0026] It should be noted that there may be multiple reference reflectors in the optical system, where the multiple reference reflectors may have higher and / or lower reflectivities than any of the sensing reflectors.

[0027] In a preferred embodiment, the reference reflector is located between the proximal end and the sensing reflector closest to the distal end. The optical fiber then also has the advantage that the dead space at the distal end of the optical fiber is reduced.

[0028] Preferably, the reference reflector is located between the proximal end and the sensing reflector closest to the proximal end, which has the advantage that if the sensor is damaged and the reference reflector is closer to the proximal end than the location of the sensor damage, the sensor will continue to operate up to the point of damage, compared to the reference reflector being at the proximal end or anywhere between the sensing reflectors.

[0029] In a preferred embodiment of an optical fiber, the reference reflector is a fiber Bragg grating. This has the advantage that the reference reflector can be placed at the proximal end of the fiber outside the active sensing area. The reference reflector can also be placed inside the housing containing the light source and the photodetector or inside the interrogator equipment. The reference reflector is therefore protected from contamination and mechanical damage. Also, fiber Bragg gratings are manufactured in a more controlled manner compared to cutting, which results in higher and more reproducible manufacturing quality of the optical fiber.

[0030] In one aspect of the invention, an attenuator is placed at the distal end of the optical fiber. This is advantageous because it suppresses unwanted reflections from the end of the fiber (e.g., the original cleavage). The attenuator can be configured to disperse the light at the distal end of the fiber.

[0031] The reference reflector preferably has a higher reflectance than any of the sensing reflectors. Therefore, since the reflectance of the reference reflector is relatively high, the mutual interference between the sensing reflectors is relatively weaker than the interference with the reference reflector. However, in order to distinguish the mutual interference between the sensing reflector and the reference reflector, it is also possible to make the reflectance of the reference reflector lower than any of the sensing reflectors. In this case, since the reflectance of the reference reflector is relatively low, the mutual interference between the reference reflector and the sensing reflector is relatively weaker than the interference between the sensing reflectors.

[0032] It should be noted that the names of the reference reflector and the sensing reflector refer only to the designated use of the respective reflectors. In general, the reflectance of any reference reflector 2 is different from the reflectance of any sensing reflector 1.i, the mutual interference is distinguishable in intensity from the mutual interference resulting from any pair of sensing reflectors 1.i, and the boundary conditions are satisfied that the distance between any sensing reflector 1.1-1.i and the respective reference reflector 2 is unique.

[0033] Further advantages and aspects of the present invention are disclosed in the embodiments.

[0034] The drawings used to explain the embodiments are schematic representations and show: [Brief description of the drawings]

[0035] [Figure 1] FIG. 1 is a schematic diagram of one prior art embodiment of a sensor system. [Diagram 2] 1 is a schematic diagram of one embodiment of a sensor system. [Diagram 3] FIG. 2 is a schematic diagram of another embodiment of a sensor system. [Figure 4] FIG. 2 is a schematic diagram of another embodiment of a sensor system. [Diagram 5] FIG. 2 is a schematic diagram of another embodiment of a sensor system. [Figure 6] FIG. 2 is a schematic diagram of another embodiment of a sensor system. [Figure 7] FIG. 2 is a schematic diagram of another embodiment of a sensor system. [Figure 8] FIG. 2 is a schematic diagram of another embodiment of a sensor system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] FIG. 1 shows a prior art sensor system 1. At least one optical fiber 7 is shown attached at its proximal end 71 to a modulatable light source 4. The light source 4 is preferably wavelength tunable. The light source is configured to emit light to a coupling device 9 (e.g. a three-port optical circulator 9). The coupling device is configured to transmit the light emitted by the light source to the fiber 7 and to transmit the reflected light from the fiber 7 to a photodetector 5. The optical fiber 7 comprises sensing reflectors 1.1 to 1.i (i>1, i∈N). A reference reflector 2 is arranged at the distal end 72 of the fiber 7.

[0037] The optical fiber 7 comprises measurement segments 3.1-3.i (i>1, i∈N). The measurement segments 3.1-3.i are arranged between a distal end 72 and a proximal end 71, each measurement segment 3.1-3.i being formed between two sensing reflectors 1.1-1.i. The light source 4 is configured to emit light having a coherence length longer than the maximum distance between any of the sensing reflectors 1.1-1.i and the reference reflector 2. In an exemplary embodiment of the state of the art, a maximum distance is shown between the sensing reflector 1.1 and the reference reflector 2. The measurement segments 3.1-3.i are arranged within a sensing area 8. The sensing area 8 is an area in which a physical parameter (e.g. temperature or strain) causing a change in the optical properties of the measurement area 3.1-3.i can be determined. To determine the physical parameters of the sensing region 3.n (n∈i), the reflection from the Fabry-Perot cavity consisting of the sensing reflector 3.n (n∈i) and the reference reflector 2 is determined as a function of the modulation wavelength, and the reflection from the Fabry-Perot cavity consisting of the sensing reflector 3.n+1 (n∈i) and the reference reflector 2 is determined as a function of the modulation wavelength.

[0038] Figure 2 shows an embodiment of a sensor system 1 according to the invention. Elements having the same function as in the state of the art described in figure 1 are given the same reference numerals.

[0039] In the embodiment of the inventive sensor system 1 shown in Fig. 2, the reference reflector 2 is disposed between the proximal end 71 and the sensing reflector 1.1 closest to the proximal end 71. This disposition of the reference reflector 2 is therefore quite different from the prior art.

[0040] Figure 3 shows an embodiment of a sensor system 1 according to the invention. Elements having the same function as in the state of the art described in figure 1 are given the same reference numerals.

[0041] In the embodiment of the sensor system 1 according to the invention shown in Figure 3, the reference reflector 2 is located between the proximal end 71 and the sensing reflector 1.1 closest to the distal end 72. In this embodiment, the reference reflector 2 is located between the sensing reflector 1.5 and the sensing reflector 1.6, but it could just as easily be located elsewhere between the sensing reflector 1.6 and the sensing reflector 1.i, or between the sensing reflector 1.i and the coupling device 9.

[0042] Figure 4 shows an embodiment of a sensor system 1 according to the invention. Elements having the same function as in the state of the art described in figure 1 are given the same reference numerals.

[0043] In the embodiment of the sensor system 1 shown in Figure 4, the reference reflector 2 is disposed between the proximal end 71 and the distal end 72. The reference reflector 2 is disposed at least 1 mm away from the distal end 72. In this embodiment, the reference reflector 2 is disposed between the sensing reflector 1.i and the distal end 72, and is approximately 1 mm away from the distal end. The optional attenuator 21 is not shown for clarity.

[0044] Figure 5 shows an embodiment of a sensor system 1 according to the invention. Elements having the same function as in the state of the art described in figure 1 are given the same reference numerals.

[0045] In the embodiment of the sensor system 1 shown in FIG. 5, the reference reflector 2 is located between the proximal end 71 and the distal end 72, but it may be located anywhere according to the embodiments shown in FIGS. 1-4. In this embodiment, the photodetector 5 is located at the distal end 72. The photodetector 5 may be directly coupled to the distal end 72, for example by splicing. The photodetector 5 may also be located spaced apart from the distal end 72 by an air gap. In the latter case, it may be beneficial to cover the distal end with an anti-reflective coating known to those skilled in the art.

[0046] Figure 6 shows an embodiment of a sensor system 1 according to the invention. Elements having the same function as in the state of the art described in figure 1 are given the same reference numerals.

[0047] In the embodiment of the sensor system 1 according to the invention shown in FIG. 6, three fibers 7, 7', 7" are shown as an example. The fibers are coupled by coupling devices 9', 9". In this example, the coupling devices 9', 9" can be three-port circulators. The reflected light injected into the first fiber 7 is guided into the second fiber 7'. The reflected light from the second fiber upstream of the coupling device 9" is guided into the third fiber 7". The reflected light from the third fiber 7'' is guided back to the photodetector 5 via all the coupling devices 9, 9', 9". Preferably, the first fiber 7 and the second fiber 7' are fitted with a high reflection coating 22, 22' or a mirror 22, 22' at their respective distal ends 72, 72' to ensure that the intensity of the light injected into all the fibers 7, 7', 7" is high, but this is not necessary. The third fiber 7" is preferably fitted with an attenuator 21 at its distal end to prevent light not reflected by either the reference reflectors 2, 2', 2" or the sensing reflectors 1.1-1.i from reaching the photodetector. In this embodiment of the sensor system 1, the fibers are arranged in a sort of "daisy chain" arrangement. In this embodiment, three reference reflectors 2, 2', 2" are also shown, each located at the location of one of the three fibers. In this way, the total optical length of the fibers can be extended beyond the coherence length of the light source 4. It should be understood that a different number of fibers 7 (e.g., two fibers 7 or more than three fibers 7) can also be used. Also, a different number of reference reflectors 2 may be used. Or, similar to the embodiment shown in Figures 2-5, the reference reflectors 2 and / or the photodetectors 5 may be arranged in a different manner within each fiber 7.

[0048] Figure 7 shows an embodiment of a sensor system 1 according to the invention. Elements having the same function as in the state of the art described in figure 1 are given the same reference numerals.

[0049] In the embodiment of the inventive sensor system 1 shown in Fig. 7, two fibers 7, 7' are shown as an example. The fibers are coupled by a coupling device 9'. In this example, the coupling device 9' can be a three-port circulator or a beam splitter. In the case of a beam splitter 9', the incident light is introduced simultaneously into the first fiber 7 and into the second fiber 7'. The sensing reflectors 1.1-1.i are placed in the first fiber 7', while one reference reflector 2 is placed in the second fiber. The reflected light from the reference reflector 2 and the reflected light from the sensing reflectors 1.1-1.i are guided back to the photodetector. The mutual interference is obtained in the same way as described in the state-of-the-art single fiber 7 embodiment. Of course, the coupling device 9' can also be chosen as a three-port optical circulator, in which case the two fibers 7, 7' are again daisy-chained, as already described in the embodiment of Fig. 6. It should be understood that a different number of fibers 7 (e.g., two fibers 7 or more than three fibers 7) can also be used. Also, a different number of reference reflectors 2 may be used. Or, similar to the embodiments shown in Figs. 2-5, the reference reflectors 2 and / or the photodetectors 5 may be arranged in different ways inside each fiber 7. Those skilled in the art will understand that the embodiments of Figs. 5 and 6 can also be combined. As already explained in the state of the art, the sensing reflectors 1.1-1.i and at least one reference reflector 2 may be distributed among multiple fibers 7 as long as the boundary conditions are met and the distance between any sensing reflector 1.1-1.i and the respective reference reflector 2 is unique.

[0050] Figure 8 shows an embodiment of a sensor system 1 according to the invention. Elements having the same function as in the state of the art described in figure 1 are given the same reference numerals.

[0051] In the embodiment of the sensor system 1 according to the invention shown in FIG. 8, one fiber 7 is also shown. The fiber 7 exemplarily comprises a total of three reference reflectors 2, 2', 2". In this way, the total optical length of the fiber 7 can be extended longer than the coherence length of the light source 4, as long as the distance between any of the sensing reflectors 1.1-1.i and its nearest reference reflector 2, 2', 2" does not exceed the coherence length of the light source. It should be understood that it is also possible that a different number of fibers 7 are used and that each fiber 7, 7', 7" has multiple reference reflectors 2, as shown in the embodiment of FIG. 6 or FIG. 7.

[0052] In the following, various aspects of the present invention are described, which are all compatible with all embodiments shown in the figures and described above, unless otherwise stated.

[0053] In one embodiment of the sensor system 1, the reference reflector 2 is a fiber Bragg grating. This has the advantage that the reference reflector 2 can be located at the proximal end 71 of the fiber 7, away from the active sensing region 8. The reference reflector can also be located inside the housing containing the light source 4 and the photodetector 5 or inside the interrogator equipment. Thus, the reference reflector 2 is protected from contamination and mechanical damage. Also, fiber Bragg gratings are manufactured in a more controlled manner compared to cleaving, which results in higher and more reproducible manufacturing quality of the optical fiber 7.

[0054] In one embodiment of the sensor system 1, the attenuator 21 is placed at the distal end 72 of the fiber 7. This is advantageous because unwanted reflections from the distal end 72 of the fiber are suppressed. The attenuator 21 can be configured to disperse the light at the distal end 72 of the fiber. Placing the attenuator at the distal end of the fiber, although advantageous, is not essential to the invention. This embodiment is not compatible with the embodiment in which the photodetector 5 is placed at the distal end 72 shown in FIG. 5. Alternatively, the reflectivity of the reference reflector 2 according to the invention is selected to be different from potential reflections coming from the distal end 72, allowing to distinguish between interference of the reference reflector 2 and the sensing reflector 1.i of the invention and interference of potential reflections from the distal end 72 and the sensing reflector 1.i. Reflections from the distal end are not used for the measurement, so that contamination of the distal end does not change the reference.

[0055] Preferably, the reference reflector 2 has a higher reflectance than any of the sensing reflectors 1.i. Therefore, since the reflectance of the reference reflector 2 is relatively high, the mutual interference between the sensing reflectors 1.i is relatively weaker than the mutual interference with the reference reflector 2. However, in order to distinguish the mutual interference between the sensing reflector 1.i and the reference reflector 2, it is also possible to make the reflectance of the reference reflector 2 lower than any of the sensing reflectors 1.i. In this case, since the reflectance of the reference reflector 2 is relatively low, the mutual interference between the reference reflector 2 and the sensing reflector 1.i is relatively weaker than the interference between the sensing reflectors 1.i.

[0056] In another embodiment of the sensor system 1, the measurement segments 3.1-3.i are arranged in the sensing region 8. The measurement segments 3.1-3.i are configured such that their properties change depending on the physical properties (e.g. temperature, pressure or strain) in the sensing region 8. The attenuator 21 is arranged in the sensing region 8. This has the advantage that the fiber 7 is shorter than a comparable state-of-the-art fiber 7. No additional dead space at the distal end needs to be considered in the sensing region 8.

[0057] According to another aspect of the sensor system 1, the attenuator 21 is formed from a coreless fiber. This has the advantage that splicing the coreless fiber to the distal end 72 can be easily accomplished and is therefore cost-effective. Splicing is the process of connecting two optical fiber elements. The coreless fiber is, for example, a glass rod.

[0058] According to another aspect of the sensor system 1, the attenuator 21 is configured to attenuate the incident light by at least -0.1 dB / mm, preferably more than -2 dB / mm, or the attenuator 21 is 100 mm or less in length, preferably 20 mm or less in length. This is advantageous because only a short excess length of fiber effectively suppresses unwanted potential reflections from the distal end 72. Possible embodiments of such absorbers to attenuate / suppress back reflections include absorbers, optical isolators or crossed polarizers, index matching materials, diffusers / scatterers, anti-reflective (dielectric) coatings, fiber tapers, melting and / or shaping of the fiber end, or multimode fibers.

[0059] According to another embodiment of the sensor system 1, the sensing reflectors 1.1-1.i are fiber Bragg gratings. This is advantageous because the same manufacturing techniques can be used to introduce the sensing reflectors 1.1-1.i and the reference reflector 2 into the fiber. This allows for an efficient manufacturing of the fiber.

[0060] According to another aspect of the sensor system 1, the bandwidth of the sensing reflector 1.1, 1.i is ideally 40 nm with a central wavelength of about 1550 nm. However, the invention is also feasible with a very wide range of bandwidths, as long as the bandwidth of the sensing reflector 1.1, 1.i is wider than the modulatable bandwidth of the light source 4, i.e. at least greater than 3 pm as described below for the difference between the start and end wavelengths of the light source 4. In general, the bandwidth of the sensing reflector 1.1, 1.i is usually not less than 1 nm. This is advantageous since it allows the bandwidth of the modulation of the light up to the bandwidth of the reflector to be used. However, the wider the modulation bandwidth, the more wavelength-dependent interferences can be determined depending on the physical properties of the measurement area 3.i, and therefore the greater the accuracy of the sensor system 1.

[0061] Of course, other central wavelengths can be used, but wavelengths around 1290 nm to 1620 nm are good choices because suitable light sources are available at relatively low cost, and optical fibers are usually optimized for minimal attenuation at these wavelengths.

[0062] According to another embodiment of the sensor system 1, the light source 4 is configured to emit light with a linewidth less than 10 GHz, preferably less than 2 MHz. The coherence length of the light source increases with the inverse of the bandwidth, and therefore with increasing distance between the reference reflector 2 and the most distal sensing reflector 1.i. The linewidth of the light source 4 depends on the application in which the sensor system 1 is used. For example, 1 MHz corresponds to a coherence length of about 200 m, which is suitable for a sensor system 1 used in bridges or tunnels. 10 GHz results in a coherence length of about 20 mm, i.e. suitable for applications in limited or narrow spaces.

[0063] According to another aspect of the sensor system 1, the light source 4 is configured to emit light modulated at a modulation rate, the light source 4 is configured to sweep from a start wavelength to an end wavelength during the intermodulation rate, the start wavelength differing from the end wavelength by at least 3 pm (picometers). The difference between the start and end wavelengths depends on the required measurement resolution. A modulation depth of 300 pm can achieve a resolution of 10 mm. For large structures that only require a resolution of 1 m, 3 pm is sufficient.

[0064] According to another aspect of the sensor system 1, the reflectivity of the reference reflector 2 is at least twice the reflectivity of any of the sensing reflectors 1.1, 1.2 to 1.i. This has the advantage that wavelength-dependent optical interference between the reference reflector 2 and any of the sensing reflectors 1.i can be easily distinguished from wavelength-dependent interference between any two sensing reflectors 1.i, 1.j (i, j>1, i≠j, i∈N).

[0065] Preferably, the reflectivity of the reference reflector 2 is at least 5 times the reflectivity of any of the sensing reflectors 1.1-1.i. This can be easily achieved by using a fiber Bragg grating as the reference reflector 2 and a fiber Bragg grating as the sensing reflector. The reflectivity of the fiber Bragg grating can be easily adjusted by the morphology and shape of the grating. It has been recognized that 5 times is advantageous to easily distinguish between interference between the reference reflector 2 and any of the sensing reflectors 1.i and interference between any two sensing reflectors 1.i, 1.j (i, j>1, i≠j, i∈N).

[0066] In another aspect of the invention, the reflectivity of the reference reflector 2 is at least half, preferably one fifth, of the reflectivity of any of the sensing reflectors 1.1, 1.2-1.i. It is recognized that a reference reflector 2 having a lower reflectivity also has the advantage that wavelength-dependent optical interference between the reference reflector 2 and any of the sensing reflectors 1.i can be easily distinguished from wavelength-dependent interference between any two sensing reflectors 1.i, 1.j (i,j>1, i≠j, i∈N).

[0067] In another embodiment, the reflectivity of any reference reflector 2 differs from the reflectivity of any sensing reflector 1.i, as long as the mutual interference is distinguishable in intensity from the mutual interference arising from any pair of sensing reflectors 1.i.

[0068] It should be noted that the names of the reference reflectors 2, 2', 2" and the sensing reflector 1.i refer only to the designated use of each reflector. Thus, in an embodiment with several reflectors with different reflectivities, the reflector with the highest reflectivity can be selected as the reference reflector 2 and all other reflectors become sensing reflectors, as long as the boundary conditions for coherence length, distinguishable mutual interference, and the characteristic length between the reference reflector 2 and the sensing reflector 1.i described above are satisfied. Alternatively, the reflector with the lowest reflectivity can be selected as the reference reflector 2. Alternatively, several reflectors with the highest or lowest reflectivities can be selected as the reference reflectors 2, 2', 2", or a combination of the highest and lowest reflectivities can be selected. Alternatively, a reflector with an intermediate reflectivity can be selected as the reference reflector 2, as long as the mutual interference between the sensing reflector 1.i and the reference reflector is distinguishable in strength from the mutual interference resulting from any pair of sensing reflectors 1.i.

[0069] According to another aspect of the sensor system 1, the sensor system 1 comprises an evaluation unit 6. The evaluation unit 6 is configured to track the relative resonance phase in time upon modulation of the light of the light source 4. This is advantageous compared to typical FBG-based sensors, since the FSI-based sensing method only needs to track the change in phase of each interference peak relative to a reference reflector.

[0070] According to another embodiment of the sensor system 1, the modulation rate of the light source 4 is at least 10 Hz, preferably at least 200 kHz. This allows for rapid data acquisition, since it is only necessary to determine the change in phase of each interference peak relative to the reference reflector. This can be realized by state-of-the-art FPGAs (Field Programmable Gate Arrays). No complex detector arrays are required. In this way, high-frequency vibrations or acoustic signals that impose high-frequency strains on the fiber 7 can also be measured in the measurement zone.

[0071] According to another embodiment of the sensor system 1, the light source 4 is a distributed feedback diode.

[0072] According to another embodiment of the sensor system 1, the photodetector 5 comprises exactly one (exactly one) photosensitive element. Distributed feedback diode lasers are relatively cheap and robust. No expensive external cavity laser is required in an FSI-based sensor system.

[0073] An optical sensing method for determining a physical property such as temperature or strain in a sensing region 8 includes the steps of providing at least one optical fiber 7 including at least two sensing reflectors 1.1, 1.2 and at least one reference reflector 2, and further providing a light source 4 having a modulatable wavelength. The light source 4 emits light into the proximal end 71 of the fiber 7, the wavelength of the light is modulated over a bandwidth of light lengths as a function of time, the light is reflected by at least two sensing reflectors 1.1, 1.2-1.i forming a measurement segment 3.1-3.i of the optical fiber 7, whereby the sensing reflectors 1.1, 1.2-1.i are arranged between the distal end 72 and the proximal end 71 of the fiber 7, the light is also reflected by a reference reflector 2, which is arranged between the proximal end 71 and the sensing reflector 1.1, 1.2-1.i closest to the proximal end 72 and is arranged at least 1 mm away from the distal end 72, the coherence length of the light source 4 is set to be longer than the maximum distance between any of the sensing reflectors 1.1, 1.2-1.i and the reference reflector 2, and the light source 4 is modulated over the bandwidth of the emitted light. The bandwidth of the light is narrower than that of reflectors 1.1, 1.2-1.i.

[0074] In another embodiment of the sensor system 1, instead of the wavelength modulatable light source, another suitable multiplexing technique can be used. The multiplexing technique can include signal multiplexing and light source, such as code division multiplexing, time division multiplexing, or other suitable multiplexing techniques. In this case, it should be noted that the light source can comprise multiple laser light sources inside the light source, and the laser light sources can be coupled to one fiber outlet of the light source. As long as the different lasers of the light source can be distinguished from each other in the interference signal, the multiplexing technique is suitable.

[0075] Possible embodiments of the sensor system 1 explicitly include combinations of aspects of the above embodiments. [Explanation of symbols]

[0076] 1.1~1.i(i>1;i∈N) Measurement reflector 2,2',2” Reference Reflector 21 Attenuator 22 Highly reflective device, mirror 3.1~3.i(i>1;i∈N) Measurement segments 4 Light source, laser 5. Photodetector 6 Evaluation Unit 7,7',7” Fiber 71,71',71” proximal end 72,72',72” Distal end 8 Sensing Area 9,9',9” coupling device, 3-port optical circulator

Claims

1. A sensor system (1) comprising at least one optical fiber (7), a modulatable light source (4) and a photodetector (5), said at least one optical fiber (7) comprises at least two sensing reflectors (1.1, 1.2 to 1.i); The at least one optical fiber (7) comprises at least one reference reflector (2), Each of the at least one optical fiber (7) further comprises a distal end (72) and a proximal end (71); A measuring segment (3.1 to 3.i) is formed between two sensing reflectors (1.1 to 1.i), The measuring segments (3.1 to 3.i) are arranged between the distal end (72) and the proximal end (71); The sensor system (1), wherein the light source (4) is disposed at the proximal end (71) of the optical fiber (7), The reference reflector (2) is disposed between the proximal end (71) and the distal end (72); The sensor system (1), characterized in that the reference reflector is at least 1 mm away from the distal end (72).

2. the reference reflector (2) is located between the proximal end (71) and the sensing reflector (1.1) closest to the distal end (72), or The sensor system (1) according to claim 1, further characterized in that preferably the reference reflector (2) is disposed between the proximal end (7.1) and the sensing reflector (1.1) closest to the proximal end (71).

3. the reference reflector (2) is a fiber Bragg grating, or The sensor system (1) according to claim 1 or 2, further characterized in that the sensing reflector (1.1, 1.2-1.i) is a fiber Bragg grating and the reference reflector (2) is a fiber Bragg grating.

4. an attenuator (21) is disposed at the distal end (72) of the fiber (7); or The sensor system according to any one of claims 1 to 3, further characterized in that the optical detector (5) is arranged at the distal end (72) of the fiber (7).

5. said measuring segments (3.1 to 3.i) being arranged within a sensing area (8); the measuring segments (3.1-3.i) are configured to change their properties in response to a physical property, such as temperature, pressure or strain, within the sensing region (8); The sensor system of claim 4, further characterized in that the attenuator (21) is disposed within the sensing area (8).

6. the attenuator is configured to attenuate incident light by at least -0.1 dB / mm (decibels per millimeter, preferably by more than -2 dB / mm); or A sensor system according to claim 4 or 5, further characterised in that the attenuator has a length of less than or equal to 100 mm, preferably less than or equal to 20 mm.

7. The light source (4) has a modulatable wavelength, The light source (4) is configured to emit light modulated at a modulation rate, 7. The sensor system according to claim 1, further characterized in that the light source (4) is configured to emit light having a coherence length longer than the maximum distance between any of the sensing reflectors (1.1, 1.2-1.i) and the closest one of the at least one reference reflector (2).

8. the bandwidth of said sensing reflector (1.1, 1.2 to 1.i) is at least 1 nm at a wavelength of about 1550 nm; The sensor system (1) according to claim 3, further characterized in that the light source (4) is configured to emit light having a linewidth less than 10 GHz, preferably less than 2 MHz.

9. the reflectivity of the reference reflector (2) is at least twice, preferably five times, the reflectivity of any of the sensing reflectors (1.1, 1.2 to 1.i), or The sensor system (1) according to any one of claims 1 to 7, further characterized in that the reflectivity of the reference reflector (2) is at least half, preferably one fifth, of the reflectivity of any of the sensing reflectors (1.1, 1.2 to 1.i).

10. the modulation rate of said light source (4) is at least 10 Hz, preferably at least 200 kHz; the light source (4) is a distributed feedback diode; The sensor system (1) according to any one of the preceding claims, further characterized in that the photodetector (5) comprises only one light sensitive element.

11. The sensor system (1) comprises an evaluation unit (6), The sensor system (1) according to any one of claims 1 to 10, further characterized in that the evaluation unit (6) is configured to track the relative resonance phase in time upon modulation of the light of the light source (4).

12. Providing at least one optical fiber (7) including at least two sensing reflectors (1.1, 1.2) and at least one reference reflector (2); Providing a modulatable light source (4); Providing a photodetector (5); An optical sensing method comprising: The light source (4) emits light into the proximal end (71) of the fiber (7); the wavelength of the light is modulated across a bandwidth of the light as a function of time; the light is reflected by at least two sensing reflectors (1.1, 1.2-1.i) forming a measurement segment (3.1-3.i) of the optical fiber (7), the sensing reflectors (1.1, 1.2-1.i) being arranged between the distal end (72) and the proximal end (71) of the fiber (7); The light is also reflected by a reference reflector (2), the reference reflector (2) being disposed between the proximal end (71) and the distal end (72) and at least 1 mm away from the distal end (72); a coherence length of the light source (4) is set to be longer than a maximum distance between any one of the sensing reflectors (1.1, 1.2 to 1.i) and the reference reflector (2); said light source (4) being modulated over a bandwidth of the emitted light; A method for optical sensing, wherein the bandwidth of the light is narrower than the bandwidth of the reflectance of the reflector (1.1, 1.2 to 1.i, 2).

13. providing an evaluation unit (6), the evaluation unit (6) tracks the relative resonance phase in time upon modulation of the light of the light source (4); The optical sensing method according to claim 11 , wherein a time-resolved interference pattern including a characteristic resonant peak in amplitude is formed from the reflected optical signal in response to a length between the reference reflector and each sensing reflector.

14. The optical sensing method according to claim 11 or 12, wherein the optical detector (5) comprises only one light sensitive element.

15. the reference reflector (2) is disposed between the proximal end (71) and the distal end (72), the reference reflector being at least 1 mm away from the distal end (72); or the reference reflector (2) is disposed between the proximal end (71) and the sensing reflector (1.1) closest to the distal end (72), the reference reflector being at least 1 mm away from the distal end (72); or The optical sensing method according to any one of claims 11 to 13, wherein the reference reflector (2) is disposed between the proximal end (7.1) and the sensing reflector (1.1) closest to the proximal end (71), and the reference reflector is at least 1 mm away from the distal end (72).

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