Fiber optic system for determining at least one physical parameter and method of using fiber optic system

Abstract

The invention relates to a sensor system (1) comprising: at least one optical fiber (7), a modulatable light source (4) and a photodetector (5); the at least one optical fiber comprises at least two sensing reflectors (1.1, 1.2 to 1. I); the at least one optical fiber comprises at least one reference reflector (2); each of the at least one optical fiber further comprises a distal end (72) and a proximal end (71); wherein the measuring section (3.1 to 3. I) is formed between the two sensing reflectors; wherein the measuring section is arranged between the distal end (72) and the proximal end (71); wherein the light source (4) is arranged at the proximal end (71) of the optical fiber (7); wherein the light source (4) is configured to emit light having a coherence length greater than a maximum distance between any one of the sensing reflectors and any one of the at least one reference reflector (2); wherein the reference reflector (2) is arranged between the proximal end (71) and the distal end (72); wherein the reference reflector is spaced from the distal end (72) by at least 1 mm.

Description

Technical Field

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

[0002] Optical fibers are well known in communication technology and are used to transmit information in the form of light over long distances. However, optical fibers can also be used to detect physical parameters of the environment due to the influence of said parameters on the optical properties of the fiber. Thus, the light emerging from the optical fiber is affected by the physical parameters of the environment through which the fiber passes. Examples of physical parameters are temperature, strain, magnetic field, electric field, etc.

[0003] One way to detect certain physical parameters is to introduce optical elements into the optical fiber that are particularly sensitive to said physical parameters. A well-known method is fiber sensing based on fiber Bragg gratings, which generally refers to the technology that directly uses physical deformations of Bragg gratings (short FBGs) that are fabricated into the core of the optical fiber. These gratings consist of a series of uniform partially reflecting / refractive defects that are spatially separated by a defined period (Λ). When the reflected light resonates with the grating structure, the reflected waves are enhanced (constructive interference). The reflected wavelength λ B (also called the Bragg wavelength) is defined as λ B =2n e Λ, where n e is the refractive index of the fiber core. For non-ideal gratings of finite length, the Bragg wavelength is the central wavelength of the characteristic bandwidth. In general, the greater the number of grating elements, the higher the reflectivity of the grating and the narrower the bandwidth of the reflected wavelengths. When the FBG is deformed due to strain, the Bragg wavelength of the grating shifts. If the grating is stretched, the shift occurs towards the red, or if the grating is compressed, the shift occurs towards the blue. In a typical FBG sensor system, each grating in the fiber sensor has a different Bragg wavelength, allowing them to be easily distinguished spectrally, usually using a diffraction grating and a linear array detector or a wavelength modulated light source and a single detector. With a sufficiently narrow bandwidth, it is possible to track many sensors within a single optical fiber (wavelength division multiplexing, WDM). However, there is usually a trade-off, whereby a higher reflectivity of the grating is required to achieve a sufficiently narrow bandwidth and attenuation of the light passing through the optical fiber, thereby reducing the signal-to-noise ratio.

[0004] Another completely different approach to determining physical properties of the environment using optical fibers and reflectors is Fiber Segment Interferometry, or FSI for short. FSI is a range-resolved interferometry technique originally developed by Cranfield University in the United Kingdom and disclosed in GB2568305A1 and GB2571575A1. FSI and other fiber optic techniques are described in detail in "Range-resolved optical interferometric signal processing", Kissinger, Thomas (thesis, Cranfield University).

[0005] Fiber optic sensors based on FSI technology can be used to measure very small (i.e., parts per billion) changes in the length of an object. It can do this by utilizing a distance-resolved interferometry peak tracking algorithm, as described in the prior art described above and in 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).

[0006] FSI is a measurement technique that uses a coherent light source (e.g., a laser). Light is guided through an optical fiber. The wavelength of the light is modulated at a desired modulation frequency (e.g., up to 200 kHz). The modulated light is preferably launched into the optical fiber via a coupling device (e.g., a 3-port optical circulator). The light is reflected from the optical fiber and returned to a photodetector through the coupling device, such as a conventional optical detector. Figure 1 Of course, those skilled in the art may implement other means of launching light into an optical fiber and directing reflected light from the optical fiber to a photodetector, such as an arrangement of a fiber coupler and / or a beam splitter.

[0007] The optical sensor system includes an optical fiber, a reference reflector at the distal end of the optical fiber, and a plurality of sensing reflectors arranged along the length of the optical fiber between the distal end and the proximal end of the optical fiber. The sensing reflectors together with the reference reflectors form a series of Fabry-Perot optical cavities, also known as Fabry-Perot (FP) or resonators. The distance from the reference reflector to each individual sensing reflector is unique, resulting in each resonator exhibiting a different resonant wavelength.

[0008] In-fiber reflectors can be created by making local periodic variations of the refractive index within the core of an optical fiber. The variation in the refractive index causes light to be reflected according to the resulting Fresnel coefficients. If the period of the structure is half the wavelength of the light passing through it, the reflected waves are in phase or resonant and satisfy the Bragg condition. When illuminated by a light source, these fiber Bragg gratings produce reflected light waves with a reflection profile according to wavelength and bandwidth, whose characteristics depend on the design parameters of the grating, such as the number of periods, refractive index contrast, toe-cut function, chirp, etc.

[0009] Of course, other reflective elements may also be used as sensing reflectors.

[0010] Therefore, the modulated light at the desired modulation frequency launched into the optical fiber is affected by the reflection spectra of the reference reflector and the sensing reflector, wherein the length between the two reflectors is affected 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 temperature, or on the physical strain applied to the fiber Bragg grating. The light is reflected from the optical fiber and returned to the photodetector through a coupling device, such as the prior art Figure 1 The detected signal is digitized, for example at 200 MHz, providing approximately 1000 samples per measurement / modulation cycle.

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

[0012] The optical path length of each measurement segment changes due to strain. This is determined by the difference in path length between the first sensing reflector of the measurement segment to the reference reflector and the second sensing reflector of the measurement segment to the reference reflector. Therefore, optical fiber can be used in strain measurement applications when the optical fiber is firmly attached to another element and the element is subjected to strain. Due to the thermo-optical coefficient (temperature-related change of refractive index) and thermal expansion of the optical fiber, the optical path length of each segment can also change depending on the temperature. For this reason, the optical fiber is usually placed in a suitable smooth-bored sheath material (such as a polymer tube or a glass capillary), which allows the optical fiber to expand freely with temperature. Of course, since the measurement segments are specified along the length of the optical fiber, the measurement segments used only for temperature measurement must be placed in such a sheath material. If the optical fiber is firmly attached to another material and is therefore subjected to strain caused by the thermal expansion and change of the thermo-optical coefficient of the other material, temperature measurement can also be used. Of course, if the optical fiber is bonded to a flexible membrane, the optical fiber can also be used to measure pressure, thereby inducing strain on the optical fiber when the pressure-induced bending of the membrane occurs.

[0013] When the wavelength of the incident light is modulated, the reflected light signal forms a time-resolved interference pattern that contains characteristic resonant peaks in amplitude corresponding to the length of each Fabry-Perot resonator, which consists of a reference reflector and a corresponding sensing reflector.

[0014] The reference reflector according to the known prior art is realized by cutting at the distal end of the optical fiber, as disclosed in Thomas Kissinger, Ricardo Correia, Thomas OH Charrett, Stephen W. James and Ralph P. Tatam, "Fiber Segment Interferometry for Dynamic Strain Measurements", J. Lightwave Technol. 34, 4620-4626 (2016). The mechanical cutting of the end of the optical fiber creates a glass-to-air interface. Such an interface has a Fresnel reflection of about 3.5%.

[0015] However, the reflectors disclosed above are very susceptible to contamination. Furthermore, the maximum reflectivity of the reference reflector is limited by Fresnel reflections unless an additional expensive coating is applied to the cut surface. Even with the coating, 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 measurement. Particularly in applications with limited space, the reference reflector at the far end of the fiber can result in a dead zone. This dead zone is a portion of the fiber that is required due to the reference reflector, but does not provide temperature or strain data by itself. Thus, the fiber is very long with no added value. The additional fiber length containing the reference reflector must be accommodated in the object being measured.

[0017] In addition, the contamination of the cut reference reflector according to the prior art by a fluid medium other than air has the disadvantage that the Fresnel reflection at the interface is affected and the reflectivity of the reference reflector changes. In the case of particle contamination, scattering losses reduce the signal-to-noise ratio of the measurement.

[0018] The object of the present invention is to reduce the above mentioned disadvantages. Another object of the present invention is to create a more compact optical fiber. Summary of the invention

[0019] At least one of these objects is achieved by the features of the independent claims.

[0020] The sensor system of the present invention includes at least one optical fiber, a modulatable light source and a photodetector. At least one optical fiber includes at least two sensing reflectors. At least one optical fiber includes at least one reference reflector. Each of the at least one optical fiber also includes a distal end and a proximal end. The measuring section is formed between the two sensing reflectors. The measuring section is arranged between the distal end and the proximal end. The light source is arranged 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 one of the sensing reflectors and the nearest one of the at least one reference reflector. Otherwise, the reference reflector and the sensing reflector will not be able to function as a Fabry-Perot resonator. Of course, in the case of incorporating more than one reference reflector, the coherence length only has to be longer than the maximum optical distance between any one of the sensing reflectors and one of the reference reflectors to which they refer. Typically, each sensing reflector is referenced by the reference reflector closest to it in terms of optical distance.

[0021] In the following, distances are always optical distances.

[0022] Obviously, the segment formed between the reference reflector and the sensing reflector may also be affected by the physical parameter and may therefore also be used to obtain information about the physical parameter. In addition, in the case of more than one reference reflector, the segment between the two reference reflectors may also be affected by the physical parameter and may therefore also be used to obtain information about the physical parameter. Therefore, the term "measurement segment" does not mean that the measurement capability is limited to the cross-interference of the reference reflector and the two involved sensing reflectors, but only shows the most commonly used segments for measuring physical parameters.

[0023] According to the invention, a reference reflector is arranged between the proximal end and the distal end. The reference reflector is spaced at least 1 mm from the distal end. Replacing the cut-out reference reflector according to the prior art with an internally arranged reference reflector, which is spaced about 1 mm (millimeters) from the distal end of the optical fiber, eliminates the risk of contamination or mechanical damage to the reference reflector. This has the advantage that the fiber-based sensor of the invention is more robust. In addition, the fiber-based sensor of the invention can be easily packaged and / or immersed in a fluid medium.

[0024] It should be noted that the optical system may have more than one reference reflector. In this case, the more than one reference reflector may have a higher and / or lower reflectivity than any sensing reflector.

[0025] In a preferred embodiment, the reference reflector is arranged between the proximal end and the sensing reflector closest to the distal end.The optical fiber then also has the advantage of reducing the dead zone at the distal end of the optical fiber.

[0026] Preferably, the reference reflector is disposed between the proximal end and the sensing reflector closest to the proximal end. This is advantageous compared to the reference reflector being located at the proximal end or in any position between the sensing reflectors because in the event of sensor damage, if the reference reflector is located closer to the proximal end than the damage location of the sensor, the sensor continues to operate before the damage point.

[0027] In a preferred embodiment of the optical fiber, the reference reflector is a fiber Bragg grating. This has the following advantages: The reference reflector can be located at the proximal end of the optical fiber, outside the active sensing area. The reference reflector can even be arranged inside a housing or interrogator device comprising the light source and the photodetector. Thus, the reference reflector is protected from contamination or mechanical damage. In addition, because the fiber Bragg grating is produced in a more controlled manner compared to a cleaved end face, the production quality of the optical fiber is higher and more reproducible.

[0028] In one aspect of the invention, the attenuator is arranged at the distal end of the optical fiber. This is advantageous because unwanted reflections from the distal end of the optical fiber (eg the original cleaved face) are suppressed. The attenuator can be configured to disperse light at the distal end of the optical fiber.

[0029] Preferably, the reference reflector has a higher reflectivity than any of the sensing reflectors. Therefore, due to the relatively high reflectivity of the reference reflector, the cross-interference between the sensing reflectors is relatively weaker than the interference with the reference reflector. However, the reference reflector may also have a lower reflectivity than any of the sensing reflectors to distinguish the cross-interference between the sensing reflector and the reference reflector. In this case, due to the relatively low reflectivity of the reference reflector, the cross-interference between the reference reflector and the sensing reflector is relatively weaker than the interference between the sensing reflectors.

[0030] It should be noted that the names of reference reflectors and sensing reflectors refer only to the designated purpose of the corresponding reflectors. Typically, the reflectivity of any reference reflector 2 is different from the reflectivity of any sensing reflector 1.i to meet the following boundary conditions: the cross-interference can be distinguished in intensity from the cross-interference from any pair of sensing reflectors 1.i, and the distance between any sensing reflector 1.1 to 1.i and the corresponding reference reflector 2 is unique.

[0031] Further advantages and aspects of the invention are disclosed in the embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The drawings used to illustrate the embodiments are schematic representations and show: Figure 1 A schematic diagram of a prior art embodiment of a sensor system, Figure 2 A schematic diagram of an embodiment of a sensor system, Figure 3 A schematic diagram of another embodiment of a sensor system, Figure 4 A schematic diagram of another embodiment of a sensor system, Figure 5 A schematic diagram of another embodiment of a sensor system, Figure 6 A schematic diagram of another embodiment of a sensor system, Figure 7 a schematic diagram of another embodiment of a sensor system, and Figure 8 Schematic diagram of another embodiment of a sensor system. DETAILED DESCRIPTION

[0033] Figure 1 The prior art of a sensor system 1 is shown. At least one optical fiber 7 is shown attached to a modulatable light source 4 with a proximal end 71. The light source 4 is preferably modulatable in wavelength. The light source is configured to emit light to a coupling device 9, such as a 3-port optical circulator 9. The coupling device is configured to transmit the light emitted from the light source into the optical fiber 7 and transmit the reflected light from the optical fiber 7 to the photodetector 5. The optical fiber 7 comprises sensing reflectors 1.1 to 1.i, where i>1, The reference reflector 2 is arranged at the distal end 72 of the optical fiber 7 .

[0034] The optical fiber 7 comprises measuring sections 3.1 to 3.i, where i>1, The measuring sections 3.1 to 3.i are arranged between the distal end 72 and the proximal end 71, wherein each measuring section 3.1 to 3.i is formed between two sensing reflectors 1.1 to 1.i. The light source 4 is configured to emit light having a coherence length longer than the maximum distance between any one of the sensing reflectors 1.1 to 1.i and the reference reflector 2. In an exemplary embodiment of the prior art, the maximum distance is shown between the sensing reflector 1.1 and the reference reflector 2. The measuring sections 3.1 to 3.i are arranged in 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 measuring areas 3.1 to 3.i can be determined. In order to determine the physical parameters in the sensing area 3.n (n∈i), the reflection from the Fabry-Perot resonator according to the modulation wavelength is determined, wherein the Fabry-Perot resonator is composed of the sensing reflector 3.n (n∈i) and the reference reflector 2, and the reflection from the Fabry-Perot resonator according to the modulation wavelength is determined, wherein the Fabry-Perot resonator is composed of the sensing reflector 3.n+1 (n∈i) and the reference reflector 2.

[0035] Figure 2 An embodiment of a sensor system 1 according to the invention is shown. Figure 1 Elements with the same functions as in the prior art described in the accompanying drawings are denoted by the same reference numerals.

[0036] exist Figure 2 In the shown embodiment of the sensor system 1 according to the invention, the reference reflector 2 is arranged between the proximal end 71 and the sensing reflector 1 . 1 closest to the proximal end 71. This arrangement of the reference reflector 2 is therefore completely different from the prior art.

[0037] Figure 3 An embodiment of a sensor system 1 according to the invention is shown. Figure 1 Elements with the same functions as in the prior art described in the accompanying drawings are denoted by the same reference numerals.

[0038] exist Figure 3 In the illustrated embodiment of the sensor system 1 according to the invention, the reference reflector 2 is arranged 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 arranged between the sensing reflectors 1.5 and 1.6, but can just as easily be arranged elsewhere between the sensing reflectors 1.6 and the sensing reflectors 1.i, or between the sensing reflectors 1.i and the coupling device 9.

[0039] Figure 4 An embodiment of a sensor system 1 according to the invention is shown. Figure 1 Elements with the same functions as in the prior art described in the accompanying drawings are denoted by the same reference numerals.

[0040] exist Figure 4 In the embodiment of the sensor system 1 shown, the reference reflector 2 is arranged between the proximal end 71 and the distal end 72. The reference reflector 2 is spaced at least 1 mm from the distal end 72. In this embodiment, the reference reflector 2 is arranged between the sensing reflector 1.i and the distal end 72 and is spaced about 1 mm from the distal end. For clarity, the optional attenuator 21 is not shown.

[0041] Figure 5 An embodiment of a sensor system 1 according to the invention is shown. Figure 1 Elements with the same functions as in the prior art described in the accompanying drawings are denoted by the same reference numerals.

[0042] exist Figure 5 In the embodiment of the sensor system 1 shown, the reference reflector 2 is arranged between the proximal end 71 and the distal end 72, but it can also be arranged according to Figures 1 to 4 The embodiment shown is placed at any position. In this embodiment, the photodetector 5 is placed at the distal end 72. The photodetector 5 can be directly coupled to the distal end 72, for example by splicing. The photodetector 5 can also be arranged to be separated 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-reflection coating known to those skilled in the art.

[0043] Figure 6 An embodiment of a sensor system 1 according to the invention is shown. Figure 1 Elements with the same functions as in the prior art described in the accompanying drawings are denoted by the same reference numerals.

[0044] exist Figure 6In the illustrated embodiment of the sensor system 1 according to the invention, three optical fibers 7, 7', 7". are shown as an example. The optical fibers are coupled by coupling devices 9', 9". In this example, the coupling devices 9', 9" may be 3-port circulators. The reflected light introduced into the first optical fiber 7 is guided into the second optical fiber 7'. The reflected light from the second optical fiber upstream of the coupling device 9" is guided into the third optical fiber 7". The reflected light from the third optical fiber 7'" is guided back to the photodetector 5 via all coupling devices 9, 9', 9". Preferably, but not necessarily, the first optical fiber 7 and the second optical fiber 7' are equipped with a high reflective coating 22, 22' or a mirror 22, 22' at the respective distal ends 72, 72' to ensure a high intensity of the light introduced into all optical fibers 7, 7', 7". The third optical fiber 7" is preferably equipped with an attenuator 21 at the distal end to prevent light that is not reflected by the reference reflector 2, 2', 2" or any of the sensing reflectors 1.1 to 1.i from reaching the photodetector. In this embodiment of the sensor system 1, the optical fibers are arranged in a "daisy chain" arrangement. This embodiment also shows three reference reflectors 2, 2', 2", each of which is arranged in the position of one of the three optical fibers. In this way, the total optical length of the optical fiber can be extended to be longer than the coherence length of the light source 4. It should be understood that a different number of optical fibers 7 can also be used, such as two optical fibers 7 or more than three optical fibers 7. In addition, a different number of reference reflectors 2 can be used. Or the reference reflector 2 and / or the photodetector 5 can be connected with Figures 2 to 5 The exemplary embodiments shown in FIG. 8 are similarly arranged differently within the corresponding optical fibers 7 .

[0045] Figure 7 An embodiment of a sensor system 1 according to the invention is shown. Figure 1 Elements with the same functions as in the prior art described in the accompanying drawings are denoted by the same reference numerals.

[0046] exist Figure 7 In the illustrated embodiment of the sensor system 1 according to the invention, two optical fibers 7, 7' are shown as an example. The optical fibers are coupled by a coupling device 9'. In this example, the coupling device 9' can be a 3-port circulator or a beam splitter. In the case of the beam splitter 9', the incident light is introduced simultaneously into the first optical fiber 7 and the second optical fiber 7'. The sensing reflectors 1.1 to 1.i are arranged in the first optical fiber 7', while one reference reflector 2 is arranged in the second optical fiber. The reflected light from the reference reflector 2 and the reflected light from the sensing reflectors 1.1 to 1.i are guided back to the photodetector. Cross-interference is obtained in the same way as described for the single optical fiber 7 embodiment in the prior art. Of course, the coupling device 9' can also be chosen as a 3-port optical circulator, in which case the two optical fibers 7, 7' are again daisy-chained, as already described for Figure 6It should be understood that a different number of optical fibers 7 may be used, for example two optical fibers 7 or more than three optical fibers 7. In addition, a different number of reference reflectors 2 may be used. Alternatively, the reference reflector 2 and / or the photodetector 5 may be connected to Figures 2 to 5 The embodiments shown in are similarly arranged differently within the corresponding optical fibers 7. A person skilled in the art will appreciate that Figure 5 and from Figure 6 Combinations of embodiments are also possible. The sensing reflectors 1.1 to 1.i and the at least one reference reflector 2 can be distributed among a plurality of optical fibers 7, as long as boundary conditions are met so that the distance between any sensing reflector 1.1 to 1.i and the corresponding reference reflector 2 is unique, as has been described for the prior art.

[0047] Figure 8 An embodiment of a sensor system 1 according to the invention is shown. Figure 1 Elements with the same functions as in the prior art described in the accompanying drawings are denoted by the same reference numerals.

[0048] exist Figure 8 In the illustrated embodiment of the sensor system 1 according to the invention, one optical fiber 7 is again illustrated. The optical fiber 7 exemplarily comprises a total of three reference reflectors 2, 2', 2". In this way, the total optical length of the optical fiber 7 can be extended to be longer than the coherence length of the light source 4, as long as the distance between any one of the sensing reflectors 1.1 to 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 a different number of optical fibers 7 can also be used, such as Figure 6 or Figure 7 In the embodiment shown in FIG. 1 , each optical fiber 7 , 7 ′, 7 ″ has more than one reference reflector 2 .

[0049] In the following, different aspects of the invention are described which, unless explicitly stated otherwise, are compatible with all the embodiments shown in the drawings and described above.

[0050] In one aspect 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 optical fiber 7, away from the active sensing area 8. The reference reflector can even be arranged inside a housing or interrogator device comprising the light source 4 and the photodetector 5. Thus, the reference reflector 2 is protected from contamination or mechanical damage. In addition, because the fiber Bragg grating is produced in a more controlled manner compared to cutting, the production quality of the optical fiber 7 is higher and more reproducible.

[0051] In one aspect of the sensor system 1, the attenuator 21 is arranged at the distal end 72 of the optical fiber 7. This is advantageous because unwanted reflections from the distal end 72 of the optical fiber are suppressed. The attenuator 21 can be configured to disperse light at the distal end 72 of the optical fiber. Although advantageous, it is not necessary for the present invention to provide an attenuator at the distal end of the optical fiber. This aspect is similar to Figure 5 The embodiment shown with the photodetector 5 arranged at the distal end 72 is not compatible. Alternatively, the reflectivity of the reference reflector 2 according to the invention can be chosen differently from the potential reflections from the distal end 72 to distinguish the interference of the reference reflector 2 and the sensing reflectors 1.i of the invention from the interference of the potential reflections and the sensing reflectors 1.i from the distal end 72. Contamination of the distal end does not change the reference, since the reflections from the distal end are not used for measurement.

[0052] Preferably, the reference reflector 2 has a higher reflectivity than any of the sensing reflectors 1.i. Therefore, due to the relatively high reflectivity of the reference reflector 2, the cross-interference between the sensing reflectors 1.i is relatively weaker than the cross-interference with the reference reflector 2. However, the reference reflector 2 may also have a lower reflectivity than any of the sensing reflectors 1.i to distinguish the cross-interference between the sensing reflectors 1.i and the reference reflector 2. In this case, due to the relatively low reflectivity of the reference reflector 2, the cross-interference between the reference reflector 2 and the sensing reflectors 1.i is relatively weaker than the interference between the sensing reflectors 1.i.

[0053] In another aspect of the sensor system 1, the measuring sections 3.1 to 3.i are arranged in the sensing region 8. The measuring sections 3.1 to 3.i are configured to change their properties depending on a physical property (e.g. temperature, pressure or strain) within the sensing region 8. The attenuator 21 is arranged in the sensing region 8. This has the advantage that the optical fiber 7 is shorter than a comparable prior art optical fiber 7. No additional dead space at the far end needs to be taken into account in the sensing region 8.

[0054] According to another aspect of the sensor system 1, the attenuator 21 is formed by a coreless optical fiber. This has the advantage of being cost-effective, since splicing the coreless optical fiber to the distal end 72 is easily achievable. Splicing is the process of connecting two optical fiber elements. The coreless optical fiber is, for example, a glass rod.

[0055] 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 has a length of no more than 100 mm, preferably no more than 20 mm. This is advantageous because only a short excess length of optical fiber is required to effectively suppress unwanted potential reflections from the distal end 72. Possible embodiments of such absorbers for attenuating / suppressing back reflections include: absorbers, optical isolators or crossed polarizers, refractive index matching materials, diffusers / scatterers, anti-reflection (dielectric) coatings, fiber tapers, melting and / or shaping the ends of optical fibers, or multimode optical fibers.

[0056] According to another aspect of the sensor system 1, the sensing reflectors 1.1 to 1.i are fiber Bragg gratings. This is advantageous because the sensing reflectors 1.1 to 1.i and the reference reflector 2 can be introduced into the optical fiber using the same production technology. This allows efficient production of the optical fiber.

[0057] According to another aspect of the sensor system 1, the bandwidth of the sensing reflectors 1.1, 1.i is ideally 40 nm at a central wavelength of about 1550 nm. However, the invention is also feasible for a very wide bandwidth range, as long as the bandwidth of the sensing reflectors 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 with respect to the difference between the start wavelength and the end wavelength of the light source 4. Typically, the bandwidth of the sensing reflectors 1.1, 1.i is typically not less than 1 nm. This is advantageous, since a light modulation bandwidth up to the bandwidth of the reflectors can be used. However, a higher modulation bandwidth increases the accuracy of the sensor system 1, since more wavelength-dependent interferences can be determined from the physical properties on the measurement area 3.i.

[0058] Of course, other central wavelengths may also be used. Wavelengths between approximately 1290 nm and 1620 nm are good choices because suitable light sources are available at relatively low cost. In addition, optical fibers are typically optimized to have the lowest attenuation at these wavelengths.

[0059] According to another aspect of the sensor system 1, the light source 4 is configured to emit light having a line width of less than 10 GHz, preferably less than 2 MHz. The coherence length of the light source increases with the inverse of the bandwidth, thereby allowing a longer distance between the reference reflector 2 and the farthest sensing reflector 1.i. The line width of the light source 4 depends on the application in which the sensor system 1 is used. For example, 1 MHz would equal a coherence length of approximately 200 m, which is suitable for sensor systems 1 used in bridges or tunnels. 10 GHz would result in a coherence length of approximately 20 mm, i.e., applications in limited or small spaces.

[0060] According to another aspect of the sensor system 1, the light source 4 is configured to emit light modulated at a modulation rate; and the light source 4 is configured to scan from a starting wavelength to an ending wavelength during the reciprocating modulation rate; and the starting wavelength differs from the ending wavelength by at least 3 pm (picometers). The difference between the starting wavelength and the ending wavelength depends on the required measurement resolution. With a 300 pm modulation depth, a resolution of 10 mm can be achieved. For large structures that only require a 1 m resolution, 3 pm is sufficient.

[0061] According to another aspect of the sensor system 1, the reflectivity of the reference reflector 2 is at least twice the reflectivity of any sensing reflector 1.1, 1.2 to 1.i. This has the advantage that the wavelength-dependent optical interference between the reference reflector 2 and any sensing reflector 1.i can be easily correlated with the wavelength-dependent optical interference between any two sensing reflectors 1.i, 1.j (i, j>1, i≠j, ) are separated by the wavelength-dependent interference zone.

[0062] Preferably, the reflectivity of the reference reflector 2 is at least five times the reflectivity of any sensing reflector 1.1 to 1.i. This can be easily achieved by using a fiber Bragg grating as the reference reflector 2 and by using a fiber Bragg grating as the sensing reflector. The reflectivity of the fiber Bragg grating can be easily adjusted by the form and shape of the grating. It has been recognized that the factor of 5 is conducive to easily distinguishing the interference between the reference reflector 2 and any sensing reflector 1.i and any two sensing reflectors 1.i, 1.j (i,j>1, i≠j, ) between them.

[0063] 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 sensing reflector 1.1, 1.2 to 1.i. It has been recognized that a reference reflector 2 having a lower reflectivity also has the advantage that the wavelength-dependent optical interference between the reference reflector 2 and any sensing reflector 1.i can be easily correlated with the wavelength-dependent optical interference between any two sensing reflectors 1.i, 1.j (i, j>1, i≠j, ) are separated by the wavelength-dependent interference zone.

[0064] In another embodiment, the reflectivity of any reference reflector 2 is different from the reflectivity of any sensing reflector 1.i, as long as the cross-interference is distinguishable in intensity from the cross-interference from any pair of sensing reflectors 1.i.

[0065] It should be noted that the names of the reference reflectors 2, 2', 2" and the sensing reflectors 1.i refer only to the designated purpose of the corresponding reflectors. Therefore, in an embodiment having several reflectors with different reflectivities, the reflector with the highest reflectivity can be selected as the reference reflector 2, and all other reflectors are sensing reflectors, as long as the above-mentioned boundary conditions of coherence length, distinguishable cross-interference and unique length between the reference reflector 2 and the sensing reflectors 1.i are met. Alternatively, the reflector with the lowest reflectivity can be selected as the reference reflector 2. Alternatively, several reflectors with the highest or lowest reflectivity can be selected as the reference reflectors 2, 2', 2", where a combination of the highest and lowest reflectivities can also be selected. Alternatively, a reflector with an intermediate reflectivity can be selected as the reference reflector 2, as long as the cross-interference of the sensing reflector 1.i with the reference reflector can be distinguished in intensity from the cross-interference from any pair of sensing reflectors 1.i.

[0066] 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 phase of the resonance over time when modulating the light of the light source 4. This is advantageous with respect to typical FBG based sensors, since for FSI based sensing methods only tracking the phase change of each interference peak with respect to a reference reflector is necessary.

[0067] According to another aspect 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 fast data acquisition, since only the phase change of each interference peak relative to the reference reflector needs to be determined. This can be implemented with a prior art FPGA (field programmable gate array). No complex detector arrays are required. In this way, high-frequency vibrations or acoustic signals can also be determined in the measurement zone, thereby applying high-frequency strains on the optical fiber 7.

[0068] According to another aspect of the sensor system 1 , the light source 4 is a distributed feedback diode.

[0069] According to another aspect of the sensor system 1, the photodetector 5 comprises exactly one single photosensitive element. Distributed feedback diode lasers are relatively cheap and robust. FSI based sensor systems do not require expensive external cavity lasers.

[0070] An optical sensing method for determining a physical property such as temperature or strain in a sensing area 8 comprises the following steps: providing at least one optical fiber 7, which comprises at least two sensing reflectors 1.1, 1.2 and at least one reference reflector 2; furthermore, providing a light source 4 with a modulatable wavelength; and providing a photodetector 5. The light source 4 emits light into the proximal end 71 of the optical fiber 7; wherein the wavelength of the light is modulated over a wavelength bandwidth according to time, wherein the light is reflected by at least two sensing reflectors 1.1, 1.2 to 1.i forming a measuring section 3.1 to 3.i of the optical fiber 7, whereby the sensing reflectors 1.1, 1.2 to 1.i are arranged between the distal end 72 and the proximal end 71 of the optical fiber 7; wherein the light is also reflected at a reference reflector 2 arranged between the proximal end 71 and the sensing reflectors 1.1, 1.2 to 1.i closest to the proximal end 71; wherein the coherence length of the light source 4 is set to be longer than the maximum distance between any one of the sensing reflectors 1.1, 1.2 to 1.i and the reference reflector 2; and wherein the light source 4 is modulated over a bandwidth of the emitted light. The bandwidth of the light is narrower than the bandwidth of the reflectors 1.1, 1.2 to 1.i.

[0071] In another embodiment of the sensor system 1, instead of a wavelength modulatable light source, another suitable multiplexing technique may be used. The multiplexing technique may include signal multiplexing, such as code division multiplexing, time division multiplexing or any other suitable multiplexing technique and light source. It should be noted that in this case, the light source may include multiple laser sources inside the light source, which are coupled to one optical fiber outlet of the light source. The multiplexing technique is suitable as long as the different lasers of the light source can be distinguished from each other in the interference signal.

[0072] Embodiments having combinations of aspects of the above-described embodiments are also explicitly included among the possible embodiments of the sensor system 1 . Reference numerals list 1.1 to 1.i(i>1; )Measurement reflector 2, 2', 2" reference reflectors 21 Attenuator 22 Highly reflective device, mirror 3.1 to 3.i(i>1; ) Measurement section 4. Light source, laser 5 Photodetector 6 evaluation units 7, 7', 7" optical fiber 71, 71', 71" near end 72, 72', 72" far end 8 sensing areas 9, 9', 9" coupler, 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); At least one optical fiber (7) includes at least two sensing reflectors (1.1, 1.2 to 1.i); and at least one optical fiber (7) includes at least one reference reflector (2); each of the at least one optical fiber (7) also includes a distal end (72) and a proximal end (71); wherein a measuring section (3.1 to 3.i) is formed between the two sensing reflectors (1.1 to 1.i); wherein the measuring section (3.1 to 3.i) is arranged between the distal end (72) and the proximal end (71); wherein the light source (4) is arranged at the proximal end (71) of the optical fiber (7); characterized in that the reference reflector (2) is arranged between the proximal end (71) and the distal end (72); and the reference reflector is spaced at least 1 mm from the distal end (72).

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

3. The sensor system (1) according to any one of claims 1 to 2, further characterized in that: The reference reflector (2) is a fiber Bragg grating; or the sensing reflectors (1.1, 1.2 to 1.i) are fiber Bragg gratings and the reference reflector (2) is a fiber Bragg grating.

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

5. The sensor system according to claim 4, further characterized in that: The measuring sections (3.1 to 3.i) are arranged in a sensing area (8); and the measuring sections (3.1 to 3.i) are configured to change their properties according to a physical property within the sensing area (8), such as temperature, pressure or strain; and the attenuator (21) is arranged in the sensing area (8).

6. The sensor system according to any one of claims 4 or 5, further characterized in that: The attenuator is configured to attenuate incident light by at least -0.1 dB / mm (decibel per millimeter), preferably greater than -2 dB / mm; or the attenuator has a length of not greater than 100 mm, preferably not greater than 20 mm.

7. The sensor system according to any one of claims 1 to 6, further characterized in that: The light source (4) has a modulatable wavelength; the light source (4) is configured to emit light modulated at a modulation rate; and the light source (4) is configured to emit light having a coherence length longer than a maximum distance between any one of the sensing reflectors (1.1, 1.2 to 1.i) and the nearest one of the at least one reference reflector (2).

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

9. 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 twice, preferably five times, the reflectivity of any sensing reflector (1.1, 1.2 to 1.i); or the reflectivity of the reference reflector (2) is at least half, preferably one fifth, the reflectivity of any sensing reflector (1.1, 1.2 to 1.i).

10. The sensor system (1) according to any one of claims 1 to 8, further characterized in that: The modulation rate of the light source (4) is at least 10 Hz, preferably at least 200 kHz; the light source (4) is a distributed feedback diode; and the photodetector (5) comprises exactly one single photosensor.

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

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

13. The optical sensing method according to claim 11, further comprising the following steps: - providing an evaluation unit (6); - wherein the evaluation unit (6) tracks the relative phase of the resonance over time when modulating the light of the light source (4); and wherein a time-resolved interference pattern is formed from the reflected light signal containing a characteristic resonance peak in amplitude, which corresponds to the length between the reference reflector and the corresponding sensing reflector.

14. The optical sensing method according to any one of claims 11 or 12; wherein: The photodetector (5) comprises exactly one single photosensitive element.

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

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