Optical sensing method and optical sensor system for optical sensing method

Abstract

To provide a method for solving a problem of a conventional sensing method, and an optical sensor system 100.SOLUTION: According to the present invention, an optical sensor system (100) adapted for a sensing method using fiber segment interferometry is provided, wherein at the distal end of the fiber (73) a sensing element (1. I) is arranged having a thermo-optic coefficient larger than the thermo-optic coefficient of the optical fiber (73. I).SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an optical sensing method and an optical sensor system for the optical sensing method. [Background technology]

[0002] Measurement of physical parameters such as temperature, pressure, strain, and force using optical sensing methods typically relies on the physical parameter-dependent measurement of optical path length. Optical fibers, commonly known in communication technology for transmitting information in the form of light over long distances, are typically subject to the temperature and other physical parameters of the surrounding environment. As a result, the light emitted from an optical fiber is affected by the physical parameters of the environment through which the optical fiber passes. Physical parameters include, for example, temperature, strain, magnetic fields, electric fields, and the like.

[0003] A way to probe a particular physical parameter is to introduce an optical element into the optical fiber that is particularly sensitive to that physical parameter. One well-known approach is fiber optic sensors based on fiber Bragg gratings, which refers to a technique that directly uses the physical deformation, due to temperature or strain, of a Bragg grating, or FBG for short, typically fabricated in the core of an optical fiber. These gratings consist of a uniform series of partially reflective / refractive defects, spatially separated by a defined period. Depending on the wavelength of the incident light, the light is either reflected or transmitted. If the light is in resonance with the grating structure, the reflected waves will constructively interfere (constructive interference). The reflected wavelength λ, also known as the Bragg wavelength, is B is λ B =2n e Λ is defined as n eis the refractive index of the fiber core. For a nonideal grating of finite length, the Bragg wavelength is the central wavelength of the characteristic bandwidth. Generally, the greater the number of grating elements, the higher the grating's reflectivity and the narrower the bandwidth of the reflected wavelengths. When the FBG deforms due to strain, the Bragg wavelength of the grating shifts. In a typical FBG sensor system, each grating in the fiber sensor has a different Bragg wavelength, and these different Bragg wavelengths can be easily distinguished spectroscopically, typically using a diffraction grating and a linear array detector, or a wavelength-modulated light source and a single detector. In these sensors, the linewidth of the Bragg grating, i.e., the reflected wavelength range, is much narrower than the wavelength range of the incident light. These sufficiently narrow bandwidths allow multiple sensors to be tracked within a single fiber. However, this fiber sensor concept is not suitable for many applications, especially temperature measurement, where more localized measurements are preferred. In particular, the need to bend the fiber to reach various measurement locations induces strain in the fiber and introduces errors into the temperature measurement. Applications such as installation in electric vehicle battery packs often require sensors to be routed around individual battery cells, requiring multiple bends.

[0004] Another entirely different method for determining the physical properties of an environment using optical fibers and reflectors is fiber segment interferometry, or FSI for short. FSI is a distance-resolved interferometry technique originally developed at Cranfield University in the UK and disclosed in GB2568305A1 and GB2571575A1. FSI and other fiber techniques are described in detail in "Range-resolved optical interferometric signal processing," by Thomas Kissinger (Cranfield University).

[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. This can be achieved by utilizing distance-resolved interferometry peak-tracking algorithms, as described in the state-of-the-art paper mentioned above and in Thomas Kissinger, Ricardo Correia, Thomas O.H. Charrett, Stephen W. James, and Ralph P. Tatam, "Fiber Segment Interferometry for Dynamic Strain Measurements," J. Lightwave Technol. 34, pp. 4620–4626 (2016).

[0006] FSI is a measurement technique that employs a coherent light source, such as a laser. The light is guided through an optical fiber. The light is wavelength-modulated at a desired modulation frequency, such as up to 200 kHz. The modulated light is then coupled into the optical fiber, preferably via a coupling device, such as a three-port optical circulator, or simply called a circulator. The light reflects from the optical fiber and returns to the photodetector through the coupling device.

[0007] The optical sensor system includes an optical fiber, a reference fiber Bragg grating (FBG) at the distal end of the optical fiber, and multiple sensing FBGs arranged along the length of the optical fiber between the distal and proximal ends of the optical fiber. The sensing FBGs, together with the reference FBG, form a series of Fabry-Perot optical cavities, also known as Fabry-Perot (FP) interferometers or FP resonators. Because the distance from the reference FBG to each individual sensing FBG is unique, each resonator exhibits a different resonant wavelength. Optical fiber is also referred to as fiber for short.

[0008] When two sensing FBGs and the fiber between them are subjected to a physical parameter, the local physical parameter is determined. In this case, the measurement is based on the interference of an FP cavity consisting of a reference FBG and a first sensing FBG, and the interference of an FP cavity consisting of a reference FBG and a second sensing FBG. From the relative change in these two interference patterns, the physical parameter-induced optical change in the fiber between the first FBG and the second FBG is determined.

[0009] In contrast to the direct effect of FBGs on the Bragg wavelength, FSI uses broadband FBGs with a reflected wavelength range wider than the modulation range of the incident light wavelength. The detected signal is digitized at, for example, 200 MHz, providing approximately 1000 samples per measurement / modulation cycle.

[0010] Tracking the relative phase of the resonances over time for each modulation cycle yields the relative distance, also known as the optical path length, between each sensing and reference FBG multiplied by the refractive index of the fiber. In what follows, the distance is always the optical distance.

[0011] 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 in the measurement segment and the path length from the second sensing reflector to the reference reflector in the measurement segment. Therefore, optical fibers can be used in strain measurement applications when they are rigidly attached to another element and that 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 refractive index change) and the thermal expansion of the fiber. For this reason, fibers are typically placed inside a suitable smooth, perforated sheath material, such as a polymer tube or glass capillary, which allows the optical fiber to expand freely with temperature.

[0012] Temperature measurement has the disadvantage that the entire measurement segment must be located within the measurement area. This means that the fiber must enter the measurement area and then exit it again. Particularly in environments where multiple temperatures are to be detected at different locations, this detection can only be achieved by bending the fiber multiple times. However, bending or curving the fiber can introduce additional strain into the measurement segment, which can interfere with temperature measurement. In addition, the thermo-optic coefficient of the fiber only changes slightly as a function of temperature, resulting in a weak measurement signal. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] GB2568305A1 [Patent Document 2] GB2571575A1 [Non-patent literature]

[0014] [Non-Patent Document 1] "Range-resolved optical interferometric signal processing," Kissinger, Thomas (Thesis, 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, pp. 4620-4626 (2016) Summary of the Invention [Problem to be solved by the invention]

[0015] It is an object of the present invention to alleviate the above-mentioned drawbacks.A further object of the present invention is to use FSI techniques to create a well-defined temperature measurement area. [Means for solving the problem]

[0016] At least one of the problems is solved by the features of the independent claims.

[0017] The optical sensing method of the present invention includes: providing at least one optical fiber segment, each optical fiber segment having a proximal end, a distal end, and an optical fiber; providing a reference reflector disposed within an optical fiber of at least one optical fiber segment between a proximal end and a distal end; providing a sensing element disposed at a distal end of at least one optical fiber segment, the sensing element having a thermo-optic coefficient greater than the thermo-optic coefficient of the fiber; providing a first reflective surface of the sensing element; providing a second reflective surface of the sensing element, the first reflective surface being parallel to the second reflective surface; providing a photodetector; providing an analyzer; Light is emitted into the proximal end of each optical fiber segment and propagates from the proximal end to the distal end. The light is characterized by a center wavelength and a linewidth. The center wavelength is wavelength-modulated as a function of time over a modulation range. The bandwidth of the modulation range is greater than the linewidth of the light. The light detected by the detector is analyzed by an analyzer, and the temperature of the sensing element is established based on the effect of the thermo-optic coefficient of the sensing element on the detected light. The sensing element may be an optical window, a so-called Fabry-Perot etalon, or a sheet of material transparent to the optical wavelength used. The term transparent refers to a material with an attenuation coefficient that follows the Beer-Lambert law, resulting in an optical loss of less than -30 dB between the light source and the detector.

[0018] The thermo-optic coefficient, also called the thermo-optic coefficient, is understood to be the change in refractive index as a function of temperature in a material, such as an optical sensing element or an optical fiber. The thermo-optic coefficient also depends on the current temperature of the material.

[0019] The first reflecting surface has a broadband reflectivity that partially reflects all wavelengths in the modulation range. The second reflecting surface has a broadband reflectivity that partially or completely reflects all wavelengths in the modulation range. The reference reflector has a broadband reflectivity that at least partially reflects all wavelengths in the modulation range. Therefore, the reflective wavelength range of the reference reflector is wider than the modulation range.

[0020] The "reflection wavelength range" is also called the "reflection range" or "reflective range" for short.

[0021] The sensing element is placed at the location where the temperature is to be determined. Because the thermo-optic coefficient of the sensing element is larger than that of the fiber, the light that propagates twice through the sensing element due to reflection at the second reflective surface is subject to the temperature-dependent thermo-optic coefficient, while the light reflected at the first reflective surface is not. In addition to the thermo-optic coefficient, the length of the sensing element also changes with temperature due to its thermal expansion coefficient. Both effects make it possible to analyze the optical distance between the reference reflector and the first reflective surface, as well as the optical distance to the second reflective surface of the reference reflector, which is strongly temperature-dependent, similar to the FSI method described above.

[0022] In contrast to methods using only one wavelength, where only the relative length change between the reference and sensing reflectors can be detected, modulating the wavelength to track the relative phase of the resonance in time as the source light is modulated is advantageous because FSI-based sensing methods only need to track the phase change of each interference peak relative to the reference reflector.

[0023] Placing a dedicated sensing element at the distal end of the optical fiber segment at the measurement location where the temperature is to be detected is advantageous because the optical fiber segment only enters the measurement location. The optical fiber segment terminates at the measurement location, so the optical fiber does not have to follow a curvature to re-exit the measurement location. Additionally, a sensing element with a thermo-optic coefficient larger than that of the fiber improves measurement accuracy compared to optical sensors where temperature is detected based on the thermo-optic coefficient of the fiber.

[0024] The optical sensor system used in the FSI-based optical sensing method may include an InGaAs photodetector, and preferably an optical fiber circulator, and preferably a light source, e.g., an InGaAs photodetector with a central wavelength of 1550.0 nm (nm refers to nanometers, 10 -9 The linewidth is 800 kHz (equivalent to 0.0064 pm, where pm stands for picometer, and 10 -12The laser may comprise an Eblana EP1550DM-B laser diode, having a wavelength of λ0 (meaning 1.5 GHz) and emitting at a center wavelength λ0 = 1550.0 nm. The laser may be injection current modulated, preferably with a sinusoidal waveform, resulting in a peak-to-peak wavelength modulation amplitude of approximately 0.3 nm as measured using an optical spectrum analyzer.

[0025] Here, it can be seen that the fabricated reference reflection gratings typically have a reflectivity of 1% to 50% and a full width at half maximum bandwidth of about 4 nm to 5 nm.

[0026] Preferably, the sensing method is implemented using a reference reflector implemented as a fiber Bragg grating (FBG). In FSI, it is essential that the reference reflector have a broadband reflectivity, such that it partially reflects all wavelengths in the modulation range. The reflection range of the reference reflector is wider than the modulation range of the light in all embodiments of the sensor system and all embodiments of the method. The introduction of the FBG into the fiber is achieved by inscribing the FBG into the photosensitive core of the fiber. Although the present invention is not limited in any way, one method is to use a frequency-quadrupled Nd:YAG laser operating at 266 nm, or a single-mode fiber-coupled diode laser, or a femtosecond inscription process in the visible wavelength range. The broadband reflectivity of the FBG is achieved, for example, by limiting the length of the FBG and thus broadening the reflection bandwidth. Preferably, the FBG has a reflectivity curve with a FWHM (full width at half maximum) of at least 2 nm, particularly preferably 4 nm, around the central wavelength of the light source. The light source may particularly preferably be a laser. This allows reflection of all wavelengths in the modulation range.

[0027] Preferably, the maximum reflectance of the reference reflector is at least 1%.

[0028] The sensing element is provided as a sheet of optically transparent material. It is proposed that the optically transparent material has an optional first reflective coating that forms a first reflective surface. This is advantageous because the reflectivity of the first reflective surface is not only given by the refractive index of the sensing element, also called refraction index, but the first reflective coating makes it possible to adjust the reflectivity of the first reflective surface.

[0029] Preferably, the reflectivity of the first reflective surface with the first reflective coating is such that the FWHM (full width at half maximum) of its reflectivity curve is at least 2 nm, and particularly preferably 4 nm, around the central wavelength of the light source. This allows at least partial reflection of all wavelengths in the modulation range. Adjusting the reflectivity of the first reflective surface is beneficial for achieving a good signal-to-noise ratio of the interference resulting from the light reflected by the reference reflector and the first reflective coating.

[0030] Also preferably, the reflectivity of the reference reflector is less than half or more than twice the reflectivity of the first reflecting surface, which is beneficial because interference that may arise from different sensing elements 1.i, first surface 2.i or second surface 3.i, can be easily distinguished from interference arising from the reference reflector 9.i together with either the first surface 2.i or the second surface 3.i.

[0031] Preferably, the optically transparent material has a second reflective coating that forms a second reflective surface. This is beneficial because the reflectivity of the second reflective surface can be adjusted by the second reflective coating, rather than being determined solely by the refractive index of the sensing element. For the second reflective surface, a reflectivity greater than 90% (900,000 ppm) is beneficial because light passing through the second reflective surface may be reflected elsewhere in the environment, creating unwanted reflections of light. However, a lower reflectivity similar to that of the reference reflector or the first reflective surface is also acceptable.

[0032] The sensing element, which is a sheet of optically transparent material, preferably has a refractive index of at least 2.00 at the central wavelength of the light, and preferably a refractive index of at least 3.4 at the central wavelength of the light. This is beneficial because the thermal expansion coefficient of the transparent material causes the length of the sensing element to change with temperature. The optical path length of the sensing element is proportional to the refractive index. Therefore, the optical path length dependent change increases by a factor proportional to the refractive index, thereby amplifying the temperature-dependent effects of thermal expansion of the sensing element.

[0033] Preferably, the sensing element has at least 10 -4 ·K -1 (K -1 The optically transparent material is provided as a sheet having a thermo-optic coefficient of 1.0 kJ / °C (where θ is per Kelvin).

[0034] In one preferred method, the sensing element is provided as a sheet of material having at least 90 wt% silicon. Silicon is particularly preferred at center wavelengths above 800 nm because it has a low absorption coefficient and a high refractive index above 3.0. In addition, silicon has a refractive index of 2.10 -6 ·K -1 From 3:10 -6 ·K -1 Since the sensing element has a thermal expansion coefficient between .gtoreq..gtoreq..times ...

[0035] Silicon is preferred because its high refractive index allows for a compact sensing element, and its high thermo-optic coefficient allows for high sensitivity in temperature measurements. However, alternative materials with high refractive indices and even higher thermo-optic coefficients than silicon are possible, such as ZnSe and germanium. However, germanium has an absorption coefficient that results in significant loss of light intensity, as well as a temperature-dependent redshift. In general, any glass with a refractive index higher than that of single-mode fiber can be used as the sensing element material, such as LASFN-35. In addition, sapphire, which is resistant to high temperatures, can also be used, but birefringence behavior must be considered. Polymers such as PMMA and SU-8, which have negative thermo-optic coefficients, can also be used as sensing element materials, allowing the effects of temperature and strain to be separated by their negative thermo-optic coefficients.

[0036] In general, although not preferred, any material having a near-zero or non-zero thermo-optic coefficient may be used for the sensing element, but may result in very low sensitivity of the temperature measurement.

[0037] In one embodiment, the sensing element is positioned a distance from the distal end such that a cavity is formed between the distal end and the first reflective surface. The cavity may be filled with a fluid medium, such as a gas or a liquid. Alternatively, the cavity may be subjected to a low pressure of less than 10 Pascals, also referred to herein as a vacuum. The fluid medium may be selected to effectively reduce reflection of light at the surface of the end of the fiber back into the cavity. Preferably, the fluid medium has a refractive index different from that of the sensing element. An additional advantage of using a fluid medium or a vacuum is that the cavity is filled with a contaminant-free medium by absorbing elements or residues that may contaminate the first reflective surface or the distal end of the fiber. Liquids generally have a higher thermal conductivity, which provides an additional advantage of allowing the sensing element to more quickly reach the temperature of the target location. However, while selecting a transparent liquid with a high thermal conductivity is beneficial, care should be taken to minimize the amount of liquid used, as the specific heat capacity of liquids is generally higher than that of gases, and large amounts can adversely affect the sensor's response time.

[0038] It should be noted that it is also possible to place a reference reflector in one optical fiber segment that does not have a sensing element, and it is also possible to place no reference reflector in an optical fiber segment that has a sensing element, as long as there is at least one reference reflector in at least one optical fiber segment.

[0039] When used in an optical sensing method in which the sensing element is positioned a distance from the distal end such that a cavity is formed between the distal end and the first reflective surface, it is proposed to provide each optical fiber segment with an optional collimator. The collimator is positioned within the cavity between the distal end of the fiber and the sensing element and collimates the light from the distal end of the fiber to the sensing element. This is beneficial because the first reflective surface is exposed to collimated light, and the light reflected from the first surface is introduced into the distal end of the fiber with minimal loss compared to an embodiment without a collimator.

[0040] According to the optical sensing method, a first interference pattern from light partially reflected by the reference reflector and partially reflected by the first reflective surface is detected by a detector as a function of time, and a second interference pattern from light partially reflected by the reference reflector and partially or completely reflected by the second reflective surface is detected by the detector as a function of time.

[0041] The first interference pattern is analyzed, and a first optical distance between the reference reflector and the first reflective surface is determined and made available as a digital value to the analyzer. The optical distance, also known as the optical path length or optical length, is often represented in equations by Λ and is the length required for light to travel in a vacuum to have the same phase difference as if it had passed through a material with a given refractive index. The analyzer can be a computer program running on a computer, or an FPGA, or any other form of computer program product. The second interference pattern is analyzed, and a second optical distance between the reference reflector and the second reflective surface is determined and made available as a digital value to the analyzer.

[0042] Preferably, the first interference pattern is analyzed for all wavelengths in the modulation range. Because the central wavelength of the light is temporally modulated, the detected first interference pattern is also a function of time. The first interference pattern is demodulated to determine the first optical distance, similar to that described in "Range-resolved optical interferometric signal processing," by Kissinger, Thomas (dissertation, Cranfield University). Like the first interference pattern, the second interference pattern is analyzed for all wavelengths in the modulation range.

[0043] The next step proposes analyzing the first and second interference patterns over a period of time. The period of time is longer than the time it takes to modulate the central wavelength and is the time period over which the temperature is to be determined. Therefore, the period of time can range from one second to several hours, or even weeks or more. The change in the first optical distance is determined as a function of time. The change in the second optical distance is also determined as a function of time. The optical distance difference between the first and second optical distances is determined as a function of time and made available to the analyzer as a digital value. Because the first and second interference patterns depend on the first and second optical distances, respectively, the optical distance difference is a direct measure of the temperature of the sensing element. The optical distance difference is the optical distance traveled by light passing through the sensing element, being reflected by the second reflective surface, and then passing back through the sensing element. Therefore, by determining the optical distance difference as a function of time, the temperature-dependent change in the optical distance of the sensing element is a direct measure of the temperature of the sensing element.

[0044] The optical sensing method may be used with a single optical fiber segment, where light is directed to the optical fiber segment through an optical circulator, while reflected light from the optical fiber segment is directed by the circulator to a detector, and the temperature is determined at the location of one of the sensing elements in the optical fiber segment.

[0045] However, if two or more temperatures are to be determined at different locations, several optical fiber segments may be used. In this case, an optical multiplexer is placed between the light source and at least two optical fiber segments. The light is distributed to the at least two optical fiber segments by the multiplexer. The multiplexer is placed either between the at least two optical fiber segments and an optical circulator, and each reference reflector has a unique optical length between the light source and each reference reflector of each optical fiber segment to distinguish reflections occurring from each optical fiber segment. The reflected light from all optical fiber segments is reflected to the photodetector by the multiplexer and the optical circulator.

[0046] However, it is also possible to place an optical circulator, multiplexer, and optical fiber segment in each optical fiber segment. In this case, each circulator directs the reflected light from its respective optical fiber segment to a detector dedicated to this optical fiber segment. In this case, a unique optical length between the light source and the reference reflector of each optical fiber segment is not required, since the distinction is achieved by the different detectors.

[0047] Note that the reference reflector and the first reflecting surface form a first Fabry-Perot cavity. Furthermore, the reference reflector and the second reflecting surface form a second Fabry-Perot cavity. Therefore, the first interference pattern and the second interference pattern are interference patterns of a Fabry-Perot cavity, also known as a Fabry-Perot interferometer.

[0048] The sensing element senses temperature, and the thermo-optic coefficient of the sensing element is temperature dependent. The thermo-optic coefficient is defined as the change in refractive index dn as a function of temperature dT, i.e., thermo-optic coefficient = dn / dT, for temperature T and refractive index n. All values ​​given for the refractive index of the thermo-optic coefficient are understood to apply over a wavelength range between 800 nm and 5000 nm, and a temperature range of at least 150 K (150 Kelvin).

[0049] The present invention also relates to an optical sensor system adapted for the sensing method. The optical sensor system includes at least one light source. The optical sensor system further includes at least optical fiber segments. Each optical fiber segment includes an optical fiber having a proximal end and a distal end. At least one optical fiber segment (7.i) further includes a reference reflector disposed within the optical fiber between the proximal end and the distal end. At least one optical fiber segment includes a sensing element having a thermo-optic coefficient greater than the thermo-optic coefficient of the optical fiber. The sensing element includes a first partially reflective surface and a second at least partially or completely reflective surface. The first reflective surface is parallel to the second reflective surface. The optical sensing system further includes at least a detector and an analyzer. The detector is adapted to detect light from the light source. Each fiber of the optical fiber segment is adapted to allow light to propagate from the proximal end to the distal end. The light is characterized by a center wavelength and a linewidth. The center wavelength is wavelength-modulatable as a function of time over a modulation range. The modulation may be sinusoidal, but other modulation types are possible, such as triangular or sawtooth. The bandwidth of the modulation range is greater than the linewidth of the light. Preferably, the modulation range is at least 500 times, preferably at least 1000 times, greater than the linewidth. The detector is adapted to detect a first interference pattern as a function of time from light partially reflected by the reference reflector and partially reflected by the first reflective surface. The detector is also adapted to detect a second interference pattern as a function of time from light partially reflected by the reference reflector (9.i) and partially reflected by the second reflective surface. The analyzer is adapted to analyze the first and second interference patterns. The analyzer may demodulate the detected light as described in "Range-resolved optical interferometric signal processing," Kissinger, Thomas (dissertation, Cranfield University). The analyzer is adapted to determine a first optical distance between the reference reflector and the first reflective surface.The analyzer is adapted to determine a second optical distance between the reference reflector and a second reflective surface.

[0050] The optical sensor system includes at least one optical circulator, preferably disposed between the light source and the optical fiber segment.

[0051] If the temperature at only one location is to be detected, the optical sensing system has exactly one optical fiber segment. If the temperature at only one location is to be detected, a multiplexer is not required to direct the light from the light source to one optical fiber segment.

[0052] The fiber optic sensing system may also be adapted to detect temperature at more than one location, in which case the fiber optic sensing system has at least one multiplexer adapted to provide light to at least two optical fiber segments, each reference reflector having a unique optical length between the light source and its respective reference reflector included in at least one optical fiber segment.

[0053] The reference reflector is preferably provided as a fiber Bragg grating. The introduction of an FBG into a fiber is achieved by inscribing the FBG into the fiber's photosensitive core. One method, although not limited to this, is to use a frequency-quadrupled Nd:YAG laser operating at 266 nm. The broadband reflection of the FBG can be achieved, for example, by limiting the length of the FBG and thus broadening the reflection bandwidth. Fiber Bragg gratings can be easily introduced into fibers, allowing for the economical manufacture of fiber sensing systems.

[0054] The sensing element is provided as a sheet of optically transparent material, which is partially reflected by the first reflective surface due to the refractive index of the sensing element according to Fresnel's law, and partially reflected by the second reflective surface due to the refractive index of the sensing element.

[0055] Preferably, the sensing element has a first reflective coating that forms the first reflective surface. Also preferably, the sensing element has a second reflective coating that forms the second reflective surface. Although not essential to the invention, it is beneficial to adjust the amount of light reflected by the first reflective surface or the second reflective surface, respectively. This is beneficial because the reflectivities of the first and second reflective surfaces are not determined solely by the refractive index of the sensing element.

[0056] Preferably, the sensing element has a refractive index of at least 3.00, preferably at least 3.4. This refractive index is advantageous because the refractive index is directly proportional to the optical distance traveled by light within the sensing element. Therefore, to achieve the same optical distance, a material with a higher refractive index can be thinner than a material with a lower refractive index. This allows the sensing element to be very compact and therefore can be installed in limited space. Additionally, because less material is required to manufacture the sensing element, less total thermal energy is required to change the temperature of the sensing element, resulting in a faster response time of the sensing element to temperature changes.

[0057] The sensing element preferably has at least 10 -4 K -1 This results in a lower optical sensitivity of the sensing element, i.e., 10 -5 K -1 The thermo-optic coefficient of a sensing element made of a material such as fused silica, which has a thermo-optic coefficient of more than 1000 .mu.m, is greater than the optical sensitivity of a sensing element with a lower thermo-optic coefficient, such as fused silica. Optical sensitivity is understood as the temperature-dependent change in the optical path length of light traveling within the sensing element, which is the basis for determining temperature as explained above.

[0058] Particularly preferably, the sensing element comprises at least 90 wt% silicon (wt% refers to weight percent). The silicon wafer and silicon window can be manufactured with high precision, i.e., the silicon material has a first reflective surface and a second reflective surface parallel to the first reflective surface, allowing for economical manufacturing of the sensor system.

[0059] Preferably, the sensing element is positioned at a distance from the distal end such that a cavity is formed between the distal end and the first reflective surface. This allows the first reflective surface to be precisely aligned perpendicular to the distal end of the fiber, i.e., perpendicular to the light emitted from the distal end of the fiber. Precisely aligned perpendicular is understood to be at least ±6 degrees, preferably ±1 degree.

[0060] Each optical fiber segment is preferably provided with a collimator. The collimator is positioned in a cavity between the distal end of the fiber and the sensing element. The collimator may be a lens, ball lens, concave mirror, or gradient-index optical element. The collimator collimates the light from the distal end (72.i) of the fiber to the sensing element. Normally, light emitted from the end of the fiber is divergent. Of the divergent light, only the light closest to the optical axis is reflected back onto the fiber by the first and second reflecting surfaces. A collimator is therefore beneficial because it collimates the light and essentially focuses all of the reflected light onto the fiber, thereby increasing the reflected light collected for temperature determination.

[0061] In one embodiment, the cavity comprises a fluid medium that is optically transparent as described above.

[0062] Preferably, the sensing element has a thickness perpendicular to the first reflective surface of 5 mm (mm refers to millimeters, 10 -3The thickness of the sensing element ranges between 1000 and 20 ...

[0063] Optionally, a mirror may be disposed within the cavity at an angle θ relative to the optical axis. The mirror is at least partially reflective and reflects light emitted from the fiber toward the sensing element. In this case, the first reflective surface of the sensing element is disposed at an angle 2θ relative to the optical axis. This advantageously allows the dimensions of the optical fiber segment to be tailored to the needs of any limited space. Preferably, an optional collimator may be disposed between the distal end of the fiber and the mirror or between the mirror and the sensing element.

[0064] Please note that all values ​​are given at a mean sea level pressure of 101.3 kPa (kilopascals).

[0065] In one aspect of the invention, an attenuator may be spaced apart from the second reflective surface, particularly when the reflectivity of the second reflective surface is less than 90%. This advantageously suppresses stray reflections originating from locations other than the first or second reflective surface. The attenuator may be configured to disperse or absorb light.

[0066] Further advantages and aspects of the present invention are disclosed in the examples.

[0067] The drawings used to explain the present embodiment are schematic representations. [Brief explanation of the drawings]

[0068] [Figure 1]FIG. 1 is a schematic diagram of one embodiment of a sensor system. [Figure 2] FIG. 2 is a partial detailed view of an embodiment of a sensing element of the sensor system according to FIG. 1. [Figure 3] FIG. 2 is a partial detailed view of another embodiment of the sensing element of the sensor system according to FIG. 1. [Figure 4] FIG. 10 is a schematic diagram of another embodiment of a sensor system. [Figure 5] FIG. 5 is a partial detailed view of an embodiment of a sensing element of the sensor system according to FIG. 4. [Figure 6] FIG. 5 is a partial detailed view of another embodiment of the sensing element of the sensor system according to FIG. 4. [Figure 7] FIG. 5 is a partial detailed view of another embodiment of the sensing element of the sensor system according to FIG. 4. [Figure 8] FIG. 5 is a partial detailed view of another embodiment of the sensing element of the sensor system according to FIG. 4. [Figure 9] FIG. 5 is a partial detailed view of another embodiment of the sensing element of the sensor system according to FIG. 4. [Figure 10] FIG. 5 is a partial detailed view of another embodiment of the sensing element of the sensor system according to FIG. 4. [Figure 11] FIG. 10 is a schematic diagram of another embodiment of a sensor system. [Figure 12] FIG. 12 is a partial detailed view of an embodiment of a sensing element of the sensor system according to FIG. 11. [Figure 13] FIG. 12 is a partial detailed view of another embodiment of the sensing element of the sensor system according to FIG. 11. [Figure 14] FIG. 12 is a partial detailed view of another embodiment of the sensing element of the sensor system according to FIG. 11. [Figure 15] FIG. 10 is a schematic diagram of another embodiment of a sensor system. [Figure 16] FIG. 16 is a partial detailed view of an embodiment of a sensing element of the sensor system according to FIG. 15. [Figure 17] 1 is a schematic diagram of an optical sensing method. [Figure 18]Schematic diagram of center wavelength, linewidth, reflection range, and modulation range. [Figure 19] FIG. 2 is a schematic diagram of first and second interference patterns. DETAILED DESCRIPTION OF THE INVENTION

[0069] FIGS. 1 to 10 illustrate different embodiments of a sensor system 100 adapted to detect a temperature T at a location. FIG. 1 illustrates a schematic concept of the sensor system 100, while FIGS. 2 to 10 illustrate different embodiments of an optical fiber segment 7 used in the optical sensor system 100 illustrated schematically in FIG. 1. In this example, the sensor system 100 includes one optical fiber segment 7, each having a proximal end 71 and a distal end 72, and an optical fiber 73. The sensor system 100 further includes a reference reflector 9 disposed within the optical fiber 73 between the proximal end 71 and the distal end 72. The sensor system 100 also includes a light source, e.g., a laser. The sensor system 100 also includes a detector 12, also referred to as a photodetector 12, and an analyzer 13.

[0070] The optical fiber segment 7 has a sensing element 1 disposed at a distal end 72 at a location 32 where the temperature T is to be determined. The sensing element has a first reflective surface 2 and a second reflective surface 3 of the sensing element 1. The first reflective surface 2 is parallel to the second reflective surface 3. The first reflective surface 2 and the second reflective surface 3 are disposed perpendicular to the optical axis 30. Light 14 travels primarily along the optical axis 30 from the distal end 72 to the second reflective surface 3, and vice versa.

[0071] Light 14 is launched into the proximal end 71 of each optical fiber segment 7 and propagates from the proximal end 71 towards the distal end 72 .

[0072] In all of the embodiments depicted in the figures, light 14 is characterized by a center wavelength 15 and a linewidth 16, as shown in Figure 18. The center wavelength 15 is wavelength modulated as a function of time over a modulation range 17, which is also displayed in Figure 18. In Figure 18, this is illustrated by showing three snapshots of the intensity of light 14 versus wavelength at three times t1 (solid line), t2 (dashed line), and t3 (dotted line), illustrating the modulation of the center wavelength over a modulation cycle. As shown in Figure 18, it is important that the bandwidth of the modulation range 17 is greater than the linewidth 16 of light 14.

[0073] Light 14 in FIG. 1 passes through circulator 18 and is subsequently partially reflected by reference reflector 9, partially reflected by first reflecting surface 71, and partially or completely reflected by second reflecting surface 72. The reflected light 14 is directed by circulator 18 to detector 12. The reflected light 14 is detected by detector 12, and the signal obtained from detector 12 is subsequently analyzed by analyzer 13. The temperature T of sensing element 1 is established based on the effect of the thermo-optic coefficient of sensing element 1 on the detected light 14.

[0074] Figures 1 to 10 show a reference reflector 9 in the preferred form as a fibre Bragg grating. In Figures 1 to 10 the sensing element 1 is shown as a sheet of optically transparent material for light 14, also called optically transparent material.

[0075] 1-3, the sensing element 1 is positioned immediately adjacent to the fiber 73 at its distal end 72. The first reflective surface 2 is reflective due to the refractive index difference between the fiber 73 and the sensing element 1. The second reflective surface 3 is reflective due to the refractive index difference between the refractive index of the sensing element and the area behind it.

[0076] 3, the second reflective surface 3 has an optional reflective coating 31 adapted to adjust the amount of light 14 reflected to the detector 12. Although a reflective coating 31 has been mentioned, it should be noted that the reflective coating 31 can also reduce reflectivity in order to adjust the reflectivity of the second reflective surface 3.

[0077] The optical sensor system embodiments shown in FIGS. 4 through 10 illustrate alternative placements of the sensing element 1 in the optical fiber segment 7. In these embodiments, the sensing element 1 is positioned a fixed distance from the distal end 72 of the optical fiber 73 such that a cavity 22 is formed between the distal end 72 and the first reflective surface 2. In these embodiments, the first reflective surface 2 of the sensing element 1 includes an optional first reflective coating 21. While the first reflective coating 21 has been mentioned, it should be noted that the first reflective coating 21 can also reduce the reflectivity to tailor the reflectivity of the first surface 2. In addition, the second reflective surface 3 includes a second reflective coating 31, which has similar advantages as the embodiment shown in FIG. 3. The cavity may include a fluid medium or a vacuum, where vacuum refers to a fluid medium at a low pressure of less than 10 Pascals. The first reflective surface 2 and the second reflective surface 3 are positioned perpendicular to the optical axis 30. Light 14 travels primarily along optical axis 30 from distal end 72 to second reflective surface 3, and vice versa.

[0078] In one embodiment shown in Figure 6, a collimator 20 is disposed within the cavity 22 of the optical fiber segment 7. The collimator is disposed between the distal end 72 and the sensing element 1. The collimator 20 may be a lens, or a ball lens, or a gradient index optical element, or a concave mirror as shown later in Figure 9. The collimator 20 collimates the light 14 from the distal end 72 of the fiber 73 so that it is parallel to the optical axis 30.

[0079] In an alternative embodiment to the embodiment of FIG. 6 , shown in FIG. 7 , the collimator 20 is positioned within the cavity 22 of the optical fiber segment 7, directly adjacent to the distal end 72 of the fiber 73. This is beneficial because the fiber has no refractive index difference between the collimator surface and the fiber-cavity interface, thereby reducing reflections at the distal end 72 of the fiber 73. This reduction in reflection is achieved because the refractive index difference between the collimator 20 and the fiber 73 is less than the refractive index difference between the fiber and the cavity, which are typically filled with gas or vacuum. The collimator 20 may be a lens, ball lens, or gradient index optical element. Similar to the embodiment of FIG. 6 , the collimator 20 in the embodiment shown in FIG. 7 collimates the light 14 from the distal end 72 of the fiber 73 so that it is parallel to the optical axis 30.

[0080] 8 includes a mirror 29 disposed within the cavity 22 of the optical fiber segment 7. The mirror is disposed between the distal end 72 and the sensing element 1 and is configured to reflect light 14 from the distal end 72 to the sensing element 1. To this end, the mirror is disposed at an angle θ 33 relative to the optical axis 30, and the first reflective surface is disposed at an angle 2θ relative to the optical axis. This allows the dimensions of the optical fiber segment 7 to be adjusted to accommodate possible boundary conditions imposed by location.

[0081] Of course, it is also possible to combine the embodiments of Figures 6 or 7 and 8 so that the collimator 20 and mirror 29 are located within the cavity 22 as shown in Figure 10. In this case, it does not matter whether the collimator 20 or the mirror 29 is preceding in the beam path of the light 14.

[0082] The embodiment of optical sensor system 100 shown in Figures 11-16 is configured to detect a temperature T at each of several locations 32.i. The reference numbers referring to various elements of the embodiment described with respect to Figures 11-16 are similar to the reference numbers shown in the embodiment of Figures 1-10, but have been amended with an additional ".i" to indicate that there may be several instances of each element within each optical fiber segment 7.i within sensor system 100 (here and hereinafter, i>1,

number

[0083] Similar to the embodiment shown in Figures 1-10, the embodiment of optical sensor system 100 includes a light source 11, a detector 12, an analyzer 13, and a circulator 18. Unlike the embodiment of optical sensor system 100 shown in Figures 11-16, the embodiment of optical sensor system 100 includes at least two optical fiber segments 7.i. A multiplexer 19 is disposed between the circulator and the optical fiber segments 7.i. The multiplexer 19 distributes light 14 from the light source to each of the optical fiber segments 7.i. Similarly, the multiplexer 19 collects reflected light 14 from each optical fiber segment 7.i and transmits it to the circulator 18 and then to the detector 12.

[0084] Each optical fiber segment 7.i has an optical fiber 73.i with a proximal end 71.i and a distal end 72.i.

[0085] At least one optical fiber segment 7.i has a sensing element 1.i disposed at a distal end 72.i at a location 32.i where a temperature Ti is to be determined. The sensing element 1.i has a first reflective surface 2.i and a second reflective surface 3.i. The first reflective surface 2.i is parallel to the second reflective surface 3.i. The first reflective surface 2.i and the second reflective surface 3.i are disposed perpendicular to the optical axis 30.i. Light 14 travels primarily along the optical axis 30.i from the distal end 72.i to the second reflective surface 3.i, and vice versa.

[0086] At least one optical fiber segment 7.i further includes a reference reflector 9.i, which in a preferred embodiment is shown as a fiber Bragg grating. In the embodiment shown in FIG. 12, the sensing element 1.i is disposed immediately adjacent to the distal end 72.i of each of the fibers 73.i. Note that FIG. 11 shows a schematic concept, while the enlarged embodiments shown in FIGS. 12 and 13 show the arrangement of each element of the optical fiber segment 7.i in more detail. Note that the optical fiber segment 7 embodiment of FIG. 2 and / or FIG. 3 may be used for at least two optical fiber segments 7.i of FIG. 12. It is also conceivable to use a combination of at least one optical fiber segment 7 of FIG. 2 and at least one optical fiber segment 7 of FIG. 3 in the same optical sensing system according to FIG. 11.

[0087] The embodiment of optical fiber segment 7.i in FIG. 13 shows details of the embodiment of sensing system 100 in FIG. 11. Sensing elements 1.i are positioned a fixed distance from distal ends 72.i of fibers 73.i such that cavities 22.i are formed between the distal ends 72.i and the respective first reflective surfaces 4.i. The first reflective surfaces 2.i of sensing elements 1.i have optional first reflective coatings 21.i. Additionally, the second reflective surfaces 3.i have second reflective coatings 31.i. Each cavity contains a fluid medium 23 or a vacuum 23. The first reflective surfaces 2.i and second reflective surfaces 3.i are positioned perpendicular to their respective optical axes 30.i. Light 14 travels primarily along the respective optical axes 30.i from the distal ends 72.i to the second reflective surfaces 3.i, and vice versa.

[0088] The embodiment of FIG. 13 shows optical fiber segments 7.i with an optional collimator 20.i disposed within the cavity 22.i of each optical fiber segment 7.i.

[0089] Of course, the sensor system 100 may also include multiple different embodiments of the optical fiber segment 7.i, as shown for some embodiments of the optical fiber segment in FIG. 14. Here, the reference numerals are also amended with ".j" and ".k" (here and below, i,j,k>1, i,j,

number

[0090] FIG. 15 shows a different embodiment of the sensor system 100, illustrating an embodiment in which the sensing element 1.1 is located in an optical fiber segment 7.1 that does not have a reference reflector 9.i, but the reference reflector 9.2 is located in another optical fiber segment 7.2 that does not have a sensing element 1.i. Reference Reflector. The first and second interference patterns are based on reflections in different optical fiber segments 7.1, 7.2. However, as shown in FIG. 15, a reference reflector may also be located in any other optical fiber segment 7.i that has a sensing element 1.i. Of course, the sensor system 100 may include any number of optical fiber segments 1 to n (n is the number of optical fiber segments, n>1,

number

number

[0091] 16 illustrates an example of optical fiber segments 7.i, 7.j, 7.k in one embodiment of sensor system 100 according to the embodiment shown in FIG. 15. Here, the reference numbers have been revised with ".j" and ".k" (here and below, i,j,k>1 ...

number

[0092] The sensor system is not limited to the shown embodiment and may have different assemblies of optical fiber segments. It should be noted that embodiments are also possible in which the material of the sensing element 1.i may be different for different optical fiber segments 7.i, so that the optimum material can be selected depending on the expected temperature Ti at each location 32.i, or the expected temperature range, or the required response time to a changing temperature Ti.

[0093] 17 shows a schematic representation of an optical sensing method comprised of at least three steps 40, 50, and 60. The first step 40 comprises providing at least one of the aforementioned embodiments of the previously described sensor system 100. The first step 40 further comprises emitting light 14 at the proximal end of each optical fiber segment 7.i, the light 14 being characterized by a center wavelength 15 and a linewidth 16, the center wavelength 15 being wavelength modulated as a function of time over a modulation range 17, the bandwidth of which is greater than the linewidth 16 of the light 14, as shown in FIG.

[0094] A second step of the method comprises detecting light 14 in the form of at least a first interference pattern 4.i and a second interference pattern 5.i and analysing the detected light 14 signal.

[0095] A third step 60 comprises establishing a temperature Ti of each sensing element 1.i based at least on the effect that the thermo-optic coefficient of the sensing element 1.i has on the detected light 14.

[0096] Preferably, the second step 50 may further include detecting a first interference pattern 4.i as a function of time from light 14 partially reflected by the reference reflector 9.i and partially reflected by the first reflecting surface 2.i, and detecting a second interference pattern 5.i as a function of modulation time from light 14 partially reflected by the reference reflector 9.i and partially or completely reflected by the second reflecting surface 3.i.

[0097] FIG. 19 shows a schematic illustration of a first interference pattern 4.i and a second interference pattern 5.i for a sensor system 100 having one sensing element 1.i. The modulated light 14 reflected by the first reflective surface 2.i and the reference reflector 9 is analyzed for a modulation range 17 of the light 14 and Fourier transformed into the frequency domain. The left peak of the signal shown in FIG. 19 corresponds to this first interference pattern 4.i. The modulated light 14 reflected by the second reflective surface 3.i and the reference reflector 9 is analyzed for a modulation range 17 of the light 14 and Fourier transformed into the frequency domain. The right peak of the signal shown in FIG. 19 corresponds to this second interference pattern 5.i.

[0098] Preferably, the second step 50 may further include analyzing the first interference pattern 4.i and determining a first optical distance 25.i between the reference reflector 9.i and the first reflective surface 2.i. The determination of the first optical distance 25.i is implemented as a computer program product that analyzes the first interference pattern 4.i and calculates the first optical distance 25.i based on the first interference pattern 4.i. The second step 50 may further include analyzing the second interference pattern 5.i and determining a second optical distance 26.i between the reference reflector 9.i and the second reflective surface 3.i, similar to the first optical distance 25.i (mutatis mutandis).

[0099] Preferably, step 50 is performed for at least all wavelengths in the modulation range 17. This may be done by demodulating the detected optical signal, as explained above.

[0100] Preferably, the second step 50 may further comprise analysing the first interference pattern 4.i and the second interference pattern 5.i over a period of time and determining the change in the first optical distance (25.i) and / or the second optical distance (26.i) as a function of time. Furthermore, the step 50 may comprise determining and making available an optical distance difference 27.i between the first optical distance 25.i and the second optical distance 26.i, determined as a function of time.

[0101] Preferably, the first step 40 may further include distributing the light 14 into at least two optical fiber segments 7.i, each optical fiber segment 7.i having a unique optical length between the light source 11 and the reference reflector 9.i of the respective optical fiber segment 7.i.

[0102] The reflected light 14 from all optical fiber segments 7.i is reflected by a multiplexer 19 and an optical circulator 18 to a photodetector 12.

[0103] The examples of suitable center wavelengths 15 given above are merely suggestions, and neither the methods 40, 50, 60 nor the sensing system 100 are intended to be limited to including only these center wavelengths 15.

[0104] Because suitable light sources 11 are relatively inexpensive to obtain, it is beneficial for the central wavelength 15 to be between approximately 1290 nm and 1620 nm. In addition, optical fibers 73.i are typically optimized for lowest attenuation at these wavelengths. Typically, a central wavelength of 1550 nm is selected.

[0105] The light source 11 is configured to emit light 14 modulated at a modulation rate. The light source 11 is configured to sweep from a start wavelength to an end wavelength during the reverse modulation rate. The start wavelength typically differs 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 sensing element 1.i in the form of a silicon etalon with a physical length of 3 mm can be resolved using a modulation range 17 of 300 pm.

[0106] According to another embodiment of the sensor system, the light source 11 is a distributed feedback diode. According to another embodiment of the sensor system, the photodetector 12 has just one single photosensitive element. Distributed feedback diode lasers are relatively inexpensive and robust. FSI-based sensor systems do not require expensive external cavity lasers.

[0107] Possible embodiments of the sensor system 100 expressly include embodiments having combinations of aspects of the embodiments described above. [Explanation of symbols]

[0108] 1.i Sensing element 2.i First reflecting surface 3.i Second reflective surface 4.i First interference pattern 5.i Second interference pattern 6.i First optical distance 7.i Optical fiber segment 8.i Second optical distance 9.i Reference Reflector 11 Light source, laser 12 Detector, photodetector 13 Analyzer 14 light 15 Center wavelength 16 Line Width 17 Modulation Range 18 Circulator 19 Multiplexer, Multiplexing Unit 20.i Collimator, lens 21.i First reflective coating 22.i Cavity 23.i Fluid media 24.i Distance 25.i First optical distance 26.i Second optical distance 27.i Distance difference 28.i Position 29.i Mirror 30.i Optical axis 31.i Second reflective coating 32.i position 33.i Angle θ 37 Reflection range, reflection range 40 First Step 50 Second Step 60 Third Step 71.i Proximal end 72.i Distal end 73.i Optical Fiber 100 Sensor Systems Ti temperature

Claims

1. - providing at least one optical fiber segment (7.i), each optical fiber segment (7.i) having a proximal end (71.i), a distal end (72.i) and an optical fiber (73.i); - providing a reference reflector (9.i) arranged within the optical fiber (73.i) of at least one optical fiber segment (7.i) between the proximal end (71.i) and the distal end (72.i); - providing, for at least one optical fiber segment (7.i), a sensing element (1.i) arranged at said distal end (72.i), said sensing element (1.i) having a thermo-optic coefficient greater than the thermo-optic coefficient of said fiber (73.i); providing a first reflective surface (2.i) of said sensing element (1.i) and a second reflective surface (3.i) of said sensing element (1.i), said first reflective surface (2.i) being parallel to said second reflective surface (3.i); providing a photodetector (12); and providing an analyzer (13). Including, 1. A method of optical sensing, comprising: emitting light (14) into the proximal end of each optical fiber segment (7.i) and propagating from the proximal end (71.i) toward the distal end (72.i); the light (14) is characterized by a center wavelength (15) and a linewidth (16); the center wavelength (15) is wavelength modulated as a function of time over a modulation range (17), the bandwidth of the modulation range (17) being greater than the linewidth (16) of the light (14); the light (14) detected by the detector is analyzed by the analyzer; and establishing a temperature (T.i) of the sensing element (1.i) based on an effect of the thermo-optic coefficient of the sensing element (1.i) on the detected light.

2. 2. The optical sensing method of claim 1, wherein the reference reflector (9.i) is provided as a fiber Bragg grating and the sensing element (1.i) is provided as a sheet of optically transparent material for the light (14), the optically transparent material having a first reflective coating (21.i) forming the first reflective surface (2.i), and the optically transparent material having a second reflective coating (31.i) forming the second reflective surface (3.i).

3. 3. The optical sensing method according to any one of claims 1 to 2, wherein the sensing element (1.i) is provided as a sheet of optically transparent material having a refractive index at the central wavelength (15) of at least 2.00, preferably a refractive index at the central wavelength (15) of at least 3.

4.

4. The sensing element (1.i) has a thermo-optic coefficient of at least 10 －４ ・K －１ 4. The optical sensing method of claim 1, wherein the optical sensing method is provided as a sheet of optically transparent material,

5. 5. The optical sensing method according to claim 1, wherein the reference reflector (9.i) is provided as a fiber Bragg grating, the reflection range (37) of the reference reflector (9.i) being larger than the modulation range (17), and the sensing element (1.i) is provided as a sheet of material having at least 90 wt % silicon.

6. 6. The optical sensing method according to claim 1, wherein the sensing element (1.i) is positioned at a distance (24.i) from the distal end (72.i) such that a cavity (22.i) is formed between the distal end (72.i) and the first reflective surface (2.i).

7. 7. The optical sensing method of claim 6, wherein each optical fiber segment (7.i) is preferably provided with a collimator (20.i), the collimator (20.i) being arranged in the cavity (22.i) between the distal end (72.i) of the fiber (73.i) and the sensing element (1.i), the collimator (20.i) collimating the light (14) from the distal end (72.i) of the fiber (73.i) to the sensing element (1.i), and the cavity (22.i) having a fluid medium (23.i) that is transparent to the light (14).

8. 8. The optical sensing method according to claim 1, wherein a first interference pattern (4.i) from light (14) partially reflected by the reference reflector (9.i) and partially reflected by the first reflective surface (2.i) is detected by the detector as a function of time, and a second interference pattern (5.i) from light (14) partially reflected by the reference reflector (9.i) and partially or completely reflected by the second reflective surface (3.i) is detected by the detector (12) as a function of modulation time.

9. The first interference pattern (4.i) is analyzed, and a first optical distance (25.i) between the reference reflector (9.i) and the first reflecting surface (2.i) is determined and made available, the second interference pattern (5.i) is analyzed, and a second optical distance (26.i) between the reference reflector (9.i) and the second reflecting surface (3.i) is determined and made available, and the first interference pattern (4.i) is analyzed to determine and make available a second optical distance (26.i) between the reference reflector (9.i) and the second reflecting surface (3.i), ...

9. The optical sensing method of claim 8, wherein the first interference pattern (4.i) and the second interference pattern (5.i) are analyzed over a predetermined period of time, a change in the first optical distance (25.i) is determined as a function of time, a change in the second optical distance (26.i) is determined as a function of time, and an optical distance difference (27.i) between the first optical distance (25.i) and the second optical distance (26.i) is determined and made available as a function of time.

10. 12. The optical sensing method according to claim 1, wherein the light (14) is distributed by a multiplexer (19) to at least two optical fiber segments (7.i), each optical fiber segment (7.i) having a unique optical length between the light source (11) and the reference reflector (9.i) of the respective optical fiber segment (7.i), and the reflected light (14) from all optical fiber segments (7.i) is reflected to the photodetector (12) by the multiplexer (19) and an optical circulator (18).

11. 11. Optical sensing method according to any one of claims 1 to 10, wherein the sensing elements (1.i) sense temperatures (T.i) and the thermo-optic coefficients of the sensing elements (1.i) depend on the temperature.

12. An optical sensor system (100) adapted for the sensing method according to any one of claims 1 to 11, comprising: the optical sensor system (100) comprises at least one light source (11) and at least optical fiber segments (7.i), each optical fiber segment (7.i) having an optical fiber (73.i) with a proximal end (71.i) and a distal end (72.i), at least one optical fiber segment (7.i) further comprising a reference reflector (9.i) disposed within the fiber (73.i) between the proximal end (71.i) and the distal end (72.i), at least one of the optical fiber segments (7.i) having a sensing element (1.i) with a thermo-optic coefficient greater than the thermo-optic coefficient of the optical fiber (73.i), a first partially reflective surface (2.i), and a second at least partially or completely reflective surface (3.i); the first reflecting surface (2.i) is parallel to the second reflecting surface (3.i), and the optical sensing system (100) further comprises at least a detector (12) and an analyzer (13); each of the fibers (73.i) of the optical fiber segment (7.i) being adapted to allow light (14) to propagate from the proximal end (71.i) towards the distal end (72.i); the light (14) is characterized by a central wavelength (15) and a linewidth (16); the central wavelength (15) is wavelength modulatable as a function of time over a modulation range (17); the bandwidth of the modulation range (17) is greater than the linewidth (16) of the light (14); the detector (12) is adapted to detect a first interference pattern (4.i) as a function of time from light (14) that is partially reflected by the reference reflector (9.i) and partially reflected by the first reflective surface (2.i); the detector (12) is adapted to detect a second interference pattern (5.i) as a function of time from light (14) that is partially reflected by the reference reflector (9.i) and partially reflected by the second reflective surface (3.i); the analyzer (13) is adapted to analyze the first interference pattern (4.i) and the second interference pattern (5.i); the analyzer (13) is adapted to determine a first optical distance (6.i) between the reference reflector (9.i) and the first reflecting surface (2.i); an optical sensor system (100) wherein the analyzer (13) is adapted to determine a second optical distance (8.i) between the reference reflector (9.i) and the second reflective surface (5.i);

13. 13. The optical sensor system (100) of claim 12, wherein the optical sensing system (100) comprises at least one optical circulator (18) and at least a multiplexer (19) adapted to provide light (14) to at least two optical fiber segments (7.i), each reference reflector (9.i) having a unique optical length between the light source (11) and a respective reference reflector (9.i) included in the at least one optical fiber segment (7.i).

14. The reference reflector (9.i) is provided as a fiber Bragg grating, a reflection range (37) of the reference reflector (9.i) is greater than the modulation range (17), the sensing element (1.i) is provided as a sheet of a material that is optically transparent to the light (14), the sensing element (1.i) has a first reflective coating (21.i) forming the first reflective surface (2.i), the sensing element (1.i) has a second reflective coating (31.i) forming the second reflective surface (3.i), the sensing element (1.i) has a refractive index of at least 3.00, preferably at least 3.4, and the sensing element (1.i) has a refractive index of at least 10 －４ ・K －１ 16. The optical sensor system (100) of any of claims 14 to 15, wherein the sensing element (1.i) has a thermo-optic coefficient of 0.1 .mu.m and / or the sensing element (1.i) comprises at least 90 wt% silicon.

15. The sensing element (1.i) is arranged at a distance from the distal end (72.i) such that a cavity (22.i) is formed between the distal end (72.i) and the first reflecting surface (4.i), and preferably each optical fiber segment (7.i) is provided with a collimator (20.i), the collimator (20.i) being arranged in the cavity (22.i) between the distal end (72.i) of the fiber (73.i) and the sensing element (1.i), and the collimator (20.i) may be a lens, or a ball, 15. The optical sensor system (100) of claim 12, wherein the collimator (20.i) is a lens, or a concave mirror, or a gradient index optical element, and the collimator (20.i) collimates the light (14) from the distal end (72.i) of the fiber (73.i) to the sensing element (1.i), the cavity (22.i) has a fluid medium (23.i) that is transparent to the light (14), the sensing element (1.i) detects temperature, and one optical property of the sensing element (1.i) depends on the temperature.

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