Interferometric optical fiber sensor, optical fiber probe and method for detecting chemical substance

Abstract

Disclosed is an interferometric fiber optic sensor for detecting chemical substances, comprising a light source, a detector and a light path arranged between the light source and the detector, further comprising a light splitting element and a fiber optic segment, the detector and the light source being connected to the light splitting element, having a measuring element forming a resonant cavity, the surface is suitable for being in contact with a chemical substance to be detected. The fiber segment and the measuring element are pressed together such that there is only a residual cavity therebetween. The fiber segment is contained within the capillary along at least a portion of its length. The first end of the capillary is connected to the measuring element, and the other part of the capillary is clamped on the optical fiber segment, so that the capillary, the optical fiber segment and the measuring element together form the optical fiber measuring probe as part of the optical path. The capillary has a length of at least 5 mm. The present disclosure also relates to a probe and a method for detecting chemical substances using an interferometric fiber optic sensor.

Description

[0001] This application is a divisional application of the Chinese patent application entitled “Interference Measurement Fiber Optic Sensor, Fiber Optic Probe and Method for Detecting Chemical Substances” filed on December 18, 2020 and with application number 201980041394.6. Technical Field

[0002] The present invention relates to an interferometric fiber optic sensor, a fiber optic probe and a method for detecting chemical substances using the interferometric fiber optic sensor with the fiber optic probe. Background Art

[0003] Interferometric fiber optic sensors for measuring parameters such as temperature, strain or detection of substances are known in the art.

[0004] Publication WO2018106134 of the international patent application discloses an interferometric fiber optic sensor comprising a Michelson interferometer, one arm of which is covered with an active substance suitable for interacting with a substance to be detected. The result of this interaction is a change in the thickness or optical thickness of the active substance, the refractive index or attenuation of the end of the measuring arm. This interferometric fiber optic sensor can be used to detect the presence of a substance in the environment surrounding the measuring arm. In particular, the measuring arm can be immersed in an unknown substance to detect whether the substance includes a predetermined one or more substances. The active substance can be selected so that it reacts only with one substance to be detected or a group of substances to be detected. A detection method includes observing interference fringes before and after contact with the environment or an unknown substance. If a specific shift of the interference fringes is detected, the presence of a predetermined substance is detected.

[0005] Other configurations of interferometric sensors are also known in the prior art. A Fabry-Perot interferometer is one of the so-called multi-beam interferometers and is used to examine light waves and utilizes the phenomenon of multiple reflections of a light beam between two parallel and partially transparent mirrors.

[0006] An example of such a structure is the fuel level sensor disclosed in the specification of EP 3054272, which can be taken as an example, in which an optical cavity is made on an optical fiber with a semiconductor material (e.g. gallium arsenide). The optical cavity is attached to the lower surface of the diaphragm, and the dimensional changes caused by the pressure changes of the diaphragm on the cavity make it possible to interpret the result as the state of the fuel reserve.

[0007] U.S. Patent Application No. US2005254062 discloses an interferometric sensor and method for detecting the presence or amount or binding rate of an analyte. The sensor works in a Fabry-Perot configuration and has a light source, a detector, and the optical path between them includes a spectroscopic element and a measuring element probe, wherein an optical fiber transmits light to the first and second optical elements through an air gap. The second optical element is covered with a substance capable of binding to an analyte. The analyte is analyzed by observing the spectrum of the reflected light. The document discloses a variety of applications that allow detection and measurement of specific substances-analytes (particularly antigens). This disclosure is not without disadvantages. It requires high-precision manufacturing of the first and second optical elements, and particularly requires accurate definition of the air gap. This is a non-optional requirement for the probe to be repeatable and for the spectral changes to be properly observed. But the cost is to complicate the mathematical model. The replacement of the probe and its manufacture are also a problem.

[0008] Sensors known in the art are difficult to mass produce, have parameters that are difficult to repeat, and are somewhat cumbersome for small samples. On the other hand, industrial sensors such as level sensors only give limited information about specific conditions that have been met. Therefore, there is a need for a sensor that can be applied to a large number of repeatable tests and inspections. The object of the present invention is to provide an interferometric fiber optic sensor, a fiber optic probe and a method for using the same that do not have the above-mentioned disadvantages of the prior art. Summary of the invention

[0009] The interferometric fiber optic sensor for detecting chemical substances according to the present invention comprises a light source, a detector and an optical path arranged between the light source and the detector. It also comprises a spectroscopic element and a fiber segment, to which the detector and the light source are connected, and a measuring element constituting a resonant cavity having a surface suitable for contacting the chemical substance to be detected. The fiber segment and the measuring element are squeezed together so that there is only a residual cavity between them. The fiber segment is contained in a capillary at least along a portion of its length. The first end (ending part) of the capillary is connected to the measuring element, and the other part of the capillary is clamped (clenched) on the fiber segment, so that the capillary, the fiber segment and the measuring element together constitute an optical fiber measuring probe as a part of the optical path. The length of the capillary is at least 5 mm. The positive result of this structure of the sensor is that the residual cavity can be ignored in the calculation. It has a small impact on the measurement results and is easy to manufacture in a stable and repeatable manner. The capillary length of 5 mm or longer makes it possible to move the clamping point on the fiber segment far enough away from the residual cavity so that its parameters are not affected in the process. Therefore, the sensor according to the present invention can be considered as having a single measurement resonant cavity constituted by the surface of the face of the measuring element. It leads to linear measurements and simple interpretation of the measurement results. The probe with a capillary can be easily replaced by cutting and welding a new probe or using an optical fiber connector. The sensor is particularly suitable for, but not only for, detecting substances in the group including acetone, refrigerants, ethanol vapor, hydrochloric acid, poly (allylamine hydrochloride), allylamine, proteins, biomarkers and antigens. Other advantages of the present invention are that the substance to be detected interacts with the face of the measuring element located in front of the optical fiber. Compared with the probe in which the substance to be detected interacts with the side surface of the optical fiber or the measuring element, this configuration improves the measurement dynamics.

[0010] Advantageously, the optical length of the residual cavity a·n 1 The optical length of the measuring element b·n 2 The ratio between falls within the range of 0 to 0.4. Setting the ratio of the length of the resonant cavity to that of the residual cavity within the range of 0 to 0.4 results in a high frequency of interference fringes and simplifies the analysis. This ratio makes it easier to use a single cavity model in the analysis. The spectrum of the reflected signal from the residual cavity is much wider than the spectrum of the measurement signal generated by the cavity formed by the measurement element. In the mathematical model, it corresponds to the multiplication of two sinusoidal signals with different periods. Providing a given ratio makes it easier to consider the influence on the spectrum generated by the residual cavity as a constant and ignore it.

[0011] Advantageously, the measuring element has a face at least partially covered with a substance which is adhesive to the substance to be detected. This configuration makes it easier to capture a sample of the substance to be detected.

[0012] Alternatively, the measuring element has a face at least partially covered with a substance which loses its adhesiveness when exposed to the substance to be detected. In this construction, the result of the exposure of the face of the measuring element to the predetermined substance can be detected, and thus the substance itself.

[0013] Alternatively, the measuring element has a face at least partially covered with a substance which changes its optical parameters when exposed to the substance to be detected. Through the use of a plurality of substances covered with substances which interact with the respective specific substance to be detected, a plurality of substances can be detected in a single measurement with the same detector and light source. The attachment / detachment of substances on the face of the measuring element or changes in the material parameters or changes in the parameters of the element itself after exposure to external factors causes a change in the optical path length in the interferometer. As a result, a shift in the interference fringes can be observed.

[0014] The measuring element is advantageously a tile made of a material comprising a substance selected from the group consisting of glass, quartz glass SiO 2 , lead glass, ZBLAN glass and polymer.

[0015] The optical fibre segment is advantageously a single mode optical fibre within the operating range of the detector.

[0016] The end of the fiber segment within the capillary is advantageously inclined and is inclined at an angle in the range of 4° to 20° relative to the axis of the fiber segment. Optimal results are obtained for a range of 7° to 8°.

[0017] The first surface of the fiber segment that bounds the residual cavity from the first side has a reflection coefficient R 1 , and the second surface of the measuring element which delimits the residual cavity from the opposite side has a reflection coefficient R 2 , and the reflection coefficient advantageously satisfies the condition

[0018] Advantageously, the fiber segment is a multi-core fiber segment and is connected to a fan-in / fan-out located in the optical path, wherein the interferometer has a first light source and a first detector connected to the fan-in / fan-out device via a first beam splitter element and a second light source and a second detector connected to the fan-in / fan-out device via a second beam splitter element. This configuration makes it possible to observe shifts of different wavelengths simultaneously, which can improve the sensitivity and specificity of detection, or enable the simultaneous detection of two different substances.

[0019] Advantageously, the interferometer also has a third light source and a third detector, which are connected to the fan-in / fan-out arrangement via a third beam splitting element. Applying a greater number of light sources and detectors operating at various wavelengths further improves sensitivity and specificity, but is associated with a higher complexity of the sensor.

[0020] Advantageously, at least one light source is selected from the group consisting of:

[0021] ●Supercontinuum source,

[0022] ●Halogen lamps that emit light in the wavelength range of 400-700nm,

[0023] ●Superluminescent diode with a central wavelength of 1310nm,

[0024] ●Superluminescent diode with a central wavelength of 1400nm,

[0025] ●Superluminescent diode with a central wavelength of 1450nm,

[0026] ●Superluminescent diode with a central wavelength of 1550nm,

[0027] - A narrow-band light source with a central wavelength in the range of 1310 to 1550 nm, capable of being tuned over a range of 20 nm or wider.

[0028] Typically, good results are obtained with sources operating in the wavelength range of 400-1600 nm. Choosing the mechanism by which the source and the face of the measuring element interact with the substance to be detected, the length of the measuring element can be easily chosen to obtain fringes at the desired wavelength corresponding to the chosen source, so that the shift of the fringes can be easily observed.

[0029] The fiber optic measuring probe according to the invention comprises a fiber segment and a measuring element having a surface for contacting a substance, the fiber segment and the measuring element being pressed together so that there is only a residual cavity between them, and the fiber segment is contained in a capillary at least along a part of its length, wherein a first end of the capillary is connected to the measuring element and the other part of the capillary is clamped on the fiber segment. The probe according to the invention can be positioned away from the light source and the detector. It can be easily replaced, can be easily sterilized and is suitable for subsequent use. The length of the capillary is at least 5 mm.

[0030] Advantageously, the optical length of the residual cavity a·n 1 The optical length of the measuring element b·n 2 The ratio between the lengths of the resonant cavity and the residual cavity falls within the range of 0 to 0.4. Setting the ratio of the length of the resonant cavity to the length of the residual cavity within the range of 0 to 0.4 results in a high frequency of interference fringes and simplifies the analysis. This ratio makes it easier to use a single cavity model in the analysis. The measured fringes are much narrower than those originating from the residual cavity.

[0031] Advantageously, the measuring element has a face at least partially covered with a substance which is adhesive to the substance to be detected. This configuration makes it easier to capture a sample of the substance to be detected.

[0032] Alternatively, the measuring element has a face at least partially covered with a substance which loses its adhesiveness when exposed to the substance to be detected. In this construction, the result of the exposure of the face of the measuring element to the predetermined substance can be detected, and thus the substance itself.

[0033] Alternatively, the measuring element has a face at least partially covered with a substance which changes its optical parameters when exposed to the substance to be detected. Through the use of a plurality of substances covered with substances which interact with the respective specific substance to be detected, a plurality of substances can be detected in a single measurement with the same detector and light source. The attachment / detachment of substances on the face of the measuring element or changes in the material parameters or changes in the parameters of the element itself after exposure to external factors causes a change in the optical path length in the interferometer. As a result, a shift in the interference fringes can be observed.

[0034] The measuring element is advantageously a brick made of a material comprising a substance selected from the group consisting of glass, quartz glass, lead glass, ZBLAN glass and polymers.

[0035] The end of the optical fiber segment within the capillary is advantageously beveled and is inclined at an angle in the range of 4° to 20° relative to the axis of the optical fiber segment.

[0036] The first surface of the fiber segment that bounds the residual cavity from the first side has a reflection coefficient R 1 , and the second surface of the measuring element which delimits the residual cavity from the opposite side has a reflection coefficient R 2 , and the reflection coefficient advantageously satisfies the condition

[0037] Advantageously, when the refractive index n of the medium in the capillary is 1 The refractive index n of the material of the measuring element 2 When the difference between is greater than 30%, that is:

[0038]

[0039] The fiber-optic measuring probe advantageously has a connector which is arranged at the end of the fiber section opposite to the end at which the measuring element is located.

[0040] Advantageously, the fiber section in the measurement probe has more than one core. It enables the probe to be used in an interferometer with multiple detectors and / or sources, thereby providing better sensitivity and specificity.

[0041] A method for detecting a chemical substance using an interferometer, the interferometer having an optical path, a detector, a light source, and an optical fiber measuring probe included in the optical path, directing light to the substance to be detected, wherein if a shift of a predetermined interference fringe is observed on the detector, the substance is considered to be detected, characterized in that the interferometer is an interferometer according to the present invention. Since the measuring element constitutes a resonant cavity, the interference fringes can be recorded in the wavelength domain. In the case where a substance is attached to the outer surface of the cavity or the thickness of the already attached substance increases, the interference fringes shift toward a larger wavelength. The size of the shift can be used to determine the thickness change. Alternatively, if the layer is separated, the interference fringes shift toward a shorter wavelength and the thickness reduction can be determined. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Embodiments of the present invention have been described in detail below with reference to the accompanying drawings, in which: Figure 1a shows a block diagram of an interferometric optical fiber sensor according to a first, second and third embodiment of the present invention, Figure 1b shows a schematic structure of a probe according to an embodiment of the present invention, Figure 1c FIG. 4 shows a schematic structure of a probe according to another embodiment of the present invention. Figure 1d FIG. 4 shows a schematic structure of a probe according to another embodiment of the present invention. Figure 2a and Figure 2b shows the shift of the interferometric fringes due to exposure to a test substance in a first embodiment of the invention, Figure 3 FIG. 4 shows a block diagram of a sensor having a multi-core (three-core) optical fiber according to a fourth embodiment of the present invention, and FIG. Figure 4 A cross-sectional view of the triple-core optical fiber is shown. DETAILED DESCRIPTION

[0043] Example 1

[0044] The block diagram of the interferometer in this embodiment is shown in Figure 1. A superluminescent diode with a central wavelength of 1400 nm and a full width at half maximum equal to 50 nm constituting the light source 1 is connected to the first port C1 of the circulator constituting the light splitting element 3 via a single-mode optical fiber. The measuring probe 13 according to the present invention is connected to the second circulator port C.2 using the single-mode optical fiber end 4. In the structure of the measuring probe 13, the other end of the optical fiber 4 is encapsulated in a quartz glass capillary 11 and is pushed toward the measuring element 5 attached to the capillary 11 by splicing.

[0045] Alternatively, the capillary can be clamped onto the measuring element 5 .

[0046] The end of the optical fiber 4 located in the capillary is cut at an angle of α = 8°. As a result, there is an air gap between the end of the optical fiber 4 and the measuring element 5, which forms a residual cavity 7 with an effective width from a = 0 μm to a = 2 μm. The measuring element 5 has a length of 20 μm.

[0047] Further tests have shown that within the entire range of cutting angles from 4° to 20°, the attenuation effect of the reflected signal can be obtained to a certain extent.

[0048] A spectrum analyzer, which is used as detector 2 in this embodiment of the invention, is connected to the third port C.3 of the circulator.

[0049] The light from the source 1 passes through the circulator constituting the beam splitter element 3 to the optical fiber 4, from where it passes through an air gap of width "a" to the measuring element 5 having the face 6. In this way, a Fabry-Perot interferometer is obtained. On the detector 2, interference fringes in the spectral domain can be observed. The contrast and / or the shift of the fringes depend on the variation of the parameters of the cavity formed along the optical path. In fact, surface variations at the boundaries of the cavity established between the face 6 and the opposite ends of the measuring element 5 are useful in the measurement. The best results are obtained if the optical path contains only one cavity. Each connection and each boundary of the medium introduces an additional cavity. In the structure of the interferometer according to the invention, the number of cavities is limited. The residual cavity 7 is negligible due to its small size. In addition, due to the oblique cutting of the fiber segment 4 and the provision of a sufficient ratio of reflection coefficients at the boundaries of the cavity, its influence on the measurement result is limited:

[0050]

[0051] in:

[0052] -R 1 is the reflection coefficient from the fiber end,

[0053] -R 2 is the reflection coefficient from the surface of the measuring cell facing the capillary.

[0054] The refractive index n of the medium in the capillary 1 The refractive index n of the material of the measuring element 2 The difference between is greater than 30%, that is This allows for better measurement of kinetics.

[0055] The measuring element 5 is a brick in the form of a glass cylinder with a height of 20 μm. The face 6 of the measuring element 5 is coated with a perfluorinated polymer with a refractive index of about 1.33. The coating is provided by immersing the measuring element 5 in a polymer solution. As a result of the exposure of the face 6 to the substance to be detected (refrigerant), which is a compound of carbon, chlorine and fluorine, such as 1,1,2-trichloro-1,2,2-trifluoroethane, the polymer layer expands. In this configuration, the expansion contributes to a change in the thickness of the substance of up to 10 nm, which corresponds to a shift of the interference fringes around 1410 nm of up to 0.6 nm, as shown in FIG. Figure 2a and Figure 2b , which shows the reflection coefficient S11 as a function of wavelength before and after the measuring element is exposed to the refrigerant.

[0056] based on Figure 2a and Figure 2b The obtained spectra shown in , determine the thickness of the layer covering the face 6 and its thickness variation.

[0057] The first step is to measure the thickness of the measuring element 5 by determining the wavelength difference of adjacent interference fringes during the measurement before exposure to the measuring substance. These fringes are Figure 2a Calibration of the interferometer is therefore performed by determining the difference in the optical paths in the interferometer and taking into account the refractive index of the cavity formed by the surface of the measuring element 5 and taking into account that the optical signal passes twice through this cavity.

[0058] Next, face 6 is exposed to the substance to be measured and one of the interference fringes is analyzed; the difference in wavelength corresponding to the fringe before and after exposure is determined. Figure 2b The fringes are shown before and after exposure. The shifting of the fringes occurs due to thickness variations of the layer that has been attached to the face 6 of the measuring element 5 .

[0059] After determining the difference in wavelength corresponding to said interference fringes before and after exposure, knowing the refractive index of the layer covering the face 6 of the measuring element 5 , the change in layer thickness ΔL is calculated:

[0060]

[0061] in:

[0062] L 2 、n 2 represent the thickness and refractive index of the measuring element 5, respectively,

[0063] L 3 、n 3 denote respectively the thickness and the refractive index of the perfluorinated polymer layer covering the face 6 of the measuring element 5,

[0064] λ is the wavelength of the analyzed fringe before and after layer expansion,

[0065] Δλ is the difference in wavelength of the analyzed interference fringes before and after layer expansion.

[0066] If the wavelength change Δλ of the interference fringes is within the range (0.1nm, 16.8nm) towards longer wavelengths, it is confirmed that the refrigerant is detected. This range is selected by experimental methods. In other embodiments of the present invention, other ranges and other active substances are suitable for being selected to detect other substances. The selection can be made in routine testing. The technician can easily provide other active substances and measure the appropriate range.

[0067] In this embodiment, the structure of the measuring probe 13 according to the present invention makes it possible to effectively shorten the residual optical cavity 7. Figure 1b Schematically shown in FIG. The end of the optical fiber 4 and the measuring element 5 are placed in a glass capillary with a U-shaped cross section. The optical fiber end 4 is cut at an angle of 20° and pressed to the measuring element 5. The capillary is sealed by welding at both ends on the measuring element 5 and the optical fiber 4. The connection point of the capillary 11 to the optical fiber 4 should be moved away from the residual optical cavity 7 to prevent an unintended increase in reflections from one of the layers. This is achieved using a 5 mm long capillary 11.

[0068] Example 2

[0069] The interferometer according to the second embodiment of the present invention is based on Figure 1a The same block diagram as shown in is made. The light source 1 is a narrowband tunable light source with a central wavelength of 1550nm and can be tuned within a range of 50nm. The source 1 is connected to the first port C1 of the circulator via a single-mode optical fiber. The measurement probe 13 is connected to the second port C.2 of the circulator.

[0070] Figure 1c The measuring probe according to this embodiment of the invention is schematically shown. It is interconnected into the optical path of the interferometer, and one end of the single-mode optical fiber 4 is provided with a connector not shown in the figure. The other end of the optical fiber 4 is encapsulated in the capillary 11 and pushed toward the measuring element 5. The end of the optical fiber 4 housed in the capillary is cut at an angle of 8°. Welding is performed between the capillary 11 and the measuring element 5. As a result, there is an air gap between the end of the optical fiber 4 and the measuring element 5, which forms a residual cavity 7 with an effective width a=2μm. The measuring element 5 has a length b=20μm. The face 6 of the measuring element 5 is covered with the same substance as in Example 1.

[0071] The detector 2 is connected to the third port C.3 of the circulator.

[0072] Example 3

[0073] The block diagram of the interferometer according to the third embodiment of the present invention is the same as that in the first and second embodiments. It is shown in FIG1a. The light source 1 is a halogen lamp emitting light in the wavelength range of 400-700 μm, connected to the first port C1 of the circulator via a birefringent optical fiber (maintaining polarization and single mode in the operating wavelength band of the light source 1). The polarization-maintaining circulator has the function of a spectroscopic element 3. The second port C.2 of the circulator is connected to the measuring probe 13. The optical fiber segment 4 of the optical probe 13 is birefringent and has a beat length of 3 mm at a wavelength λ=550 nm. It maintains polarization and is single mode in the wavelength operating band of the source 1. The measuring element 5 is made of lead glass with a refractive index equal to 1.8. The third port C.3 of the circulator is connected to the spectrometer constituting the detector 2.

[0074] Figure 1d The measuring probe 13 schematically shown in FIG. 1 comprises a capillary 11 having an O-shaped cross section and a length of 5 mm, into which a polarization-maintaining optical fiber segment 4 is inserted, and a measuring element 5 made of lead glass and having a length of b = 200 μm. In this configuration, the residual cavity 7 obtained has a length of a = 0.1 μm.

[0075] Alternatively, the measuring element 5 may be made of other substances selected from the group comprising glass, quartz glass, lead glass, ZBLAN glass and polymers, which also offer relatively low losses.

[0076] Quartz glass is particularly convenient in processing. Using quartz glass, a cylindrical measuring element 5 with a desired length and smooth surfaces and a base that is parallel to each other and perpendicular to the axis can be easily manufactured.

[0077] Adhesion of the substance to be detected to the face 6 of the measuring element 5 is obtained by placing the face 6 in a piranha solution (i.e. a solution comprising concentrated sulfuric acid and 30% hydrogen peroxide mixed in a volume ratio of 3:1). The face 6 thus treated is sufficiently active to attract and be covered by a polymer layer having a refractive index of 1.5 and a thickness of 2 nm when exposed to poly(allylamine hydrochloride).

[0078] The surface of lead glass has a chemical bond of O-Si-O-Si-O. After exposure to the piranha solution, the structure Si-O-Si is transformed into 2x Si-OH, which has a stronger polarity, so the ionomer can more easily attach to the surface.

[0079] A 2 nm layer of a substance with a refractive index of 1.5 on face 6 of measuring element 5 results in a shift of 5 pm in the spectral refraction fringes at a wavelength of 550 nm. Thus, the sensor in this embodiment can be applied to detect poly(allylamine hydrochloride), confirming its presence when the spectral interference fringes shift towards longer wavelengths by a value between 2 pm and 0.2 nm.

[0080] Example 4

[0081] exist Figure 3 , a block diagram of a fourth embodiment of the present invention is shown. Using a measuring probe 9 with a multi-core optical fiber segment 4 makes it possible to simultaneously observe spectra of different wavelengths of a light source. Figure 4 A three-core optical fiber and a set of three light sources are schematically shown in cross section.

[0082] In the set of light sources 1.1, 1.2, 1.3, three wide spectrum superluminescent diodes are used, whose wavelengths are 1310 nm, 1450 nm and 1550 nm, respectively, and whose full width is 50 nm. In the configuration of the reflection measurement, each source is connected to a detector 2.1, 2.2, 2.3, which is connected to the source via a circulator 3.1, 3.2, 3.3. As detectors 2.1, 2.2, 2.3, a spectrophotometer is used. The free ports of the circulators 3.1, 3.2, 3.3 are connected to the ports of a fan-in / fan-out device 8, which connects these ports to the core of the three-core fiber segment 4 of the measurement probe 9.

[0083] The measuring element 5 has a length b=50 μm and is made of glass ZBLAN (ZrF 4 -BaF 2 -LaF 3 -AlF 3 -NaF). There is an air gap between the ZBLAN glass and the input multicore fiber 4, with a residual cavity 7 of length a=1 μm. In this configuration, a Fabry-Perot resonator is formed for each of the cores 9.1, 9.2, 9.3.

[0084] The face 6 of the measuring element 5 is sensitive to the detection of acetone by immersion in a 1% solution of polystyrene in dichloromethane. The active layer remaining on the face 6 after drying has certain properties: it swells due to contact with acetone. Therefore, the sensor according to the invention equipped with such a layer can be used as an acetone sensor.

[0085] The skilled person can easily provide sets with other sources having other medium wavelengths and other spectral widths allowing measurements in the range from 1300 to 1500 nm, wherein the sensor of the invention is already able to detect and measure the desired substances.

[0086] Light from the source is guided to the circulator 3 through a single-mode fiber, and then to the fan-in / fan-out device 8 to the input three-core fiber 4. This fiber has Figure 3The cross section of the cores 9.1, 9.2, 9.3 shown is a triangular arrangement and has a lattice constant Λ=80μm. The cores 9.1, 9.2, 9.3 have a diameter of 8.2μm and a numerical aperture of 0.12. From the optical fiber 4, the light is guided to the residual cavity 7 filled with air and the measuring element 5. The light reflected from the edge of the measuring element returns to the detector via the optical fiber segment 4, through the fan-in / fan-out element, through the single-mode fiber and the circulator, where the fringes generated by the interference of the light reflected from the opposite end of the measuring element 5 are observed. Immersion in acetone at room temperature causes a change in the layer thickness on the measuring probe by about 200nm and causes a shift of the fringes near the wavelength of 1550nm by about 6.4nm, 6nm near the wavelength of 1450nm, and about 5.4nm near the wavelength of 1310nm. If the interference fringe shift obtained at all source wavelengths is in the range of 0.1nm to 8.0nm, it is considered that acetone has been detected. Therefore, an increase in the specificity of the detection method has been achieved.

[0087] The use of a fan-in / fan-out element makes it possible to observe the deviations of all these fringes simultaneously. When observing two fringes, an improvement in the operation of the sensor, namely a higher sensitivity and specificity, is already obtained. For some substances to be detected, it is advisable to use optical fibers with more than three cores and a correspondingly higher number of light sources, up to 10, 12 or more.

[0088] In an embodiment of the sensor according to the invention, a multi-core fiber segment 4 with a greater number of cores and a corresponding number of light sources and detectors can be used. Such sensor systems and measuring probes are more complex, but they offer the possibility of more certain identification of the detected substance.

[0089] In some embodiments of the invention, a fiber segment can be used as the measuring element 5. By using a fiber segment as the measuring element 5, diffraction of the light beam is avoided due to the presence of a core in the fiber. Using a fiber with positive dispersion in the light source bandwidth avoids the occurrence of nonlinear phenomena.

[0090] A person skilled in the art will be able to routinely suggest other advantageous modifications of the optical path between source and detector in certain circumstances without departing from the invention as defined in the claims.For example, the circulator used in the above embodiments may be easily replaced by a coupler.

[0091] Similarly, a person skilled in the art who is familiar with the above description will have no difficulty in coming up with various methods for coating or sensitizing the face 6 of the measuring element 5 with various substances to be detected, and will also be able to select appropriate optical parameters of the source and the optical path. Sensitivity to hydrochloric acid can be obtained by preparing a measuring element made of quartz glass with a small porosity and a refractive index of about 1.8 and covering its surface with yttrium oxide, for example by laser heating the yttrium oxide and depositing it on the glass. Contacting the cut element with hydrochloric acid causes a shift of the fringes at a wavelength of 1550 nm by about 4 nm.

[0092] A person skilled in the art will also be able to propose various techniques for connecting the measurement probe to the optical path, in particular by splicing optical fibers and using dedicated fiber optic connectors.

[0093] A person skilled in the art will be able to easily select the length of the measuring element to the light source strip and replace the sources mentioned in the specification with various alternatives. The creators have also successfully used various sources, including:

[0094] ●Supercontinuum source,

[0095] ●Halogen lamps emitting light from the 400-700nm range,

[0096] ●Superluminescent diode with a central wavelength of 1310nm,

[0097] ●Superluminescent diode with a central wavelength of 1400nm,

[0098] ●Superluminescent diode with a central wavelength of 1450nm,

[0099] ●Superluminescent diode with a central wavelength of 1550nm,

[0100] - A narrow-band tunable source with a central wavelength in the range of 1310-1550 nm and a tuning range of at least 20 nm.

[0101] The present invention can be implemented using various measurement techniques known to those skilled in the art. A broadband light source can be used, such as a SLED diode, a supercontinuum source or a halogen, and as a detector, a spectrum analyzer or a spectrophotometer can be used. Alternatively, a tunable source can be used, which switches to the next wavelength during the measurement. In this configuration, more original detectors can be used. There is the possibility of analyzing a broadband optical signal using several detectors or a source of several narrowband light sources to analyze using one broadband detector.

[0102] The examples should be understood as illustrating the invention rather than limiting the invention.

Claims

1. An interferometric optical fiber sensor for detecting chemical substances, comprising a light source (1, 1.1, 1.2, 1.3), a detector (2, 2.1, 2.2, 2.3) and an optical path arranged between the light source (1, 1.1, 1.2, 1.3) and the detector (2, 2.1, 2.2, 2.3), a beam splitter (3, 3.1, 3.2, 3.3) and an optical fiber segment (4), the detector (2, 2.1, 2.2, 2.3) and the light source (1, 1.1, 1.2-1.3) being connected to the beam splitter (3, 3.1, 3.2, 3.3), a measuring element (5) constituting a resonant cavity, the measuring element having a surface (6) suitable for contacting the chemical substance to be detected, Features The fiber segment (4) and the measuring element (5) are pressed together so that there is only a residual cavity (7) between them, and the fiber segment (4) is contained in a capillary (11) at least along a part of its length, wherein a first end of the capillary (11) is connected to the measuring element (5) and the other part of the capillary (11) is clamped on the fiber segment (4), so that the capillary (11), the fiber segment (4) and the measuring element (5) together constitute an optical fiber measurement probe (13) as part of the optical path, wherein the capillary (11) has a length of at least 5 mm, and wherein a first surface of the fiber segment (4) that limits the residual cavity from a first side has a reflection coefficient R1, and a second surface of the measuring element that limits the residual cavity from an opposite side has a reflection coefficient R2, and the reflection coefficients satisfy the condition 2. The interferometric optical fiber sensor according to claim 1, characterized in that: The ratio between the optical length a·n1 of the residual cavity (7) and the optical length b·n2 of the measuring element (5) falls within the range of 0 to 0.

4.

3. The interferometric optical fiber sensor according to claim 1 or 2, characterized in that: The measuring element (5) has a surface (6) which is at least partially covered with a substance which is adhesive to the substance to be detected.

4. The interferometric optical fiber sensor according to claim 1 or 2, characterized in that: The measuring element (5) has a face (6) which is at least partially covered with a substance which loses its adhesiveness when exposed to a substance to be detected.

5. The interferometric optical fiber sensor according to claim 1 or 2, characterized in that: The measuring element (5) has a face (6) which is at least partially covered with a substance which changes its optical parameters when exposed to a substance to be detected.

6. The interferometric optical fiber sensor according to claim 1 or 2, characterized in that: The measuring element (5) is a brick made of a material comprising a substance selected from the group consisting of glass, quartz glass, lead glass, ZBLAN glass and polymer.

7. The interferometric optical fiber sensor according to claim 1 or 2, characterized in that: The end of the optical fiber segment (4) within the capillary (11) is inclined and is inclined at an angle in the range of 4° to 20° relative to the axis of the optical fiber segment (4).

8. The interferometric optical fiber sensor according to claim 1 or 2, characterized in that: The optical fiber segment (4) is a multi-core optical fiber segment and is connected to a fan-in / fan-out device (8) located in the optical path, wherein the interferometric optical fiber sensor has a first light source (1.1) and a first detector (2.1) connected to the fan-in / fan-out device (8) via a first beam splitter element (3.1), and a second light source (1.2) and a second detector (2.2) connected to the fan-in / fan-out device (8) via a second beam splitter element (3.2).

9. Fiber optic measuring probe, comprising a fiber optic segment (4) and a measuring element (5) having a surface (6), characterized in that: The fiber segment (4) and the measuring element (5) are pressed together so that there is only a residual cavity (7) between them, and the fiber segment (4) is contained in a capillary (11) at least along a part of its length, wherein a first end of the capillary (11) is connected to the measuring element (5) and the other part of the capillary (11) is clamped on the fiber segment (4), wherein the capillary (11) has a length of at least 5 mm, and wherein the reflection coefficient R1 from a first surface of the fiber segment (4) limiting the residual cavity from a first side and the reflection coefficient R2 from a second surface of the measuring element limiting the residual cavity from an opposite side satisfy the condition 10. The optical fiber measuring probe according to claim 9, characterized in that: The ratio between the optical length a·n1 of the residual cavity (7) and the optical length b·n2 of the measuring element (5) falls within the range of 0 to 0.

4.

11. The optical fiber measuring probe according to claim 9 or 10, characterized in that: The measuring element (5) has a surface (6) which is at least partially covered with a substance which is adhesive to the substance to be detected.

12. The optical fiber measuring probe according to claim 9 or 10, characterized in that: The measuring element (5) has a face (6) which is at least partially covered with a substance which loses its adhesiveness when exposed to a substance to be detected.

13. The optical fiber measuring probe according to claim 9 or 10, characterized in that: The measuring element (5) has a face (6) which is at least partially covered with a substance which changes its optical parameters when exposed to a substance to be detected.

14. The optical fiber measuring probe according to claim 9 or 10, characterized in that: The measuring element (5) is a brick made of a material comprising a substance selected from the group consisting of glass, quartz glass, lead glass, ZBLAN glass and polymer.

15. The optical fiber measuring probe according to claim 9 or 10, characterized in that: The end of the optical fiber segment (4) within the capillary (11) is inclined and is inclined at an angle in the range of 4° to 20° relative to the axis of the optical fiber segment (4).

16. The optical fiber measuring probe according to claim 9 or 10, characterized in that: The optical fiber measuring probe has a connector which is arranged on the end of the optical fiber section (4) opposite to the end where the measuring element (5) is located.

17. The optical fiber measuring probe according to claim 9 or 10, characterized in that: The optical fiber section (4) has more than one core (9.1, 9.2, 9.3).

18. A method for detecting a chemical substance using an interferometer, the interferometer being equipped with an optical path, having a detector, a light source, and an optical fiber measuring probe included in the optical path, guiding light to the substance to be detected, wherein: If a shift of predetermined interference fringes is observed on the detector, the substance is considered to be detected, characterized in that the interferometer is an interferometric fiber optic sensor according to any one of claims 1-8.

Images