Mach-Zehnder interferometer optical fiber sensor based on arc-shaped concave cavity single-mode optical fiber

Abstract

The invention provides a Mach-Zehnder interferometer optical fiber sensor based on an arc-shaped concave cavity single-mode optical fiber. The device is characterized by comprising an incident optical fiber 1, a beam splitter 2, an interference arm 3, a beam combiner 4 and an emergent optical fiber 5 which are connected in sequence. The incident optical fiber 1 is used for transmitting an input optical signal, the beam splitter 2 couples the optical signal in the incident optical fiber 1 into the interference arm 3, the interference arm 3 is used as a transmission waveguide, three arc-shaped concave cavities with different depths and curvature radiuses are arranged in the interference arm 3, and when a solution to be detected is immersed in the concave cavities, the refractive index change modulates an interference spectrum through an optical path difference and a phase difference; according to the design of the three concave cavities, refractive index distribution of three independent positions in the reaction process can be measured at the same time, the beam combiner 4 recombines optical signals in the interference arm 3 and couples the optical signals into the emergent optical fiber 5, and the emergent optical fiber 5 is responsible for outputting the optical signals and transmitting the optical signals to detection equipment for analysis. According to the invention, multi-position synchronous detection is realized through the multi-cavity gradient sensitive characteristic and in combination with the femtosecond laser micromachining technology, the problem of accidental errors of a traditional single-position sensor is solved, and the sensor is suitable for scenes such as microfluidic reaction requiring dynamic monitoring of refractive index change.

Description

(1) Technical Field

[0001] The present invention relates to a Mach-Zehnder interferometer fiber optic sensor based on an arc-shaped concave cavity single-mode fiber, which can monitor the refractive index change of a liquid to be measured in real time by analyzing interference fringes and is applicable to the dynamic monitoring of the solution reaction process, belonging to the field of fiber optic sensing technology. (2) Background Art

[0002] As a precise optical measurement tool, fiber optic refractive index sensors have shown great application potential in many fields such as environmental monitoring, food safety, biomedicine, and industrial manufacturing. The demand for real-time monitoring and accurate measurement of the refractive index of substances in these fields is increasing day by day because the refractive index is an important optical parameter of substances and can reflect various physical and chemical properties such as the composition, concentration, and temperature of substances.

[0003] The Mach-Zehnder interference structure is widely used in fiber optic sensing systems. When light is split into two beams by a beam splitter and propagates along the first interference arm and the second interference arm respectively, one beam serves as a reference optical path and the other as a modulation optical path. The two beams of light will recombine and interfere in the subsequent combiner. If the modulation optical path is affected by the external environment, the optical path will change, resulting in a change in the optical path difference between the two beams of light, and the interference fringes will also change accordingly. By observing the change in the interference fringes, the change in environmental parameters can be deduced, thus achieving precise measurement of the environment.

[0004] With the continuous progress of fiber optic sensing technology, the application of the Mach-Zehnder interference structure in fiber optic sensors is also constantly changing. The optical fiber not only undertakes the traditional transmission function but also has a sensing function. By processing the optical fiber, a specific structure can be formed to couple part of the light from the core to the cladding and then recouple it back to the core in the next fiber coupler, thus realizing the interference of the light propagating in the core and the cladding, which is the Mach-Zehnder interference principle. When the external environment affects the core and the cladding differently, the light propagating in the two will produce interference changes due to different responses to the environment, which is reflected in the change of the spectrogram. By analyzing the relationship between the spectral change and the environmental change, the external environment can be monitored using the spectral change to achieve the sensing function, and the side-polished structure enhances the influence of the external environment on light propagation. Fiber optic sensors have significant advantages in environmental monitoring with strong electromagnetic interference or narrow space.

[0005] Most existing fiber optic refractive index sensors based on Mach-Zehnder interferometers adopt a single sensing point design. This single-position measurement method has significant limitations: First, during the dynamic reaction process (such as the solution mixing reaction in a microfluidic chip), a single monitoring point cannot capture the spatial distribution differences in the reaction system, resulting in the lack of key kinetic parameters such as concentration gradients and diffusion rates; Second, the single-position data is easily affected by accidental factors such as local turbulence and bubble interference, and the reliability of the measurement results is insufficient; Third, in complex reaction systems (such as multiphase catalytic reactions), the differences in mass transfer efficiency in different regions will lead to uneven refractive index distribution, and traditional sensors cannot achieve in-situ multi-point synchronous monitoring, making it difficult to accurately evaluate the overall reaction process.

[0006] Traditional single-position sensors only obtain the refractive index data at a single point in the reaction system and cannot distinguish the concentration gradient distribution of reactants (such as the formation of the diffusion front in a microfluidic chip), the dynamics of the multiphase reaction interface (such as local refractive index mutations at the gas-liquid interface), and the localization and identification of abnormal interferences (such as bubbles only affecting a single monitoring point), etc. Existing technologies rely on the calculation of concentration based on the absolute value of the refractive index at a single position, resulting in the inability to characterize the mass diffusion rate (requiring at least two-point concentration differences for calculation) and the difficulty in evaluating the mixing uniformity (requiring the analysis of the data dispersion at multiple points).

[0007] In order to overcome the deficiencies of the existing technologies, the present invention provides a fiber optic sensor based on a Mach-Zehnder interferometer with an arc-shaped concave cavity single-mode fiber, which has significant innovative advantages and practical value. Through the topological layout of three arc-shaped concave cavities, the present invention can synchronously capture the initial state in the inlet region, the transient changes in the mixing region, and the final state characteristics in the equilibrium region. The time series correlation analysis of the data from the three concave cavities can effectively distinguish systematic errors (synchronous offsets in the three channels) from local interferences (abnormal fluctuations in a single channel). The all-fiber design eliminates the need for external packaging, has a mechanical strength equivalent to that of ordinary single-mode fibers (tensile strength > 5N), and has outstanding electromagnetic interference resistance, making it suitable for narrow spaces or strong electromagnetic environments (such as industrial production line monitoring), demonstrating broad application prospects. (III) Summary of the Invention

[0008] The purpose of the present invention is to solve the spatial resolution defect of single-position sensors through multi-cavity collaborative monitoring, and to provide a fiber optic sensor based on a Mach-Zehnder interferometer with an arc-shaped concave cavity single-mode fiber that can synchronously obtain the refractive index distribution at multiple points in the reaction system. Three arc-shaped concave cavities with a depth gradient design can construct a dynamic concentration field model by correlating the refractive index change trends at three positions in real time, providing high spatio-temporal resolution data for reaction kinetics research.

[0009] The purpose of the present invention is achieved as follows:

[0010] The incident optical fiber 1 is used to transmit the input optical signal; the first coreless optical fiber arranged in front of the incident optical fiber 1, that is, the beam splitter 2, splits the optical signal in the incident optical fiber 1 and couples it to the interference arm 3 in front of the beam splitter 2; the interference arm 3 is used as a transmission waveguide, which is used to divide the light beam coupled by the beam splitter 2 into a first part of light entering the first interference arm 31 and a second part of light entering the second interference arm 32. When the solution to be tested is immersed in the cavity, the refractive index change of the light beam modulates the interference spectrum through the optical path difference and phase difference. The design of the three cavities can simultaneously measure the refractive index distribution of three independent positions in the reaction process; the beam combiner 4 recombines the optical signal in the interference arm and couples it to the output optical fiber 5; the output optical fiber 5 is responsible for outputting the optical signal and transmitting it to the detection equipment for analysis. The overall shape is straight and the appearance is cylindrical, similar to the Mach-Zehnder refractive index sensor with a cylinder-side throw-cylinder structure.

[0011] The Mach-Zehnder interferometer fiber optic sensor based on arc-shaped concave cavity single-mode fiber, wherein the input fiber 1 is composed of a single-mode fiber, the input end is connected to an external broadband light source, and the output light of the broadband light source is transmitted from the core of the single-mode fiber.

[0012] The Mach-Zehnder interferometer optical fiber sensor based on arc-shaped concave cavity single-mode optical fiber. The structure of the beam splitter 2 is the same as that of the beam combiner 4, both of which are composed of coreless optical fiber with a diameter of 125 μm and a length of 1-2 mm.

[0013] The Mach-Zehnder interferometer fiber optic sensor based on arc-shaped concave cavity single-mode fiber. The interference arm 3 is composed of a single-mode fiber, and includes a first interference arm 31 and a second interference arm 32, and the first interference arm 31 is composed of the cladding in the single-mode fiber, and the second interference arm 32 is composed of the core in the single-mode fiber; the length of the first interference arm 31 is equal to the length of the second interference arm 32, which is 12 mm. The structure of the first interference arm 31 of the single-mode fiber is to form three arc-shaped concave cavities with different depths on the cladding using femtosecond laser micromachining technology, with a radius of curvature of 12 mm and an axial length of 12 mm, and the arc-shaped concave cavities are arranged at equal intervals along the axial direction of the optical fiber, with a spacing of 1.2-1.5 times the reaction characteristic length, and the depth of the concave cavity is 30%, 50% and 70% of the thickness of the single-mode fiber cladding, respectively. The side-throw structure is that the first interference arm 31 enables the optical signal in the cladding to interact with the measured liquid.

[0014] The Mach-Zehnder interferometer optical fiber sensor based on arc-shaped concave cavity single-mode optical fiber can simultaneously measure the refractive index changes at three positions during the reaction process by combining a phase decoupling algorithm with a three-channel interference spectrum.

[0015] The Mach-Zehnder interferometer fiber optic sensor based on an arc-shaped cavity single-mode fiber. Among them, the beam combiner 4 is used to recouple the optical signal modulated by the measured liquid back into the fiber core in the side-polished structure.

[0016] The Mach-Zehnder interferometer fiber optic sensor based on an arc-shaped cavity single-mode fiber. Among them, the output fiber 5 is composed of a single-mode fiber, and the output end is connected to an external spectrometer to obtain an interference spectrum containing sensing information. The generation of interference fringes is due to the optical path or phase change caused by the path difference when the optical signals are superimposed. This change will cause the synthesized optical intensity to show a periodic distribution, forming a pattern of alternating light and dark. The change of environmental conditions will directly act on the optical path difference or phase difference, thus changing the characteristic performance of the interference pattern.

[0017] Specifically, due to the concave cavity structures of different sizes and positions, the change in the concentration C of the measured liquid will linearly or non-linearly modulate its refractive index n, satisfying the relationship: Δn = k e ·C where k c is the refractive index-concentration sensitivity coefficient. For a non-uniform concentration field (such as a microfluidic reaction zone), the local concentrations C 1 , C 2 , C 3 at the three concave cavities can respectively correspond to the refractive index changes Δn 1 , Δn 2 , Δn 3 .

[0018] The depth of each arc-shaped cavity determines the coupling strength between the light and the evanescent field of the liquid, and its effective interaction length L i can be approximated as: L i = η·d i (i = 1, 2, 3) In the formula, d i is the cavity depth, and η is the coupling efficiency factor. When the liquid refractive index changes by Δn i , the propagation path of the light in the arc-shaped cavity changes due to the enhancement of the evanescent field, generating an optical path difference: ΔL i = L i ·Δn i = ηd i ·k c C i

[0019] The optical path difference ΔL i directly modulates the phase difference between the two interference arms satisfying: where λ is the central wavelength of the light source. According to the Mach-Zehnder interference condition, the change in the phase difference will cause a wavelength shift Δλ in the output spectrum i , and the relationship is: (IV) DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a schematic structural diagram of a Mach-Zehnder interferometer fiber optic sensor based on an arc-shaped concave cavity single-mode fiber. It consists of an incident fiber 1, a beam splitter 2, an interference arm 3, a combiner 4, and an output fiber 5.

[0021] Figure 2 is a schematic diagram of the optical path transmission principle in a Mach-Zehnder interferometer fiber optic sensor based on an arc-shaped concave cavity single-mode fiber.

[0022] Figure 3 is a schematic diagram of the application of a Mach-Zehnder interferometer fiber optic sensor based on an arc-shaped concave cavity single-mode fiber to an in-situ detection device for a microchannel reaction system. (V) SPECIFIC EMBODIMENTS

[0023] The following combines specific embodiments to further elaborate on how the present invention detects the liquid to be measured.

[0024] Please refer to Figure 1 , Figure 1 is a schematic structural diagram of a preferred embodiment of a Mach-Zehnder interferometer fiber optic sensor based on an arc-shaped concave cavity single-mode fiber.

[0025] This embodiment combines a fiber optic coupled microchannel reaction system (for in-situ detection of ammonia nitrogen and nitrite) with a Mach-Zehnder interferometer sensor based on an arc-shaped concave cavity single-mode fiber to form a multi-parameter in-situ monitoring system, as Figure 3 shown. The core of the system includes: a light source module; a Y-type fiber optic beam splitter divides the light source into two paths, where path 1 is directly connected to the signal processing module and set as a reference optical path, and path 2 inputs a Mach-Zehnder interferometer sensor based on an arc-shaped concave cavity single-mode fiber, which is embedded in a Y-type cell for refractive index detection; the detection module is a Y-type microchannel reaction cell with a built-in constant temperature device (25 °C) to suppress the drift of fluorescence and MZI signals caused by temperature (temperature drift < 0.025 nm), where the beam splitter 2, interference arm 3, and combiner 4 of the sensor are embedded in the side wall of the flow channel, and carrier liquid 1 and carrier liquid 2 are respectively injected into the two channel openings of the Y-type cell, and the two liquids react in the reaction zone and finally flow out from the bottom of the Y-type cell; the signal processing module acts on the spectrometer and compensation algorithm to jointly process fluorescence, absorbance, and refractive index signals.

[0026] As Figure 3As shown, the sensor is embedded in the side wall of the Y-shaped microchannel reaction cell. The three arc-shaped cavities are located in the inlet area A, the mixing reaction area B, and the equilibrium area C of the microfluidic reaction cell respectively. By real-time monitoring the refractive index change trends in the three areas, the completion degree of the reaction process can be calculated (such as the local concentration changes during the ammonia nitrogen and nitrite reactions), and a dynamic concentration field model can be constructed by combining multi-point data, providing in-situ data with high spatio-temporal resolution for reaction kinetics research.

[0027] When the solution to be measured flows through the Y-shaped microchannel, the three arc-shaped cavities capture the initial concentration in the inlet area, the transient concentration during the reaction in the mixing area, and the stable concentration in the equilibrium area respectively. The spectral drift amounts Δλ 1 、Δλ 2 、Δλ 3 of the three cavities are synchronously obtained by the spectrometer to establish a three-dimensional data set of refractive index - position - time. The correlation analysis of the data of the three cavities can reduce the accidental error compared with the single-position monitoring, and the mixing efficiency can be quantitatively characterized by the ratio Δλ 2 / Δλ 1 , and the stability of Δλ 3 can judge the reaction end point.

[0028] In the above experiment, the data of the three cavities show a typical progressive law: the refractive index change rate in the inlet area A is the fastest, the oscillation characteristics appear in the mixing area B, and the equilibrium area C is finally stable. This multi-position response difference can effectively distinguish fluid stagnation anomalies (synchronous stagnation of the three cavities) from local bubble interference (mutation of the data of a single cavity).

[0029] Through the signal collector (signal processing module), the combined optical signal of the outgoing optical fiber 5 can be detected in real time, and the interference light intensity can be converted into spectral data for quantitative analysis of the refractive index.

[0030] To sum up, a fiber optic refractive index sensor based on a Mach-Zehnder interferometer fiber optic sensor with arc-shaped cavities is disclosed, including: the incident light is divided into two parts after passing through the beam splitter 2, one part propagates along the fiber core and the other part propagates along the fiber cladding. After the light passes through the combiner 4, the light in the core and the cladding interferes. When the external refractive index changes, due to the different positions and depths of the three arc-shaped grooves in the cladding, the change Δn i of the refractive index at different positions of the liquid to be measured can be directly reflected by the wavelength drift Δλ i of the interference spectrum. The present invention is applicable to various practical engineering applications such as microfluidic reaction systems, biomedical detection, environmental monitoring, food safety, and industrial manufacturing.

[0031] The specific implementation cases described here are the preferred solutions of the invention, but the protection scope of the invention is not limited thereto. Any equivalent improvement based on the design shape or structure of the present invention belongs to the scope of the claims of the present invention.

Claims

1. A Mach-Zehnder interferometer optical fiber sensor based on arc-shaped concave cavity single-mode optical fiber. Its characteristics are: It consists of an incident optical fiber 1, a beam splitter 2, an interference arm 3, a beam combiner 4 and an output optical fiber 5. The incident optical fiber 1 in the sensor is used to transmit the input optical signal, and the beam splitter 2 couples the optical signal in the incident optical fiber 1 to the interference arm 3. The interference arm 3 serves as a transmission waveguide, in which three arc-shaped cavities with different depths and curvature radii are arranged. When the solution to be tested is immersed in the cavity, its refractive index changes through the optical path difference and phase difference modulation interference spectrum. The design of the three cavities can simultaneously measure the refractive index distribution of three independent positions in the reaction process. The beam combiner 4 recombines the optical signals in the interference arm and couples them to the output optical fiber 5; The output optical fiber 5 is responsible for outputting the optical signal and transmitting it to the detection equipment for analysis.

2. The Mach-Zehnder interferometer optical fiber sensor based on arc-shaped concave cavity single-mode optical fiber according to claim 1, characterized in that: The incident optical fiber 1 is composed of a single-mode optical fiber, and the input end is connected to an external broadband light source, and the output light of the broadband light source is transmitted into the core of the single-mode optical fiber.

3. The Mach-Zehnder interferometer optical fiber sensor based on arc-shaped concave cavity single-mode optical fiber according to claim 1, characterized in that: The structure of the beam splitter 2 is the same as that of the beam combiner 4, both of which are composed of coreless optical fibers with a diameter of 125 μm and a length of 1-2 mm.

4. The Mach-Zehnder interferometer optical fiber sensor based on arc-shaped concave cavity single-mode optical fiber according to claim 1, characterized in that: The interference arm 3 is composed of a single-mode optical fiber, and includes a first interference arm 31 and a second interference arm 32, and the first interference arm 31 is composed of the cladding in the single-mode optical fiber, and the second interference arm 32 is composed of the core in the single-mode optical fiber; the first interference arm 31 and the second interference arm 32 are equal in length, and the length is 12 mm. The structure of the first interference arm 31 of the single-mode optical fiber is formed by femtosecond laser micromachining technology to form three arc-shaped concave cavities with different depths, and the side-throw structure is that the first interference arm 31 enables the optical signal in the cladding to interact with the measured liquid.

5. The Mach-Zehnder interferometer optical fiber sensor based on arc-shaped concave cavity single-mode optical fiber according to claim 4, characterized in that: The arcuate cavity has a curvature radius of 2-3mm and an axial length of 2-3mm. The arcuate cavities are arranged at equal intervals along the axial direction of the optical fiber, and the interval is 1.2-1.5 times the reaction characteristic length. The depth of the cavity is 30%, 50% and 70% of the thickness of the single-mode optical fiber cladding, respectively.

6. The Mach-Zehnder interferometer optical fiber sensor based on arc-shaped concave cavity single-mode optical fiber according to claim 4, characterized in that: The three cavities correspond to three monitoring points in the solution reaction area respectively. By combining the phase decoupling algorithm of three-channel interference spectroscopy, the refractive index changes at three positions during the reaction process can be measured simultaneously.

7. The Mach-Zehnder interferometer optical fiber sensor based on arc-shaped concave cavity single-mode optical fiber according to claim 1, characterized in that: The beam combiner 4 is used to recouple the optical signal modulated by the measured liquid in the side-throw structure back to the fiber core.

8. The Mach-Zehnder interferometer optical fiber sensor based on arc-shaped concave cavity single-mode optical fiber according to claim 1, characterized in that: The output optical fiber 5 is composed of a single-mode optical fiber, and the output end is connected to an external spectrometer to obtain an interference spectrum containing sensing information. The generation of interference fringes originates from the optical path or phase change caused by the path difference when the optical signal is superimposed. This change will cause the synthetic light intensity to be distributed periodically, forming a pattern of alternating light and dark. The change of environmental conditions will directly act on the optical path difference or phase difference, thereby changing the characteristic performance of the interference pattern.

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