Ammonia concentration detection system, method, and preparation method of interferometer FPI1 and FPI2

Abstract

The present invention belongs to the technical field of ammonia concentration detection, and discloses an ammonia concentration detection system, method, interferometer FPI1 and preparation method of FPI2, including: a light source, a first circulator, a second circulator, an interferometer FPI1, an interferometer FPI2 and a spectrometer; the light source emits a detection light, enters the A port of the first circulator and outputs the detection light to the interferometer FPI1 through the B port of the first circulator, the interferometer FPI1 reflects the detection light to the B port of the first circulator, and outputs the detection light to the A port of the second circulator through the C port of the first circulator, the detection light is output to the interferometer FPI2 through the B port of the second circulator, the interferometer FPI2 reflects the detection light to the B port of the second circulator, and outputs the detection light to the spectrometer through the C port of the second circulator. The technical solution of the present invention can accurately control the free spectral range of the two interferometers to produce a vernier effect, thereby improving the sensitivity of ammonia detection.

Description

Technical Field

[0001] The present invention belongs to the technical field of ammonia concentration detection, and in particular relates to an ammonia concentration detection system, method, and a preparation method of interferometers FPI1 and FPI2. Background Art

[0002] Ammonia is widely used in various industrial processes, agricultural activities and biotechnology, including ammonia (NH3) and free ammonium ions (NH4 + ), whose composition mainly depends on the pH of water. In most natural water environments, the pH value of water is below 9, so ammonia in water mainly exists in the form of free ammonia ions. Ammonia and its salts are easily soluble in water, and even low concentrations can cause harm to aquatic organisms. When the dissolved ammonia content is too high, it will cause the death of fish and crabs, seriously threatening the ecological balance of water bodies and fishery production. In addition, ammonia can cause chemical pharyngitis, pneumonia and other diseases when ingested by humans, posing a potential threat to human health. Therefore, detecting the concentration of ammonia in water is very necessary to maintain the ecological balance of water bodies and human health.

[0003] In recent years, a variety of ammonia sensors based on electrochemical reactions have been proposed to detect trace ammonia in water. Although this method has high accuracy and sensitivity, the electrolyte is easily contaminated, the sensor is large in size, and the stability is poor, which limits the development of such sensors. In contrast, fiber optic sensors have attracted more and more attention due to their advantages such as small size, low cost, and resistance to electromagnetic interference. Among the many fiber optic sensors, fiber Fabry-Perot interferometer (FPI) is particularly attractive in fiber optic sensing applications due to its compact structure, flexible sensitivity adjustment, and easy preparation.

[0004] To achieve specific sensing of ammonia using optical fiber, a layer of sensitive material needs to be deposited on the surface of the optical fiber. The sensitive material attached to the surface of the optical fiber reacts physically or chemically with ammonia, changing the optical properties of the sensitive material, thereby realizing the detection of ammonia concentration. Metal oxides, nanomaterials, and organic dyes have been widely used to detect ammonia in water, but the former two have limited ammonia absorption capacity and are not sensitive enough at low ammonia concentrations; although the latter has higher sensitivity, the dye slowly dissolves into water and needs to be coated with a protective layer to obtain a stable sensing signal, but the protective layer hinders the reaction of the dye with ammonia, reducing the sensitivity of the sensor, and cannot achieve stable sensing of trace ammonia in water. Summary of the invention

[0005] The object of the present invention is to provide an ammonia concentration detection system, method, and method for preparing interferometers FPI1 and FPI2 to solve the problems existing in the above-mentioned prior art.

[0006] To achieve the above object, the present invention provides an ammonia concentration detection system, comprising a light source, a first circulator, a second circulator, an interferometer FPI1, an interferometer FPI2 and a spectrometer;

[0007] The light source emits a detection light, which enters the A port of the first circulator and outputs the detection light to the interferometer FPI1 through the B port of the first circulator. The interferometer FPI1 reflects the detection light to the B port of the first circulator, and outputs the detection light to the A port of the second circulator through the C port of the first circulator. The detection light passes through the B port of the second circulator and is output to the interferometer FPI2. The interferometer FPI2 reflects the detection light to the B port of the second circulator, and outputs the detection light to the spectrometer through the C port of the second circulator.

[0008] Optionally, the interferometer FPI1 is disposed in the container to be tested, and the container to be tested and the interferometer FPI2 are both disposed in a heat preservation box.

[0009] Optionally, the light source is a broadband light source.

[0010] An ammonia concentration detection method, applied to the ammonia concentration detection system, comprises:

[0011] Obtaining an ammonia solution to be tested, and placing the ammonia solution to be tested in a container to be tested;

[0012] The temperature of the heat preservation box is set, and the light source is started to emit a detection light. The detection light passes through the first circulator and enters the interferometer FPI1 for a first reflection to obtain a first reflected light. The first reflected light passes through the second circulator and enters the interferometer FPI2 for a second reflection to obtain a second reflected light.

[0013] Performing spectral analysis on the second reflected light by a spectrometer to obtain an interference spectrum shift;

[0014] The ammonia concentration of the ammonia solution to be tested is calculated based on the interference spectrum shift.

[0015] Optionally, calculating the ammonia concentration of the ammonia solution to be tested based on the interference spectrum movement specifically includes: calculating refractive index change data of the interferometer FPI1 based on the interference spectrum movement, and calculating the ammonia concentration of the ammonia solution to be tested based on the refractive index change data.

[0016] A method for preparing an interferometer FPI1, comprising:

[0017] Obtaining a first single-mode optical fiber, a second single-mode optical fiber, and a third single-mode optical fiber;

[0018] The first single-mode optical fiber and the third single-mode optical fiber are staggered and fused at two ends of the second single-mode optical fiber to form an open FP cavity between the first single-mode optical fiber and the third single-mode optical fiber, and a high refractive index material is filled in the FP cavity with a fourth single-mode optical fiber and cured to obtain a cured FP cavity;

[0019] The cured FP cavity is immersed in a deionized water solution to absorb water. After the absorption of water is completed, the elastic body PDMS is coated on the top of the FP cavity using a fifth single-mode optical fiber and cured to complete the preparation of the interferometer FPI1.

[0020] Optionally, the high refractive index material is a mixture of oxazine perchloric acid and a high refractive index photosensitive adhesive.

[0021] A method for preparing an interferometer FPI2, comprising:

[0022] Obtaining a hollow core optical fiber, a sixth single-mode optical fiber, and a seventh single-mode optical fiber;

[0023] The sixth single-mode optical fiber and the seventh single-mode optical fiber are respectively core-fused to the two ends of the hollow-core optical fiber to form a closed cavity between the sixth single-mode optical fiber and the seventh single-mode optical fiber, thereby completing the preparation of the interferometer FPI2.

[0024] The technical effects of the present invention are:

[0025] The present invention realizes ammonia concentration detection by utilizing the principle that the change of refractive index in the cavity of the sensor interferometer causes the wavelength shift of the interference spectrum. By cascading reference interferometers, the free spectral range of the two interferometers is precisely controlled to produce a vernier effect, thereby improving the ammonia detection sensitivity and realizing stable sensing. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0027] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0028] Figure 1 Schematic diagram of optical transmission of an ammonia sensing system in an embodiment of the present invention;

[0029] Figure 2 is a flow chart of the preparation of the sensor interferometer FPI1 in an embodiment of the present invention;

[0030] Figure 3 is a flow chart of the preparation of the reference interferometer FPI2 in an embodiment of the present invention;

[0031] Figure 4 It is a micrograph of the prepared interferometers FPI1 and FPI2 in the embodiment of the present invention;

[0032] Figure 5 is a schematic diagram of the interference spectrum of the cascade structure in an embodiment of the present invention;

[0033] Figure 6 Schematic diagram of interference spectra of FPI1, FPI2 and cascade sensors at different refractive indices in an embodiment of the present invention;

[0034] Figure 7 This is a schematic diagram of the interference spectrum of FPI1 when the ammonia concentration is 5-40 ppm in an embodiment of the present invention;

[0035] Figure 8 Schematic diagram of the interference spectrum of the cascade sensor when the ammonia concentration is 5-40 ppm in an embodiment of the present invention. DETAILED DESCRIPTION

[0036] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0037] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0038] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those of ordinary skill in the art generally understood by the present invention. Although the present invention describes only preferred methods, any method similar or equivalent to that described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods related to the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0039] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present application description and examples are exemplary only.

[0040] The words “include,” “including,” “have,” “contain,” etc. used in this article are open-ended terms, meaning including but not limited to.

[0041] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0042] Embodiment 1

[0043] like Figure 1 - Figure 8 As shown, this embodiment provides an ammonia concentration detection system, including a light source, a first circulator, a second circulator, an interferometer FPI1, an interferometer FPI2 and a spectrometer;

[0044] The light source emits a detection light, which enters the A port of the first circulator and outputs the detection light to the interferometer FPI1 through the B port of the first circulator. The interferometer FPI1 reflects the detection light to the B port of the first circulator, and outputs the detection light to the A port of the second circulator through the C port of the first circulator. The detection light passes through the B port of the second circulator and is output to the interferometer FPI2. The interferometer FPI2 reflects the detection light to the B port of the second circulator, and outputs the detection light to the spectrometer through the C port of the second circulator.

[0045] An ammonia concentration detection method, applied to the ammonia concentration detection system, comprises:

[0046] Obtaining an ammonia solution to be tested, and placing the ammonia solution to be tested in a container to be tested;

[0047] The temperature of the heat preservation box is set, and the light source is started to emit a detection light. The detection light passes through the first circulator and enters the interferometer FPI1 for a first reflection to obtain a first reflected light. The first reflected light passes through the second circulator and enters the interferometer FPI2 for a second reflection to obtain a second reflected light.

[0048] Performing spectral analysis on the second reflected light by a spectrometer to obtain an interference spectrum shift;

[0049] The ammonia concentration of the ammonia solution to be tested is calculated based on the interference spectrum shift.

[0050] This embodiment designs an open FPI interferometer for detecting ammonia concentration in water. The interferometer is prepared by staggered fusion of single-mode optical fibers. The open cavity is filled with a mixture of high-refractive index photosensitive adhesive (NOA170) and oxazine perchloric acid (O17), and a layer of PDMS protective film is coated on its surface to extend the life of the sensor. Ammonia concentration detection is achieved based on the reaction between ammonia and O17 to change the refractive index in the cavity. In order to further improve the sensitivity of ammonia detection, single-mode optical fiber, hollow-core optical fiber, and single-film optical fiber are fused in sequence to prepare a closed air cavity. A sensor is formed by cascading a closed cavity and an open cavity. The free spectral range of the two interferometers is accurately controlled to produce a cursor effect in the interference spectrum, thereby improving the sensitivity of ammonia detection.

[0051] Schematic diagram of optical transmission of ammonia sensing system Figure 1 As shown, it consists of a broadband light source (wavelength range: 1450-1650nm), a spectrum analyzer (wavelength range: 600-1700nm; maximum resolution: 0.02nm), a temperature control box (accuracy 0.1℃), a fiber circulator, FPI1 and FPI2. The two FPIs are placed in a temperature control box, and FPI1 is connected to FPI2 through the first circulator and the second circulator. The light from the broadband light source enters FPI1 through the first circulator, the reflected light of FPI1 enters FPI2 through the first circulator and the second circulator, and the light reflected by FPI2 is received by the spectrometer through the second circulator.

[0052] FPI1 is used as a sensor interferometer, and its preparation process is as follows Figure 2 As shown, the specific steps are as follows:

[0053] (1) Manually adjust the welding machine motor parameters to weld SMF1 and SMF2 with a misalignment of 75 μm. Cut SMF2 to the required length under a microscope, as shown in the figure. Figure 2 As shown in (a), Figure 2 (a) is a schematic diagram of the offset welding of SMF1 and SMF2 in an embodiment of the present invention;

[0054] (2) The cut end of SMF2 is welded to SMF3 by offset welding, and the offset amount selected is the same as the offset amount in step (1). At this point, an open FP cavity is formed between SMF1 and SMF3, as shown in Figure 2 As shown in (b), Figure 2 (b) is a schematic diagram of the staggered welding of the cut end of SMF2 and SMF3 in an embodiment of the present invention;

[0055] (3) Mix 5 mg of oxazine perchloric acid (O17) and 5 ml of high refractive index photosensitive adhesive (NOA170) thoroughly, fill the mixture into the open cavity with SMF4, and irradiate the cavity with a UV curing lamp for about 3 minutes. Figure 2 As shown in (c), Figure 2(c) is a schematic diagram of the process of filling the mixture into the open cavity in an embodiment of the present invention;

[0056] (4) Immerse the cured open cavity in 50 ml of deionized water. After the cavity absorbs water, the spectrum shifts to the short-wave direction. When the spectrum in the spectrometer no longer moves, it means that the cavity has fully absorbed water. At this time, take out the cavity. Figure 2 As shown in (d), Figure 2 (d) is a schematic diagram of water absorption in the cavity in an embodiment of the present invention;

[0057] (5) Prepare PDMS by mixing the elastic polymer Sylgard184-A and the curing agent Sylgard184-B in a ratio of 10:1. Use SMF5 to dip a small amount of PDMS and evenly apply it on the surface of the curing cavity. Use alcohol cotton to wipe off the excess PDMS. Figure 2 As shown in (e), Figure 2 (e) is a schematic diagram of applying PDMS in an embodiment of the present invention;

[0058] (6) Place the PDMS in a temperature-controlled box at 80°C and heat for 1 hour to solidify the PDMS. Under a microscope, it was observed that due to lateral restrictions, the colloid and PDMS protective layer in the open cavity could only expand in the radial direction.

[0059] FPI2 is used as a reference interferometer, and its preparation process is as follows Figure 3 As shown, the specific steps are as follows:

[0060] (1) Splice the SMF6 with a smooth end face and the hollow-core fiber (inner diameter of about 75µm, outer diameter of about 150μm) together, and cut the hollow-core fiber to the required length under a microscope. Figure 3 As shown in (a), the cut hollow-core fiber is fused with the SMF7 core, and the signal light is at the interface. M 3 and M The 4 reflections form FPI2, such as Figure 3 As shown in (b); Figure 3 (a) is a schematic diagram of the core-to-core fusion splicing of SMF6 and hollow-core optical fiber in an embodiment of the present invention. Figure 3 (b) is a schematic diagram of the core-to-core fusion splicing of the hollow core optical fiber and SMF7 in an embodiment of the present invention.

[0061] Micrographs of the prepared interferometers FPI1 and FPI2 are shown in Figure 2. Figure 4 As shown, Figure 4 (a) is a micrograph of FPI1 in an embodiment of the present invention, Figure 4 (b) is a micrograph of FPI2 in an embodiment of the present invention.

[0062] Beam transmission principle: Figure 1As shown, the incident light (beam 1) emitted by the broadband light source enters the sensor interferometer FPI1 through the first circulator, is reflected by FPI1, and enters the reference interferometer FPI2 through the second circulator. The light reflected by FPI2 is received by the spectrometer through the second circulator, that is, the signal received by the spectrometer is the superposition of the reflection spectra of the two interferometers. Figure 2 As shown, the open cavity interface M 1 and M The reflected light of 2 is superimposed and enters the second circulator through the first circulator. This part of the signal light is called beam 2. M 1 and M Between 2 L 1 is 200µm. Figure 3 As shown, the light received by the second circulator is M 3 and M 4 reflection, the reflected light is superimposed and enters the spectrometer after passing through the second circulator. This part of the signal light is called beam 3. M 3 and M Between 4 L 2 is 300µm.

[0063] Beam 1 and beam 2 form a Fabry-Perot interferometer FPI1, and beam 2 and beam 3 form another Fabry-Perot interferometer FPI2. The interference spectra of FPI1 and FPI2 are expressed as:

[0064] (1)

[0065] (2)

[0066] in, λ m is the wavelength of incident light, I 1 and I 2 are interfaces M 1 and M The reflected light intensity at 2, I 3 and I 4 are interfaces M 3 and M The reflected light intensity at 4, L 1 and L 2 are the cavity lengths of interferometers FPI1 and FPI2, n 1 and n 2 respectively interferometers FPI1 and FPI2 refractive index. The two interferometers are cascaded through the first circulator and the second circulator. When the interference spectrum intensity is expressed in dB, the interference spectrum received by the spectrometer is the superposition of the interference spectrum of FPI1 and FPI2, that is:

[0067] (3)

[0068] in, A is the DC component, B and C are the amplitudes of the interference spectra of FPI1 and FPI2 respectively. When the free spectral range of FPI1 ( FSR 1 = λ m 2 / 2 n 1 L 1) Free spectral range of FPI2 ( FSR 2 = λ m 2 / 2 n 2 L 2) When they are close but not equal, a cursor effect will occur after cascading, that is, an envelope will appear in the cascade interference spectrum. The envelope of the cascade interference spectrum is as follows: Figure 5 As shown, the envelope And cursor magnification M Can be expressed as:

[0069] (4)

[0070] in, D is the interference spectrum envelope amplitude, M is the cursor magnification.

[0071] Ammonia sensing principle: Oxazine perchloric acid (O17) is a sensitive material for ammonia detection. When exposed to ammonia in a water environment, an acid-base neutralization reaction occurs, as shown in formula (5):

[0072] (5)

[0073] This reaction will cause the refractive index of the sensor interferometer FPI1 to change, which will cause the drift of the reflection spectrum. The ammonia concentration can be detected by tracking the movement of the interference spectrum. I When the peak wavelength of 1(λ) satisfies condition (6), the interference spectrum fringes will have a minimum value:

[0074] (6)

[0075] It can be seen that the refractive index sensitivity of FPI1 is:

[0076] (7)

[0077] It can be seen that the refractive index sensitivity of FPI1 is determined by the ratio of the working wavelength to the refractive index in the cavity. When the ammonia concentration changes, the refractive index of FPI1 changes, and the interference spectrum I1(λ) moves with the change of refractive index, and the envelope of the cascade interference spectrum moves accordingly, and the envelope movement is approximately the movement of a single sensor interferometer. M times, that is, the cascade vernier effect amplifies the ammonia sensitivity, and the ammonia sensitivity of the cascade interference spectrum envelope It can be expressed as:

[0078] (8)

[0079] The above analysis is numerically simulated, and the simulation parameters are: I 1=0.0005, I 2=0.0001, I 3=0.001, I 4=0.0001, n 1=1.6900, n 2=1.0, λ m =1500nm, L 1=200µm, L 2=301µm. n When 1 increases from 1.6900 to 1.6913, the interference spectrum of FPI1 at constant temperature is as follows: Figure 6 As shown in (a), Figure 6 (a) is a schematic diagram of the interference spectrum when the refractive index of FPI1 in the embodiment of the present invention increases from 1.6900 to 1.6913, and it can be seen that the free spectrum range of FPI1 is 3.55nm. As the refractive index increases, the interference spectrum of FPI1 redshifts to the long-wave direction, with a shift of about 1.14nm. Figure 6 (b) is the interference spectrum of FPI2 at constant temperature. Figure 6 (b) is a schematic diagram of the interference spectrum when the refractive index of FPI2 is 1 at a constant temperature in an embodiment of the present invention. It can be seen that the free spectrum range of FPI2 is 3.99nm. The cascade interference spectrum of interferometers FPI1 and FPI2 is shown in FIG. Figure 6 As shown in (c), Figure 6 (c) is a schematic diagram of the interference spectrum of the cascade sensor when the refractive index of FPI1 in the embodiment of the present invention increases from 1.6900 to 1.6913. After the two interferometers are cascaded, an obvious envelope phenomenon is shown. The free spectral range of the interference spectrum envelope is 32.46nm. When the refractive index increases from 1.6900 to 1.6913, the envelope red shifts by 10.83nm, which is 9.5 times the movement of a single sensor interferometer FPI1. That is, the ammonia sensitivity of the sensor after cascading is increased by 9.5 times compared with a single sensor interferometer.

[0080] Ammonia concentration test: In the ammonia concentration test experiment, 0.01% standard ammonia solution was diluted with deionized water and stirred thoroughly with a temperature-controlled magnetic stirrer to prepare an ammonia solution with a concentration between 5-40ppm. Since NOA170 photosensitive adhesive has a high thermo-optic coefficient, the sensor was placed in a temperature-controlled box and Figure 1 The constructed test device was used to carry out ammonia sensing experiments at a controlled temperature of 26 °C. Figure 7 The interference spectrum of FPI1 changes with ammonia concentration when the ammonia concentration is 5-40ppm. The results show that when the ammonia concentration increases from 5ppm to 40ppm, the interference spectrum red shifts 1.1nm (moves to the long-wave direction), and the peak wavelength of the interference spectrum has a good linear relationship with the change of ammonia concentration. After fitting, the ammonia sensitivity of FPI1 is 0.031nm / ppm. Figure 8 The interference spectrum of the sensor changes with the ammonia concentration when the ammonia concentration is 5-40ppm. The results show that when the ammonia concentration is 5-40ppm, the interference spectrum envelope of the sensor is red-shifted by 10.5nm, and the ammonia sensitivity of the sensor after fitting is 0.29nm / ppm. The sensitivity is increased by 9.35 times after cascading, which is slightly lower than the simulated vernier magnification, which is due to the slight difference between the simulation parameters and the experimental parameters.

[0081] This embodiment uses the reaction between dissolved ammonia and oxazine perchloric acid to change the refractive index in the cavity of the sensor interferometer, thereby causing the wavelength of the interference spectrum to move to achieve ammonia concentration detection. Since the dye is soluble in water, a PDMS protective film is coated on the surface of the sensor interferometer to extend the life of the sensor. In addition, by cascading the reference interferometer, the free spectral range of the two interferometers is accurately controlled to produce a vernier effect, thereby improving the sensitivity of ammonia detection.

[0082] The above is only a preferred specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. An ammonia concentration detection system, characterized in that: It includes a light source, a first circulator, a second circulator, an interferometer FPI1, an interferometer FPI2 and a spectrometer; The light source emits a detection light, which enters the port A of the first circulator and outputs the detection light to the interferometer FPI1 through the port B of the first circulator; the interferometer FPI1 reflects the detection light to the port B of the first circulator, and outputs the detection light to the port A of the second circulator through the port C of the first circulator; the detection light passes through the port B of the second circulator and is output to the interferometer FPI2; the interferometer FPI2 reflects the detection light to the port B of the second circulator, and outputs the detection light to the spectrometer through the port C of the second circulator; The interferometer FPI1 is arranged in the container to be tested, and the container to be tested and the interferometer FPI2 are both arranged in a heat preservation box; the light source adopts a broadband light source; the refractive index change data of the interferometer FPI1 is calculated based on the interference spectrum movement, and the ammonia concentration of the ammonia solution to be tested is calculated based on the refractive index change data; The preparation method of the interferometer FPI1 comprises: (1) Manually adjust the motor parameters of the fusion splicer to perform misaligned fusion splicing of the first single-mode optical fiber SMF1 and the second single-mode optical fiber SMF2, with a misalignment of 75 μm; (2) The cut end of the second single-mode optical fiber SMF2 is staggered and fused with the third single-mode optical fiber SMF3, and the selected staggered amount is the same as the staggered amount in step (1), so that an open FP cavity is formed between the first single-mode optical fiber SMF1 and the third single-mode optical fiber SMF3; (3) 5 mg of oxazine perchlorate O17 and 5 ml of high refractive index photosensitive adhesive NOA170 were fully mixed to obtain a mixture, the mixture was filled into the open cavity using the fourth single-mode optical fiber SMF4, and the cavity was irradiated with a UV curing lamp for 3 minutes; (4) Immerse the cured open cavity in 50 ml of deionized water. After the cavity absorbs water, the spectrum will blue-shift toward the short-wave direction. When the spectrum in the spectrometer no longer moves, it means that the cavity has fully absorbed water. At this time, take out the cavity; (5) The elastic polymer Sylgard184-A and the curing agent Sylgard184-B were mixed in a ratio of 10:1 to prepare PDMS. A small amount of PDMS was dipped into the fifth single-mode optical fiber SMF5 and evenly applied on the surface of the curing cavity. The excess PDMS was wiped off with alcohol cotton. (6) Place the sample in a temperature-controlled box at 80 °C and heat for 1 hour to solidify the PDMS. Under a microscope, it is observed that due to lateral restrictions, the colloid and the PDMS protective layer in the open cavity can only expand in the radial direction, thereby obtaining the interferometer FPI1. The interferometer FPI2 is used as a reference interferometer, and the specific preparation steps are as follows: Obtain a hollow-core optical fiber, a sixth single-mode optical fiber, and a seventh single-mode optical fiber; respectively fuse the sixth single-mode optical fiber and the seventh single-mode optical fiber to the two ends of the hollow-core optical fiber to form a closed cavity between the sixth single-mode optical fiber and the seventh single-mode optical fiber, thereby completing the preparation of the interferometer FPI2, wherein the inner diameter of the sixth single-mode optical fiber is 75µm, and the outer diameter is 150μm.

2. An ammonia concentration detection method, applied to an ammonia concentration detection system according to claim 1, characterized in that: include: Obtaining an ammonia solution to be tested, and placing the ammonia solution to be tested in a container to be tested; The temperature of the heat preservation box is set, and the light source is started to emit a detection light. The detection light passes through the first circulator and enters the interferometer FPI1 for a first reflection to obtain a first reflected light. The first reflected light passes through the second circulator and enters the interferometer FPI2 for a second reflection to obtain a second reflected light. Performing spectral analysis on the second reflected light by a spectrometer to obtain an interference spectrum shift; The refractive index change data of the interferometer FPI1 is calculated based on the interference spectrum movement, and the ammonia concentration of the ammonia solution to be measured is calculated based on the refractive index change data.

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