Temperature self-compensating ammonia nitrogen all-fiber detection sensor and preparation method thereof

Abstract

The invention belongs to the field of optical fiber sensing, and provides a temperature self-compensation ammonia nitrogen all-optical fiber detection sensor and a preparation method thereof, the temperature self-compensation ammonia nitrogen all-optical fiber detection sensor is formed by cascading an SMF-TCF-NCF-SMF structure generating Mach-Zehnder interference and an SMF-HHF-SMF structure generating an anti-resonance effect, and STNS and SHS simultaneously excite an MZI valley and an AR valley in the same spectrum. The refractive index of the TCF coating is changed by utilizing the change of ammonia concentration, so that MZI valley wavelength drift is caused, and the ammonia concentration in water is detected; and the refractive index of air in the HCF is changed by temperature change, so that the valley wavelength drift of the AR is caused, and temperature detection is realized. The ambient temperature is directly obtained by demodulating the AR valley, then the ambient temperature and the MZI valley are jointly demodulated to obtain the ammonia concentration after temperature interference compensation, and finally high-precision measurement of the ammonia concentration in water is achieved.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing, specifically relating to a temperature-self-compensated all-fiber optic sensor for ammonia nitrogen detection and its preparation method. Background Technology

[0002] Ammonia nitrogen is a key indicator for water quality monitoring. Excessive levels can lead to rapid algal blooms and eutrophication, disrupting the entire aquatic ecosystem. Dissolved ammonia in water oxidizes at room temperature to produce nitrates. Long-term consumption of water with excessive nitrate levels can cause methemoglobinemia, posing a potential threat to human health. Therefore, high-precision measurement of ammonia nitrogen in aquatic environments is crucial for water quality ecological assessment and protecting human health.

[0003] Among established methods for ammonia nitrogen detection in water, fiber optic sensing technology has attracted significant attention in the field of water quality testing due to its substantial advantages, including simple equipment, resistance to electromagnetic interference, corrosion resistance, high sensitivity, ease of miniaturization, and distributed multi-point measurement. However, the parameters that can be measured by the silica-based optical fiber are limited, especially its weak response to trace parameters in the aquatic environment. Therefore, the method of functionalizing the optical fiber surface with deposited sensitive coatings has been widely adopted. However, coating materials such as ammonia-sensitive metal oxides, nanofilms, and organic dyes are cross-sensitive to ambient temperature, severely affecting measurement accuracy. Limited by this, current fiber optic ammonia sensors can only achieve measurements under isothermal conditions, and the problem of temperature cross-sensitivity has not yet been solved, making it difficult to meet the high-precision detection requirements of practical aquatic environments.

[0004] An effective method for high-precision detection of ammonia concentration in water is temperature compensation. This method compensates for the cross-interference of ambient temperature on the sensitive coating by simultaneously measuring and demodulating the two parameters of temperature and ammonia concentration, thereby achieving accurate ammonia concentration measurement. To this end, this invention designs an all-fiber ammonia sensor with temperature self-compensation function. This sensor consists of a single-mode optical fiber (… SMF ) and thin-core optical fiber ( TCF Small offset fusion splicing, followed by sequential fusion splicing of coreless optical fibers ( NCF )- SMF - Hollow-core optical fiber ( HCF )- SMF Thus, TCF The surface is coated with an ammonia-responsive silica sol-gel coating containing bromocresol violet. Light in... TCF and HCF During transmission, Mach-Zehnder interferometry is excited respectively. MZI ) and anti-resonance effect ( AR The sensor output spectrum is generated simultaneously. MZI Taniwa AR Valley. Among them, MZI Glutamate is sensitive to both ammonia concentration and temperature, while ARThe valley is only sensitive to temperature. The refractive index of the coating is altered by changes in ammonia concentration, thereby causing… MZI Valley wavelength shift enables ammonia concentration detection in water; temperature changes alter... HCF The refractive index of the internal air thus leads to AR Valley wavelength shift enables temperature detection. This is achieved through demodulation. AR The valley directly obtains the ambient temperature, and then... MZI The ammonia concentration in water is obtained by joint demodulation after compensating for the temperature effect, thus achieving high-precision measurement of ammonia concentration in water. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a temperature-self-compensated all-fiber optic ammonia nitrogen detection sensor and its fabrication method, thereby resolving the issues in the prior art. The technical solution adopted by this invention is as follows: A temperature-compensated all-fiber optic sensor for ammonia nitrogen detection, comprising, end to end, connected... SMF 1. TCF , NCF , SMF 2. HCF , SMF 3; SMF 1 and TCF Offset misalignment welding TCF , NCF , SMF 2. HCF , SMF 3. Cascaded and fused together, each optical fiber forming a single unit. STNS and SHS Two-part structure; STNS The structure includes SMF 1. TCF , NCF , SMF 2; SHS The structure includes SMF 2. HCF , SMF 3; TCF The surface is coated with an ammonia-sensitive coating; in, SMF 1. SMF 2. SMF 3 is a single-mode fiber. TCF Thin-core optical fiber, NCF It is a coreless optical fiber. HCF It is a hollow-core optical fiber.

[0006] A method for fabricating a temperature-compensated all-fiber optic ammonia nitrogen detection sensor includes the following steps: Step 1, cut the... SMF 1 andTCF A single-end small offset method is used for staggered welding, with an offset distance of 4 at the welding point. µm ; Step 2, weld the pieces together. SMF 1 and TCF Placed under a microscope, and then cleaved with a fiber optic cleaver. TCF Cut to 35 cm in length mm ; Step 3, cut the... TCF and NCF Welding, microscopic cutting NCF Make its length 15 mm ,exist NCF Cutting end welding SMF 2. Composition STNS structure; Step 4, SMF 2. SMF 3 are welded together on a section with an inner diameter of 40. µm Length is 650 µm of HCF Both ends constitute SHS structure; Step 5, cut the... STNS Structure and SHS Structural alignment cascade welding; Step 6, TCF The surface is coated with an ammonia-sensitive coating.

[0007] Furthermore, the ammonia-sensitive coating is a silica sol-gel ammonia-sensitive coating containing bromocresol purple.

[0008] Furthermore, step 6 includes: Step 6.1, take 30 mlTEOS With 15 ml Anhydrous ethanol was mixed and stirred for 20 minutes at 25°C using a temperature-controlled magnetic stirrer. min ; Step 6.2, add 0.1 mol / L of HCl Solution 3 ml Continue stirring for 40 minutes. min ; Step 6.3, add dissolved 5 mgBCP 10g of anhydrous ethanol ml Continue stirring for 60 seconds. min Prepare a silica sol solution, then seal the prepared silica sol solution and store it in a refrigerator; Step 6.4: Fix the prepared sensor onto the three-dimensional moving platform, and move the platform by controlling its movement. TCFA drop of silica sol solution is passed from one end of the area to the other end, completing one coating step; Step 6.5: Dry the sensor at room temperature for 10 minutes before applying the next coating. min Repeat the same coating process as in step 6.4; Step 6.6: After applying the target number of layers, cure at room temperature for 24 hours. h Then at 120° C Lower heating 2 h Then dry at room temperature for 24 hours h .

[0009] Furthermore, for this sensor, the incident light emitted by the light source is transmitted to... SMF 1 and TCF At the misaligned weld joint, the two parts of light I 1 and I 2 Enter separately TCF The light is transmitted in the fiber core and cladding, and the two parts of the light are... NCF Coupling into SMF Mach-Zehnder interference occurs after the fiber core is 2. When the external measurement changes, the two parts of the light accumulate a certain optical path difference after passing through the sensing area, causing a shift in the interference wavelength of the transmission spectrum. The measured parameter is detected by tracking the amount of interference wavelength shift. The output intensity of the Mach-Zehnder interference transmission spectrum is expressed as: (1) in, λ The incident light wavelength, L t for TCF length, I 1 and I 2 are respectively TCF Light intensity in core and cladding modes Δn eff = n core - n clad for TCF The effective refractive index difference between the core and the cladding n core and n clad They are respectively TCF Core and cladding refractive indices; When the ammonia concentration in the water changes, the ammonia-sensitive coating adsorbs ammonia, leading to... TCF The cladding refractive index is from n clad Change to n clad + Δn The change in refractive index alters the optical path difference between the two paths, thus changing the interference wavelength in the transmission spectrum.λ Drift to λ + Δλ The relationship between wavelength drift and refractive index is described as follows: (2) Simplifying equation (2), we get MZI Sensitivity relationship between spectral wavelength shift and refractive index change: (3) From equation (3), it can be seen that tracking MZI The drift of a certain valley wavelength in the spectrum is used to detect the concentration of ammonia in water.

[0010] Furthermore, HCF Inner diameter is r Thickness is d The refractive index of the hollow core is n air The cladding refractive index is n clad , n air < n clad ;beam from SMF 2. Oblique incidence HCF Behind the inner wall, when the incident light wavelength does not meet the requirements FP At resonance, this portion of the light is reflected back. HCF The light continues to propagate along the axial direction, i.e., the first reflected light from the air-cladding interface. I 3; When the wavelength of the incident light satisfies FP At resonance conditions, the light is transmitted into the cladding. Part of the light leaks out to the outside, exhibiting a periodic loss decrease in the transmission spectrum at resonance valleys. The other part of the light is reflected by the outer wall of the cladding and refracted at the air-cladding interface, returning to the hollow core, forming a tortuous, back-and-forth refracted light. I 4; when HCF Length greater than beam I 4. At the critical length of axial transmission, I 3 and I 4. Interference of two beams of light AR effect, AR The transmission spectrum is: (4) in, F for AR Fineness of the beam's stripes φ are adjacent AR The phase difference between beams. When the phase difference satisfies the condition of equation (5), the cladding transmittance is at its maximum and the reflectivity is at its minimum, resulting in an anti-resonance effect: (5) in,m The resonance order is... λ m For the first m First resonant wavelength, θ Let be the transmission angle of the light entering the cladding from the hollow core; when the light in the hollow core is approximately incident perpendicularly to the cladding surface, according to Snell's law: (6) Derive the first condition that satisfies the anti-resonance condition m The first resonant wavelength is: (7) The position of the resonant wavelength and the cladding thickness can be obtained. d air refractive index n air and cladding refractive index n clad Relevant; when sensing ammonia concentration, only the ambient refractive index changes. λ m , n air and n clad Nothing changes. AR Valley is not sensitive to changes in ammonia concentration; however, when performing temperature sensing... d and n air Neglecting the changes in temperature, consider the effect of temperature on the cladding refractive index. n clad Due to the influence of temperature, the derivative of the resonant wavelength with respect to temperature is: (8) in, It is the thermo-optic coefficient of the cladding; when the temperature increases... n clad Increasing the wavelength shifts the resonant wavelength towards longer wavelengths; thus obtaining... STNS and SHS The total output spectrum of the sensor after structural cascading: (9).

[0011] The beneficial effects of this invention are: This invention is based on the generation of Mach-Zehnder interference. SMF - TCF - NCF - SMF Structure and generation of anti-resonance effect SMF - HCF - SMF Structure cascaded, STNS and SHS Simultaneous excitation in the same spectrum MZI Taniwa ARValley. Utilizing changes in ammonia concentration to alter... TCF The refractive index of the coating thus causes MZI Valley wavelength shift enables ammonia concentration detection in water; temperature changes alter... HCF The refractive index of the internal air thus leads to AR Valley wavelength shift enables temperature detection. This is achieved through demodulation. AR The valley directly obtains the ambient temperature, and then... MZI The ammonia concentration after temperature interference is obtained by joint demodulation, and finally high-precision measurement of ammonia concentration in water is achieved. Attached Figure Description

[0012] Figure 1 This is an ammonia concentration sensing system.

[0013] Figure 2 It is a temperature sensing system.

[0014] Figure 3 This is a schematic diagram of a temperature-compensated all-fiber optic sensor for ammonia nitrogen detection according to the present invention.

[0015] Figure 4 Microscopic images of the sensor fabrication process and the five-coat sensor; among which, ( a Sensor fabrication process, b ) SMF - TCF Welded surface, ( c )sol-gel coating micrographs, ( d ) SHS Microscopic image; Figure 5 This is the spectral evolution process; where, ( a ) STNS Transmission spectrum, ( b ) SHS Transmission spectrum, ( c Cascade transmission spectrum, d ) Spectral comparison before and after coating, ( e Transmission spectrum after coating; Figure 6 The results are spectral simulation results; where, ( a ) MZI spectrum( b ) AR spectrum( c Cascaded output total spectrum; Figure 7 The light intensity distribution along the fiber axis; where, ( a ) STNS Internal energy transfer diagram of the structure, b Different offsets MZI spectrum,( c )differentTCF Length MZI spectrum,( d )different NCF Length MZI spectrum; Figure 8 The relationship between ammonia concentration and wavelength; where, ( a ), ( b )for dip 3. Wavelength evolution process as ammonia concentration increases, ( c )for dip M Sensitivity fitting curve of wavelength as ammonia concentration increases; Figure 9 The relationship between wavelength and ammonia concentration under the influence of temperature; where, ( a ) dip M Wavelength shift with increasing temperature, ( b ) dip 3. Wavelength shift with increasing temperature, ( c ) dip M Temperature sensitivity fitting curve, ( d ) dip 3. Temperature sensitivity fitting curve. Detailed Implementation

[0016] The following will be based on embodiments of the present invention. Figures 1 - 9 The technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0017] Ammonia concentration sensing system such as Figure 1 As shown. The sensor is straightened and then... UV The sensor is fixed to a custom concave mold with adhesive. STNS The structural lead-end jumper is connected to a wavelength of 1400-1600 nm. nm On broadband light sources ( GolightOS - EB - S - D -1550-17- S - FA ), SHS The structural lead-end jumper is connected to the spectrometer. Ceyear 6362 D Peristaltic pump separator (Rongbai) BT100) Ammonia solutions of different concentrations are pumped into a concave channel, allowing them to flow slowly around the sensing area. A spectrometer records the transmission spectrum at each concentration measurement point, and the spectral data is processed by a computer. Matlab Software analysis and processing.

[0018] Temperature sensing system such as Figure 2 As shown, the groove for fixing the sensor is placed in the temperature control box ( WGL -30 B Inside, the spectrometer records the transmission spectrum at each temperature measurement point, and the spectral data is processed by a computer. Matlab Software analysis and processing.

[0019] like Figure 3 A temperature-compensated all-fiber optic sensor for ammonia nitrogen detection, comprising, end to end, connected... SMF 1. TCF , NCF , SMF 2. HCF , SMF 3; SMF 1 and TCF Offset misalignment welding TCF , NCF , SMF 2. HCF , SMF 3. Cascaded and fused together, each optical fiber forming a single unit. STNS and SHS Two-part structure; STNS structure( SMF - TCF - NCF - SMF )include SMF 1. TCF , NCF , SMF 2; SHS structure( SMF - HCF - SMF )include SMF 2. HCF , SMF 3; specifically, such as Figure 3 , STNS The structure includes SMF 2. Front section SHS The structure includes SMF 2. The latter part.

[0020] TCF The surface is coated with an ammonia-sensitive coating; In this invention, SMF 1. SMF 2. SMF3 is a single-mode fiber. TCF Thin-core optical fiber, NCF It is a coreless optical fiber. HCF It is a hollow-core optical fiber.

[0021] A method for fabricating a temperature-compensated all-fiber optic ammonia nitrogen detection sensor includes the following steps: Step 1, cut the... SMF 1 and TCF A single-end small offset method is used for staggered welding, with an offset distance of 4 at the welding point. µm ; Step 2, weld the pieces together. SMF 1 and TCF Placed under a microscope, and then cleaved with a fiber optic cleaver. TCF Cut to 35 cm in length mm ; Step 3, cut the... TCF and NCF Welding, microscopic cutting NCF Make its length 15 mm ,exist NCF Cutting end welding SMF 2. Composition STNS structure; Step 4, SMF 2. SMF 3 are welded together on a section with an inner diameter of 40. µm Length is 650 µm of HCF Both ends constitute SHS structure; Step 5, cut the... STNS Structure and SHS Structural alignment cascade welding; Step 6, TCF The surface is coated with an ammonia-sensitive coating.

[0022] Sensor fabrication using standards SMF (Corning, 8.2) μm / 125 μm ), HCF (Corning, 40) µm / 125 µm ), TCF ( Nufern 2.5 μm / 125 μm )and NCF (Corning, 125) μm ).

[0023] Furthermore, the ammonia-sensitive coating is a silica sol-gel ammonia-sensitive coating containing bromocresol purple.

[0024] Furthermore, the ammonia-sensitive coating is prepared using highly chemically active tetraethyl orthosilicate (TOS). TEOS (99% purity) as a precursor, bromocresol purple ( BCP (This refers to an ammonia-sensitive dye.) A sol-gel coating is prepared using the hydrolysis and condensation reaction of the raw materials in the liquid phase. The specific preparation and coating process, specifically step 6, includes: Step 6.1, take 30 mlTEOS With 15 ml Anhydrous ethanol was mixed and stirred for 20 minutes at 25°C using a temperature-controlled magnetic stirrer. min ; Step 6.2, add 0.1 mol / L of HCl Solution 3 ml Continue stirring for 40 minutes. min ; Step 6.3, add dissolved 5 mgBCP 10g of anhydrous ethanol ml Continue stirring for 60 seconds. min Prepare a silica sol solution, then seal the prepared silica sol solution and store it in a refrigerator; Step 6.4: Fix the prepared sensor onto the three-dimensional moving platform, and move the platform by controlling its movement. TCF A drop of silica sol solution is passed from one end of the area to the other end, completing one coating step; Step 6.5: Dry the sensor at room temperature for 10 minutes before applying the next coating. min Repeat the same coating process as in step 6.4; Step 6.6: After applying the target number of layers, cure at room temperature for 24 hours. h Then at 120° C Lower heating 2 h Then dry at room temperature for 24 hours h .

[0025] Before the ammonia test, immerse the coated sensor in deionized water for 12 minutes. h This eliminates the impact of water penetration into the coating on sensing performance. Microscopic images of the sensor fabrication process and the five-coat sensor are shown below. Figure 4 As shown.

[0026] The spectrometer records individual data. STNS structure, SHS The structure, the cascaded structure of the two components, and the transmission spectrum of the sensor after five coatings. From... Figure 5 The spectral evolution process shown reveals that the cascaded spectrum exhibits two distinguishable dips, among which... dip M 1558 nm Nearby MZIvalley, dip 1. dip 2. dip 3 are 1514 respectively nm 1541 nm and 1568 nm Nearby AR The coated sensor exhibits a blue shift in spectral wavelength and a significant decrease in spectral extinction ratio, indicating successful sol-gel coating on the fiber surface. Multiple peaks of varying intensities are present in the coated sensor's spectral spectrum, suggesting the participation of multiple cladding modes in intermodal interference; however, only one dominant cladding mode exhibits the highest peak intensity, indicating… MZI This is caused by interference between the primary cladding mode and the core mode; the weaker cladding mode is mainly caused by uneven cladding thickness in hollow fibers. Because... AR The higher the valley order, the higher the sensitivity. In the experiment, [the following was chosen]. dip M and dip 3 is the object of observation.

[0027] like Figure 3 middle STNS As shown in the structure, for this sensor, the incident light emitted by the broadband light source is transmitted to... SMF 1 and TCF At the misaligned weld joint, the two parts of light I 1 and I 2 Enter separately TCF The light is transmitted in the fiber core and cladding, and the two parts of the light are... NCF Coupling into SMF Mach-Zehnder interference occurs when the fiber core is 2mm thick. When the external measurement is changed, the two parts of light accumulate a certain optical path difference after passing through the sensing area, causing a shift in the interference wavelength of the transmitted spectrum. The measured parameter is detected by tracking the amount of interference wavelength shift. (Mach-Zehnder interference...) MZI The transmission spectrum output intensity is expressed as: (1) in, λ The incident light wavelength, L t for TCF length, I 1 and I 2 are respectively TCF Light intensity in core and cladding modes ​ eff = n core - n clad for ​ The effective refractive index difference between the core and the cladding n core and nclad They are respectively ​ Core and cladding refractive indices. When the ammonia concentration in water changes, the ammonia-sensitive coating's adsorption of ammonia leads to... ​ The cladding refractive index is from n clad change n clad + ​ The change in refractive index alters the optical path difference between the two paths, thus changing the interference wavelength in the transmission spectrum. ​ Drift to ​ + ​ The relationship between wavelength drift and refractive index can be described as follows: (2) Simplifying equation (2), we get ​ Sensitivity relationship between spectral wavelength shift and refractive index change: (3) From equation (3), it can be seen that tracking ​ The drift of a certain valley wavelength in the spectrum is used to detect the concentration of ammonia in water.

[0028] like ​ middle ​ As shown in the structure, ​ Inner diameter is r Thickness is d The refractive index of the hollow core is n air The cladding refractive index is n clad , n air < n clad The high-refractive-index cladding can be considered as radial... ​ Resonant cavity. ​ The beam (Mach-Zehnder interference beam) from ​ 2. Oblique incidence ​ Behind the inner wall, when the incident light wavelength does not meet the requirements ​ At resonance, this portion of the light is reflected back into the hollow fiber and continues to propagate axially; this is the first reflection at the air-cladding interface. I 3. When the wavelength of the incident light satisfies ​ At resonance conditions, the light is transmitted into the cladding. Part of the light leaks out to the outside, exhibiting a periodic loss decrease in the transmission spectrum at resonance valleys. The other part of the light is reflected by the outer wall of the cladding and refracted at the air-cladding interface, returning to the hollow core, forming a tortuous, back-and-forth refracted light. I 4. When ​ Length greater than beam I 4. At the critical length of axial transmission, I 3 andI 4. Interference of two beams of light ​ effect, ​ The transmission spectrum is: (4) in, F for ​ Fineness of the beam's stripes ​ are adjacent ​ The phase difference between beams. When the phase difference satisfies the condition of equation (5), the cladding transmittance is at its maximum and the reflectivity is at its minimum, resulting in an anti-resonance effect: (5) in, m It is the resonance order (a non-zero integer). ​ m For the first m First resonant wavelength, ​ Let be the transmission angle of the light entering the cladding from the hollow core. When the light in the hollow core is approximately incident perpendicularly to the cladding surface, according to Snell's law: (6) This leads to the derivation of the first condition that satisfies the anti-resonance condition. m The first resonant wavelength is: (7) It can be seen that the position of the resonant wavelength is mainly related to the cladding thickness. d air refractive index n air and cladding refractive index n clad Relevant. When sensing ammonia concentration, only the change in ambient refractive index is considered. ​ m , n air and n clad Nothing changes, therefore, ​ Valleys are not sensitive to changes in ammonia concentration. When used for temperature sensing, both silicon dioxide and air have very low coefficients of thermal expansion and thermo-optical coefficients. d and n air The change is negligible; only the effect of temperature on the cladding refractive index needs to be considered. n clad Due to the influence of temperature, the derivative of the resonant wavelength with respect to temperature is: (8) in, This is the thermo-optic coefficient of the cladding. When the temperature increases... n clad As the wavelength increases, the resonant wavelength shifts towards longer wavelengths. Therefore, we can conclude that... ​ and​ The total output spectrum of the sensor after structural cascading: (9).

[0029] use ​ For the simulation of the above cascaded sensor structure, the parameter settings are as follows: I 1 = 0.0005 I 2 = 0.001 I 3 = 0.005 I 4 = 0.0006 n core =1.4502、 n clad =1.445、 n air =1.0、 d =42.5 µm , m =40、 L t =35000 µm , L s =650 µ m。 alone ​ spectrum, ​ The simulation results of the spectrum and the cascaded total spectrum are as follows: ​ ( a ), ( b ), ( c As shown in the figure. It can be seen that 1550 ​ nearby ​ and ​ The free spectral range of the effect is 4.5. ​ and 27.1 ​ The total cascade spectrum is at 1525 ​ 1552 ​ 1579 ​ obvious ​ Valley, adjacent ​ Several appeared in the valley ​ valley.

[0030] Sensor numerical simulation and spectral simulation: caused by misaligned welding ​ At this time, different eccentric distances and fiber lengths directly affect the spectral extinction ratio. To obtain the best spectral quality, using... ​ software ​ Module pair ​ Simulation of the optical field variation inside the structure and the transmission spectrum under different lengths and offsets. Incident light wavelength: 1500-1600 nm. ​ At that time, the light intensity distribution along the fiber axis is as follows ​ (a As shown in the diagram. Light is transmitted to... ​ and ​ When welding the surfaces, enter separately ​ Light energy loss occurs during transmission within the fiber core and cladding. ​ Energy decreases in the fiber core and continues to be transmitted forward. ​ The redistribution of light energy leads to output ​ The optical energy of the fiber core was further reduced. Different misalignment offsets were simulated sequentially using the controlled variable method. ​ Length and NCF The sensor spectrum at various lengths, simulation results are as follows: Figure 7 ( b ), ( c ), ( d As shown in the figure. It can be seen that when the offset is 4... µm , TCF Length is 35 mm , NCF Length is 15 mm hour, MZI The spectral quality is optimal. Therefore, a sensor was fabricated based on the above parameters, and its sensing performance was experimentally studied.

[0031] Ammonia concentration sensing test: 0.01% standard ammonia solution was diluted with deionized water and thoroughly stirred with a temperature-controlled magnetic stirrer to prepare a concentration of 5-40%. ppm Ammonia solution between. According to Figure 1 The experimental setup shown was used to conduct an ammonia concentration sensing experiment at a constant temperature of 25℃. (The last part, "5," appears to be a fragment and doesn't translate directly. It's likely a typo or incomplete sentence.) ppm To increase the ammonia concentration in the step size, the 5-coated sensor was at 1558. nm Nearby dip M and 1568 nm Nearby dip 3. The wavelength evolution process with increasing ammonia concentration is as follows: Figure 8 ( a ), ( b As shown in the figure. dip M The redshift is approximately 5.7. nm , dip 3. The wavelength hardly shifts with increasing ammonia concentration, that is... AR The grain is not sensitive to changes in ammonia concentration. dip M The sensitivity fitting curve of wavelength increasing with ammonia concentration is shown below. Figure 8 ( c As shown in ), in 5-40 ppm Within the concentration range, dip M The ammonia sensitivity of the valley is 162. pm / ppm .

[0032] Temperature sensing test: Press Figure 2 The device shown performs 20 ppm Temperature sensing experiment under constant ammonia concentration. The temperature was increased in increments of 5°C. dip M and dip The changes of the three wavelengths with increasing temperature are as follows: Figure 9 ( a ), ( b As shown in the figure. dip M The wavelength is blue-shifted by approximately 0.40. nm , dip The redshift of wavelength 3 is approximately 0.44. nm . dip M and dip The sensitivity fitting curves of the three wavelengths as ammonia concentration increases are shown below. Figure 9 ( c ), ( d As shown in the figure. Within the temperature range of 30-60℃, dip M The temperature sensitivity is -13.2. pm / ° C , dip The temperature sensitivity of 3 is 14.6. pm / ° C .

[0033] Dual-parameter demodulation: caused by changes in ammonia concentration and ambient temperature dip M and dip The wavelength shift of 3 can be expressed by constructing a sensitivity matrix as follows: (10) in, Δλ dipM and Δλ dip3 They represent dip M and dip Wavelength shift at point 3 k T1 and k C They represent dip M Sensitivity coefficients to temperature and ammonia concentration. k T2 for dip 3. Sensitivity coefficients to temperature. Since the sensitivity coefficients differ, this matrix is ​​invertible. Therefore, the changes in temperature and ammonia concentration are derived as follows: (11) in, The results obtained from the experiment dip M Temperature sensitivity -13.2 pm / ° C Ammonia sensitivity 162 pm / ppm as well as dip Temperature sensitivity 14.6 Ω pm / ° C Substituting into equation (11) in sequence, we can obtain: (12) By introducing wavelength shifts with arbitrary variations in ammonia concentration and temperature, the accuracy of equation (12) in the measurement is verified. (5) ppm Set 25℃ as the initial value, 30 ppm 40℃ is set as the value after the ammonia concentration and temperature change simultaneously. Initial conditions and conditions after the change. dip M and dip The measured value of 3 is 1557.6. nm 1569.2 nm 1561.93 nm and 1569.42 nm The demodulated values ​​for ammonia concentration and temperature under these changing conditions are 30.3. ppm The demodulation errors were 1% and 0.25% respectively, at 35.9℃, which are close to the actual values, indicating that the sensor has good demodulation accuracy when simultaneously measuring ammonia concentration and temperature. Therefore, when changes in ammonia concentration and temperature act on the sensor simultaneously, AR Valley wavelength offset is used as a reference, and is monitored AR The wavelength shift of the valley value directly demodulates the ambient temperature, and thus affects... MZI Temperature compensation is applied to the valley to achieve precise demodulation of ammonia concentration.

[0034] This invention relates to an all-fiber optic sensor for detecting low concentrations of ammonia in water, featuring temperature self-compensation, and utilizing Mach-Zehnder interferometry. SMF - TCF - NCF - SMF Structure and generation of anti-resonance effect SMF - HCF - SMF Structure cascaded, STNS and SHS Simultaneous excitation in the same spectrum MZI Taniwa AR Valley. Utilizing changes in ammonia concentration to alter... TCF The refractive index of the coating thus causes MZI Valley wavelength shift enables ammonia concentration detection in water; temperature changes alter...HCF The refractive index of the internal air thus leads to AR Valley wavelength shift enables temperature detection. This is achieved through demodulation. AR The valley directly obtains the ambient temperature, and then... MZI The ammonia concentration after temperature interference is obtained by joint demodulation, and finally high-precision measurement of ammonia concentration in water is achieved.

[0035] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Any modifications, alterations, substitutions, or variations made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention shall fall within the protection scope defined by the claims of the present invention.

Claims

1. A temperature-compensated all-fiber optic sensor for ammonia nitrogen detection, characterized in that, Including those connected end to end SMF 1. TCF , NCF , SMF 2. HCF , SMF 3; SMF 1 and TCF Offset misalignment welding TCF , NCF , SMF 2. HCF , SMF 3. Cascaded and fused together, each optical fiber forming a single unit. STNS and SHS Two-part structure; STNS The structure includes SMF 1. TCF , NCF , SMF 2; SHS The structure includes SMF 2. HCF , SMF 3; TCF The surface is coated with an ammonia-sensitive coating; in, SMF 1. SMF 2. SMF 3 is a single-mode fiber. TCF Thin-core optical fiber, NCF It is a coreless optical fiber. HCF It is a hollow-core optical fiber.

2. A method for fabricating a temperature-self-compensated ammonia nitrogen all-fiber optic detection sensor, applied to the temperature-self-compensated ammonia nitrogen all-fiber optic detection sensor described in claim 1, characterized in that, Includes the following steps: Step 1, cut the... SMF 1 and TCF A single-end small offset method is used for staggered welding, with an offset distance of 4 at the welding point. µ m ; Step 2, weld the pieces together. SMF 1 and TCF Placed under a microscope, and cleaved with a fiber optic cleaver TCF Cut to 35 cm in length mm ; Step 3, cut the... TCF and NCF Welding, microscopic cutting NCF Make its length 15 mm ,exist NCF Cutting end welding SMF 2. Composition STNS structure; Step 4, SMF 2. SMF 3 are welded together on a section with an inner diameter of 40. µm Length is 650 µm of HCF Both ends constitute SHS structure; Step 5, cut the... STNS Structure and SHS Structural alignment cascade welding; Step 6, TCF The surface is coated with an ammonia-sensitive coating.

3. The method for fabricating a temperature-self-compensated all-fiber optic ammonia nitrogen detection sensor according to claim 2, characterized in that, The ammonia-sensitive coating is a silica sol-gel ammonia-sensitive coating containing bromocresol purple.

4. The method for fabricating a temperature-self-compensated all-fiber optic ammonia nitrogen detection sensor according to claim 2, characterized in that, Step 6 includes: Step 6.1, take 30 mlTEOS With 15 ml Anhydrous ethanol was mixed and stirred for 20 minutes at 25°C using a temperature-controlled magnetic stirrer. min ; Step 6.2, add 0.1 mol / L of HCl Solution 3 ml Continue stirring for 40 seconds. min ; Step 6.3, add dissolved 5 mgBCP 10g of anhydrous ethanol ml Continue stirring for 60 seconds. min Prepare a silica sol solution, then seal the prepared silica sol solution and store it in a refrigerator; Step 6.4: Fix the prepared sensor onto the three-dimensional moving platform, and move the platform by controlling its movement. TCF A drop of silica sol solution is passed from one end of the area to the other end, completing one coating step; Step 6.5: Dry the sensor at room temperature for 10 minutes before applying the next coating. min Repeat the same coating process as in step 6.4; Step 6.6: After applying the target number of layers, cure at room temperature for 24 hours. h Then at 120° C Lower heating 2 h Then dry at room temperature for 24 hours h .

5. The method for fabricating a temperature-self-compensated all-fiber optic ammonia nitrogen detection sensor according to claim 2, characterized in that, For this sensor, the incident light emitted by the light source is transmitted to... SMF 1 and TCF At the misaligned weld joint, the two parts of light I 1 and I 2 Enter separately TCF The light is transmitted in the fiber core and cladding, and the two parts of the light are... NCF Coupling into SMF Mach-Zehnder interference occurs behind the fiber core of 2; When the external environment is altered, the two parts of light accumulate a certain optical path difference after passing through the sensing area, causing a shift in the interference wavelength of the transmission spectrum. The measured parameter is detected by tracking the amount of interference wavelength shift. The output intensity of the Mach-Zehnder interferometer transmission spectrum is expressed as: （1） in, λ The incident light wavelength, L t for TCF length, I 1 and I 2 are respectively TCF Light intensity in core and cladding modes Δn eff = n core - n clad for TCF The effective refractive index difference between the core and the cladding n core and n clad They are respectively TCF Core and cladding refractive indices; When the ammonia concentration in the water changes, the ammonia-sensitive coating adsorbs ammonia, leading to... TCF The cladding refractive index is from n clad Change to n clad + Δ n The change in refractive index alters the optical path difference between the two paths, thus changing the interference wavelength in the transmission spectrum. λ Drift to λ + Δ λ The relationship between wavelength drift and refractive index is described as follows: （2） Simplifying equation (2), we get MZI Sensitivity relationship between spectral wavelength shift and refractive index change: （3） From equation (3), it can be seen that tracking MZI The drift of a certain valley wavelength in the spectrum is used to detect the concentration of ammonia in water.

6. The method for fabricating a temperature-self-compensated all-fiber optic ammonia nitrogen detection sensor according to claim 2, characterized in that, HCF Inner diameter is r Thickness is d The refractive index of the hollow core is n air The cladding refractive index is n clad , n air < n clad ;beam from SMF 2 oblique incidence HCF Behind the inner wall, when the incident light wavelength does not meet the requirements FP At resonance, this portion of the light is reflected back. HCF The light continues to propagate along the axial direction, i.e., the first reflected light from the air-cladding interface. I 3; When the wavelength of the incident light satisfies FP At resonance conditions, the light is transmitted into the cladding. Part of the light leaks out to the outside, exhibiting a periodic loss decrease in the transmission spectrum at resonance valleys. The other part of the light is reflected by the outer wall of the cladding and refracted at the air-cladding interface, returning to the hollow core, forming a tortuous, back-and-forth refracted light. I 4; when HCF Length greater than beam I 4. At the critical length of axial transmission, I 3 and I 4. Interference of two beams of light AR effect, AR The transmission spectrum is: （4） in, F for AR Fineness of the beam's stripes φ are adjacent AR The phase difference between beams. When the phase difference satisfies the condition of equation (5), the cladding transmittance is at its maximum and the reflectivity is at its minimum, resulting in an anti-resonance effect: （5） in, m The resonance order is... λ m For the first m First resonant wavelength, θ Let be the transmission angle of the light entering the cladding from the hollow core; when the light in the hollow core is approximately incident perpendicularly to the cladding surface, according to Snell's law: （6） Derive the first condition that satisfies the anti-resonance condition m The first resonant wavelength is: （7） The position of the resonant wavelength and the cladding thickness can be obtained. d air refractive index n air and cladding refractive index n clad Relevant; when sensing ammonia concentration, only the ambient refractive index changes. λ m , n air and n clad Nothing changes. AR Valley is not sensitive to changes in ammonia concentration; however, when performing temperature sensing... d and n air Neglecting the changes in temperature, consider the effect of temperature on the cladding refractive index. n clad Due to the influence of temperature, the derivative of the resonant wavelength with respect to temperature is: （8） in, It is the thermo-optic coefficient of the cladding; when the temperature increases... n clad Increasing the wavelength shifts the resonant wavelength towards longer wavelengths; thus obtaining... STNS and SHS The total output spectrum of the sensor after structural cascading: （9）。

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