A highly sensitive nickel ion optical fiber sensor based on reflective intermodal interference

Abstract

The application discloses a high-sensitivity nickel ion fiber sensor based on reflection inter-mode interference, which comprises a broadband light source, a fiber Bragg grating, an optical circulator, a sensing element and a spectrometer; the broadband light source outputs an optical signal, which passes through the fiber Bragg grating and then enters the optical circulator from a first port of the optical circulator and is output from a second port of the optical circulator to the sensing element; then the optical signal obtains nickel ion concentration information through the sensing element, is reflected again, reenters the optical circulator from the second port of the optical circulator, is output from a third port of the optical circulator and is received by the spectrometer; the optical signal is received by the spectrometer, and the concentration of a nickel ion solution is measured by analyzing the change of the spectrum. The application can detect the nickel ion without causing secondary pollution and can improve the sensitivity of the fiber in detecting the nickel ion; the application has the characteristics of high-sensitivity nickel ion sensing and low detection limit; and the application can be used for a long time in a water environment with different nickel ion concentrations and can be kept stable for a long time.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber sensing, and in particular to a highly sensitive nickel ion optical fiber sensor based on reflective intermodal interference. Background Art

[0002] Nickel is a metallic element, ranked 28th in the periodic table, with a relative atomic mass of 58.69. It belongs to the first transition series, Group VIII, in the periodic table. According to its content in the earth's crust, nickel ranks 24th. There are five natural nickel isotopes in nature, of which 58 Nihe 60 Nickel is the most abundant. Nickel exists in several oxidation states, ranging from -1 to +4. However, the +2 oxidation state is the most prevalent form of nickel.

[0003] Nickel can be polished, forged, welded, and drawn, and is inert to air, water, non-oxidizing acids, bases, and many organic solvents. Nickel and its compounds are widely used in industry due to their favorable physical and chemical properties. Industrial nickel waste is discharged into rivers, and nickel and its compounds in the soil and atmosphere circulate through the atmosphere and enter rivers. Large amounts of nickel in various forms easily enter the human body through breathing and ingestion, where they are deposited. Because nickel is not yet considered an essential element for the human body, most nickel and its compounds cannot be metabolized in the body. Therefore, exposure to nickel and its compounds can have adverse effects on human health. Therefore, it is extremely important to monitor whether nickel ion concentrations in domestic water, nickel ion emissions, and rivers exceed standards.

[0004] Researchers have proposed various methods for detecting heavy metal nickel and other heavy metals. The main measurement methods can be divided into two categories: chemical-based detection methods and fiber-optic sensing-based detection methods. Chemical-based detection methods mainly include atomic absorption spectroscopy, inductively coupled plasma atomic emission spectroscopy, and fluorescence probe analysis. Another type of detection method is fiber-optic sensing technology. Due to its various advantages, more and more fiber-optic sensing technologies for heavy metal ion detection have been reported.

[0005] Atomic absorption spectrometry (AAS) is a method for determining the content of an element by detecting the characteristic spectrum produced by atoms absorbing specific wavelengths. However, AAS can only detect single elements, and heavy metal ions often exist not in isolation but in combination with other substances. Therefore, heavy metal ion detection requires pretreatment, including sampling, separation, drying, evaporation, and atomization. This requires numerous methods and steps, making it complex and tedious. Inductively coupled plasma atomic emission spectrometry (ICP-AES) is a method for multi-element determination. Similar to AAS, it requires pretreatment of the sample and requires bulky instrumentation. Fluorescence probe methods detect heavy metal ions by adding specific chemical fluorescent reagents or fluorescein to the heavy metal ions. These chemical reactions, which occur by complexing or reacting with the heavy metal ions, alter the properties of the fluorescent probe or fluorescein, such as the luminescence intensity. Common fluorescent probes include rhodamine, pyrene, and coumarin. However, since fluorescent probes are chemical substances, they are prone to secondary contamination.

[0006] In recent years, researchers have also proposed fiber optic sensing methods, which measure heavy metal ions by designing structures on optical fibers to form sensing elements. This newly proposed fiber optic sensing method is compact, easy to install and apply, and does not require the addition of chemical reagents or dyes, thus preventing secondary contamination. Furthermore, fiber optic sensing does not require sampling and can be applied directly in the environment where measurement is required, improving measurement efficiency. However, fiber optic sensing technology is still immature and has shortcomings or areas for improvement, particularly in the detection of nickel ions. In this area, sensitivity needs to be further improved. Summary of the Invention

[0007] In view of this, and in response to the shortcomings of existing chemical detection methods and optical fiber sensing detection methods, the present invention proposes a highly sensitive nickel ion optical fiber sensor based on reflective intermodal interference. This optical fiber sensor detects nickel ions without causing secondary pollution and can also improve the sensitivity of optical fiber detection of nickel ions.

[0008] The present invention solves the above problems through the following technical means:

[0009] A highly sensitive nickel ion optical fiber sensor based on reflective intermodal interference, comprising a broadband light source, a fiber Bragg grating, an optical circulator, a sensing element, and a spectrometer;

[0010] The broadband light source outputs light signals, which pass through the fiber Bragg grating, then enter the optical circulator from the first port of the optical circulator, and are output from the second port of the optical circulator to the sensing element; then the light signals obtain the nickel ion concentration information by passing through the sensing element, and are reflected to re-enter the optical circulator from the second port of the optical circulator; finally, the light signals are output from the third port of the optical circulator, and are received by the optical spectrum analyzer, and the concentration of the nickel ion solution is measured by analyzing the change of the spectrum.

[0011] Further, the fiber Bragg grating can monitor the temperature of the environment, and transmit the temperature information to the optical spectrum analyzer, so that the temperature of the solution environment is obtained by analyzing the transmission peak of the spectrum of the fiber Bragg grating.

[0012] Further, the sensing element is formed by fusion splicing a multimode optical fiber and a single-mode optical fiber, the surface of the multimode optical fiber is corroded, part of the cladding is removed, and then a sensing film sensitive to nickel ions is coated on the multimode optical fiber.

[0013] Further, the sensing film sensitive to nickel ions is formed by layer-by-layer plating of chitosan and polyacrylic acid, and the hydroxyl and amino groups of the two high molecular materials can effectively absorb nickel ions.

[0014] Further, the sensing film sensitive to nickel ions is formed by layer-by-layer plating of 2% chitosan and 4% polyacrylic acid.

[0015] Further, in the sensing element, the light signal propagates to the end face of the single-mode optical fiber and the multimode optical fiber, and high-order modes are excited due to the mismatch of the modes of the single-mode optical fiber and the multimode optical fiber; the light signal forms a reflection at the end face of the multimode optical fiber, re-propagates to the end face of the single-mode optical fiber and the multimode optical fiber, and part of the high-order modes and the fundamental mode are transmitted to the single-mode optical fiber, and the initial phase, the direction of electric field vibration and the frequency of the two modes are the same, and inter-mode interference is generated; the light signal is excited to generate high-order modes, and the change of the refractive index of the sensing film sensitive to nickel ions will cause the change of the optical path and the phase of the light signal; the refractive index of the sensing film changes when the sensing film combines with nickel ions, and the light signal received by the optical spectrum analyzer also changes.

[0016] Further, in the sensing element, when the light signal is transmitted from the single-mode optical fiber to the multimode optical fiber, multiple modes of light field are excited due to the mismatch of the modes, and the expression of the mth mode of light field is:

[0017]

[0018] wherein A is the amplitude, i is an imaginary number, β m is the mth attempt, and z is the propagation distance of the light field; when the light field of the multimode optical fiber is reflected back to the single-mode optical fiber, it is coupled back to the single-mode optical fiber with a certain coupling coefficient and inter-mode interference occurs, and the expression of the light field is:

[0019]

[0020] Where a is the optical field coupling coefficient coupled into the single-mode fiber, N represents infinity, and R is the reflectivity of the multimode fiber end face; its expression is:

[0021]

[0022] Among them, n eff and n are the effective refractive index of the multimode optical fiber and the refractive index of the solution medium, respectively;

[0023] The mth and m-1th modes interfere with each other, and their phase difference is:

[0024]

[0025] in, and are the effective refractive indices of the mth and m-1th modes, λ is the wavelength of the optical signal, and L is the geometric length of the multimode fiber; when When the kth order interference is destructive, the wavelength expression is:

[0026]

[0027] Where C is the concentration of the nickel ion solution to be measured, T is an arbitrary temperature, and the k-th order interference cancellation peak drift is:

[0028] △λ k =λ k (C,T)-λ k (0,T0) (6)

[0029] Among them, λ k (0, T0) is the wavelength of the k-th interference peak in pure water at temperature T0. Due to the thermo-optical effect, the coated sensing film and optical fiber are affected by temperature to a certain extent. The effective refractive indices of the m-th and m-1-th modes are:

[0030]

[0031]

[0032] in, and are the effective refractive indices of the mth and m-1th modes when C=0 and T=T0, respectively; k c2 and k c1 are the concentration coefficients of the mth and m-1th mode sensing films, respectively, k T1 、 k T2 and k'T2 are the first-order and second-order thermo-optical coefficients of the corresponding modes for the optical fiber, respectively.

[0033] Furthermore, by substituting equations 7 and 8 into equation 5, we can obtain:

[0034]

[0035]

[0036]

[0037]

[0038] Equations 10 and 11 respectively express the single relationship between the wavelength of the sensor's interference spectrum and the temperature and solution nickel ion concentration. Equation 12 expresses the relationship between the wavelength of the sensor's interference spectrum and the temperature and solution nickel ion concentration, and is expanded into a second-order Taylor series. Substituting Equation 12 and Equation 9 into Equation 6 yields:

[0039]

[0040] It can be seen from Equation 13 that the spectral interference cancellation peak drift is related to the nickel ion solution concentration and temperature.

[0041] Furthermore, the sensor monitors the temperature through cascaded fiber Bragg gratings, which can compensate for the influence of temperature on the sensor. The reflection wavelength of the fiber Bragg grating is:

[0042] λ FBG =2n eff Λ (14)

[0043] Among them, n eff The effective refractive index of the grating, Λ is the period of the grating; the fiber Bragg grating has a certain response to temperature. The fiber Bragg grating in the sensor can monitor the temperature changes in the environment and measure the impact of temperature on the sensor at the same time. The impact of temperature on the sensor is compensated by the response of the fiber Bragg grating to temperature.

[0044] Furthermore, in the principle of the sensor, it is analyzed that small changes in temperature have an impact on spectral interference. When the nickel ion concentration remains unchanged, the expression of the interference cancellation peak and trough and temperature is:

[0045]

[0046] Where d is a constant, k = 0, 1, 2...k T1 、 k T2 and k' T2are the first-order thermo-optic coefficient and the second-order thermo-optic coefficient of the multimode optical fiber core and the sensing film in the sensing element, respectively.

[0047] Compared with the prior art, the beneficial effects of the present invention include at least:

[0048] The nickel ion optical fiber sensor of the present invention detects nickel ions without causing secondary pollution and can also improve the sensitivity of optical fiber detection of nickel ions; it has the characteristics of high-sensitivity nickel ion sensing and low detection limit.

[0049] The nickel ion optical fiber sensor of the present invention cascades a fiber Bragg grating to monitor the ambient temperature, can measure the temperature of the solution environment, and then calibrate the temperature to compensate for the cross sensitivity of the temperature to the measurement, thereby obtaining a more accurate result.

[0050] The nickel ion optical fiber sensor of the present invention has different responses to nickel ions, calcium ions, cadmium ions and barium ions, and the optical fiber sensor has certain specificity in detecting nickel ions.

[0051] The nickel ion optical fiber sensor of the present invention can remain stable for a long time in a water environment with different nickel ion concentrations and can be used for a long time. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. 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 creative work.

[0053] Figure 1 This is a schematic diagram of a highly sensitive nickel ion optical fiber sensor based on reflective intermodal interference according to the present invention;

[0054] Figure 2 Spectra of the sensor of the present invention at different nickel ion concentrations (T = 24 ° C);

[0055] Figure 3 is the linear fit of the sensor's response to nickel ions;

[0056] Figure 4 is the linear fit of the temperature response of the sensor of the present invention in pure water;

[0057] Figure 5 is the linear fit of the temperature response of the FBG of the present invention;

[0058] Figure 6 The sensor of the present invention is Ni 2+ , Ca 2+ 、Cd 2+ and Ba2+ Linear fitting of response characteristics;

[0059] Figure 7 80-minute continuous measurement results (T=24° C.) of nickel ion solutions with three concentrations according to the present invention. DETAILED DESCRIPTION

[0060] To make the above-mentioned objectives, features, and advantages of the present invention more clearly understood, the technical solutions of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that the embodiments described are only some of the embodiments of the present invention, not all of them. All other embodiments obtained by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are also within the scope of protection of the present invention.

[0061] Principle of nickel ion fiber optic sensor:

[0062] The experimental principle of the high-sensitivity nickel ion optical fiber sensor based on reflective intermodal interference is as follows: Figure 1 As shown, it includes a broadband light source, a fiber Bragg grating, an optical circulator, a sensing element and a spectrometer;

[0063] The broadband light source outputs an optical signal, which passes through a fiber Bragg grating (FBG) and then enters the optical circulator from port 1. It is then output to the sensor element from port 2. The optical signal then passes through the sensor element to obtain nickel ion concentration information, and after reflection, it re-enters the optical circulator from port 2. Finally, the optical signal is output from port 3 of the optical circulator and then enters the spectrometer, where it is received and the concentration of the nickel ion solution is measured by analyzing changes in the spectrum. In addition, the FBG can monitor the temperature of the environment, acting as a temperature sensor, and transmits the temperature information to the spectrometer. The temperature of the solution environment is determined by analyzing the transmission peak of the FBG spectrum.

[0064] The sensing element is formed by splicing multimode optical fiber and single-mode optical fiber, such as Figure 1The dotted line indicates this. The surface of a multimode optical fiber is etched to remove a portion of the cladding. A nickel-ion-sensitive film is then applied to the multimode fiber. The nickel-ion-sensitive film is formed by a layer-by-layer coating process using chitosan (2%) and polyacrylic acid (4%). The hydroxyl and amino groups of these two polymers effectively absorb nickel ions. A light signal propagates to the end faces of a single-mode fiber and a multimode fiber. Due to the mode mismatch between the single-mode and multimode fibers, higher-order modes are excited. The light signal is reflected at the multimode fiber end faces and propagates back to the single-mode and multimode fiber ends. Parts of the higher-order modes and the fundamental mode are transmitted to the single-mode fiber. Because the two modes originate from the same source, their initial phase, electric field vibration direction, and frequency are identical, resulting in intermodal interference. The excited higher-order modes, due to the evanescent wave, cause changes in the refractive index of the nickel-ion sensing film, which in turn changes the optical pathlength and phase of the light signal. The nickel-ion-sensing film binds nickel ions, causing a change in the refractive index, which in turn changes the light signal received by the spectrometer.

[0065] When light waves are transmitted from a single-mode fiber to a multimode fiber, due to mode mismatch, multiple modes of light fields are excited. The expression of the m-th order mode light field is:

[0066]

[0067] Where A is the amplitude, i is the imaginary number, β m is the mth attempt, z is the light field propagation distance; when the light field of the multimode fiber is reflected back to the single-mode fiber, it is coupled back to the single-mode fiber with a certain coupling coefficient and inter-mode interference occurs, the light field expression is:

[0068]

[0069] Where a is the coupling coefficient of the light field coupled into the single-mode fiber, N represents infinity, and R is the reflectivity of the multimode fiber end face, which is expressed as:

[0070]

[0071] Among them, n eff and n are the effective refractive index of the multimode optical fiber and the refractive index of the solution medium, respectively.

[0072] The mth and m-1th modes interfere with each other, and their phase difference is:

[0073]

[0074] in, and are the effective refractive indices of the mth and m-1th modes, λ is the wavelength of the optical signal, and L is the geometric length of the multimode fiber. When the kth order interference is destructive, the wavelength expression is:

[0075]

[0076] Where C is the concentration of the nickel ion solution to be measured, T is an arbitrary temperature, and the k-th order interference cancellation peak drift is:

[0077] △λ k =λ k (C,T)-λ k (0,T0) (6)

[0078] Among them, λ k (0, T0) is the wavelength of the kth interference peak in pure water at temperature T0. Due to the thermo-optical effect, the coated sensing film and optical fiber are affected by temperature to a certain extent. The effective refractive indices of the mth and m-1th modes are:

[0079]

[0080]

[0081] in, and k are the effective refractive indices of the mth and m-1th modes when C = 0 and T = T0, respectively. c2 and k c1 are the concentration coefficients of the mth and m-1th mode sensing films, respectively, k T1 、 k T2 and k' T2 are the first-order and second-order thermo-optical coefficients of the corresponding modes for the optical fiber. Substituting equations (7) and (8) into equation (5) yields:

[0082]

[0083]

[0084]

[0085]

[0086] Equations (10) and (11) respectively express the single relationship between the wavelength of the sensor's interference spectrum and the temperature and solution nickel ion concentration. Equation (12) expresses the relationship between the wavelength of the sensor's interference spectrum and the temperature and solution nickel ion concentration, and is expanded into a second-order Taylor series. Substituting equations (12) and (9) into equation (6) yields:

[0087]

[0088] It can be seen from equation (13) that the spectral interference cancellation peak drift is related to the concentration and temperature of the nickel ion solution.

[0089] The sensor monitors temperature through cascaded fiber Bragg gratings (FBGs), which can compensate for the effect of temperature on the sensor, making the sensor measurement more accurate. Fiber Bragg gratings are short-period gratings that have extremely high reflectivity at specific wavelengths, so they are also called reflection gratings. The reflection wavelength is:

[0090] λ FBG =2n eff Λ (14)

[0091] Among them, n e ff The effective refractive index of the grating, Λ, is the period of the grating. FBGs have a certain response to temperature. The FBG in the nickel ion sensor can monitor changes in ambient temperature and measure the effect of temperature on the sensor. The FBG's response to temperature can then be used to compensate for the effect of temperature on the sensor.

[0092] Analysis of the sensor's nickel ion sensing characteristics:

[0093] In order to verify the sensing characteristics of the sensor for nickel ion concentration, a series of nickel ion solutions were prepared for experimental measurement. Nickel ion solution was prepared by accurately weighing a certain amount of nickel chloride solid, taking an appropriate amount of ultrapure water to make 1×10 -1 mol / L nickel ion solution was diluted to obtain a series of nickel ion solution concentrations. To control the variables and maintain a single variable, all experiments were conducted in a constant temperature and humidity chamber to maintain constant humidity and temperature. The temperature was controlled at room temperature (24°C) and the humidity was controlled at ambient humidity (67%).

[0094] Figure 2 The spectra of the sensor in nickel ion solutions with different concentrations are shown in a constant temperature and humidity chamber controlled at room temperature of 24°C and humidity of 67%. Figure 2 It can be clearly seen that Dip1 in the nickel ion solution increases with the concentration of nickel ion solution from 0 to 7×10 -11 When the concentration of mol / L changes, the spectrum will red-shift.

[0095] In order to make the relationship between the drift of the interference cancellation trough and the nickel ion concentration more intuitive, the drift of the spectral interference cancellation trough Dip1 was obtained through Origin software. By fitting, the relationship between the spectral interference cancellation peak Dip1 and the nickel ion concentration was obtained, as shown in the figure: Figure 3 As shown. The black dots are experimental data values, and the straight line is the linear fit of the experimental data. This experiment set eight concentrations, one of which was ultrapure water without any heavy metal ions and impurities, and the other seven groups were set at 1-7×10-11 mol / L, the relationship between the spectral interference cancellation peak Dip1 and the nickel ion concentration is obtained by linear fitting:

[0096] Δλ=3.00×10 10 C-0.003 (15) Among them, the fitting degree R 2 =0.991. The sensitivity of the sensor is defined as:

[0097] S C =dλ / dC=3.00×10 10 (nm) / mol / L (16)

[0098] The minimum resolution of the spectrometer used in the experiment is 0.02 nm, and the detection limit of the sensor can be calculated to be: 6.67×10 -13 mol / L. The experiment verified that the sensor has the characteristics of high sensitivity of nickel ion sensing and low detection limit.

[0099] Sensor temperature cross sensitivity analysis:

[0100] Both the fiber cladding and the nickel-ion sensing film will undergo slight changes with temperature. In actual sensor applications, temperature changes are inevitable, and temperature is a major factor affecting sensor measurements.

[0101] In order to compensate for the influence of temperature on the sensor, the influence of temperature on the sensor is studied and eliminated. In the principle of the sensor, it is analyzed that a small change in temperature has an effect on spectral interference. When the nickel ion concentration remains unchanged, the expression of the interference cancellation peak and valley and temperature is:

[0102]

[0103] Where d is a constant, k = 0, 1, 2...k T1 、k T 1 1. k T2 and k' T2 are the first-order thermo-optic coefficient and the second-order thermo-optic coefficient of the multimode optical fiber core and the sensing film in the sensing element, respectively.

[0104] In order to verify the temperature response characteristics of the sensor, the present invention controls the nickel ion concentration and measures the relationship between wavelength drift and temperature. The sensor is placed in pure water, that is, the nickel ion concentration is 0, and the temperature is changed to observe the wavelength drift. The experimental results are as follows Figure 4 As shown in the figure, the black points are experimental data, and the straight line is the linear fitting of the wavelength drift when the temperature changes. Its expression is:

[0105] Δλ=-1.69×10 -2 (T-24)-0.063 (18)

[0106] Among them, the fitting degree R 2 = 0.975, the temperature range is from 20°C to 45°C, with an interval of 5°C. As can be seen from the formula, the temperature response of the sensor is a linear relationship, indicating that the second-order thermo-optical effect of the sensor is small and can be ignored. By combining the linear fitting equation of formula (15), the dependence of the wavelength drift on nickel ion concentration and temperature can be obtained as:

[0107] Δλ=3.00×10 10 C-1.69×10 -2 (T-24)-0.066 (19)

[0108] Formula (20) verifies the existence of a temperature cross-sensitivity factor when measuring nickel ion concentration in aqueous solution. Combining the formula, it can be seen that when the temperature changes by 5°C, the wavelength drift can reach 0.08nm, which is equivalent to 3.67×10 -12 mol / L nickel ion concentration deviation. The results show that temperature changes have a certain impact on the measurement of nickel ions. In order to eliminate the influence of temperature on the sensor, this experiment cascaded FBG to measure the temperature. Figure 5 The experimental data and linear fitting of Dip_FBG wavelength drift at different temperatures are as follows:

[0109] λ=-9.94×10 -3 T+1534.05 (20)

[0110] Among them, the fitting degree R 2 =0.975. In actual measurement, the FBG is cascaded to monitor the ambient temperature, and the temperature of the solution environment can be measured. Then, the temperature is calibrated by formula (20) to compensate for the cross-sensitivity of the temperature to the measurement, thereby obtaining a more accurate result.

[0111] Analysis of sensor detection specificity:

[0112] In the actual polluted environment, there are many kinds of heavy metal pollution. Accurately detecting the content of one heavy metal requires excluding the influence of other heavy metal ions. Nickel ion pollution mainly comes from the combustion of chemical materials such as petroleum and coal mines. The combustion of these chemical materials may also bring other heavy metals or metal pollution, such as calcium ions, cadmium ions and barium ions. In order to prove the specificity of the sensor in detecting nickel ions, the concentrations of three solutions of calcium ions, cadmium ions and barium ions were set from zero (i.e. ultrapure water) to 5×10 -11 mol / L, immerse the sensor in a series of solutions of these three ions respectively, and observe the drift of the interference spectrum. Figure 6 The concentration ranges of nickel ions, calcium ions, cadmium ions and barium ions (0-5×10 -11 mol / L). It can be seen that at each concentration, the drift of nickel ions is larger than that of the other three ions, while the drift of the other three ions is relatively small.

[0113] To further analyze the differences in the sensor's responses to nickel ions, calcium ions, cadmium ions, and barium ions, linear fitting was performed on the wavelength shifts of the four ions. From the figure, we can see that the sensor's responses to nickel ions, calcium ions, cadmium ions, and barium ions are: 3.00×10 10 nm / (mol / L), 0.73×10 10 nm / (mol / L), 0.54×10 10 nm / (mol / L) and 0.70×10 10 nm / (mol / L), the response ratios of nickel ion and the other three ions are: Ni 2+ / Ca 2+ =4.11、Ni 2+ / Cd 2 + =5.56、Ni 2+ / Ba 2+ =4.29. Therefore, the sensor has a certain specificity for the detection of nickel ions.

[0114] Long-term stability analysis of sensors:

[0115] Long-term stability is an important indicator for evaluating a sensor. For optical fiber sensors, especially those with chemical functional membranes, a measurement time of more than 1 hour is required to truly meet the sensor standard. In order to prove that the sensor can maintain long-term stability and long-term use in water environments with different nickel ion concentrations, three sets of experiments were conducted. The experimental settings were 2×10 -11 mol / L、4×10 -11 mol / L、6×10 -11 mol / L nickel ion solution, the sensor was placed in the solution for continuous measurement, the spectral drift was continuously observed, and the spectrum was recorded every 10 minutes. Combined with the previously obtained relationship between the sensor's spectral interference peaks and valleys and nickel ion concentration, that is, Equation (15), the measurement value of the continuous measurement of the same nickel ion solution was obtained. Figure 7 Shows 2×10 -11 mol / L、4×10 -11 mol / L、6×10 - 11The standard deviation of the continuous measurement values ​​of nickel ion solution with a concentration of mol / L within 80 minutes is 2.8×10 -13 mol / L、4.7×10 - 13 mol / L、8.0×10 -13 mol / L, the standard deviations of the three concentrations were all close to the minimum detection limit of the sensor, 6.67×10 -13 mol / L is relatively close. Therefore, the sensor is relatively stable, verifying the long-term stability of the sensor.

[0116] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A highly sensitive nickel ion optical fiber sensor based on reflective intermodal interference, characterized in that: Including broadband light source, fiber Bragg grating, optical circulator, sensor element and spectrometer; The broadband light source outputs an optical signal, which passes through the fiber Bragg grating, enters the optical circulator from the first port of the optical circulator, and is output to the sensor element from the second port of the optical circulator; The optical signal then passes through the sensor element to obtain nickel ion concentration information, and then, after reflection, enters the optical circulator again from the second port of the optical circulator; Finally, the optical signal is output from the third port of the optical circulator and then enters the spectrometer to be received. The concentration of the nickel ion solution is measured by analyzing the changes in the spectrum. In the sensor element, when the optical signal is transmitted from a single-mode optical fiber to a multi-mode optical fiber, due to mode mismatch, multiple modes of light fields are excited. The expression of the m-th order mode light field is: Where A is the amplitude, i is the imaginary number, β m is the mth attempt, z is the light field propagation distance; when the light field of the multimode fiber is reflected back to the single-mode fiber, it is coupled back to the single-mode fiber with a certain coupling coefficient and inter-mode interference occurs, the light field expression is: Where a is the optical field coupling coefficient coupled into the single-mode fiber, N represents infinity, and R is the reflectivity of the multimode fiber end face; its expression is: Among them, n eff and n are the effective refractive index of the multimode optical fiber and the refractive index of the solution medium, respectively; The mth and m-1th modes interfere with each other, and their phase difference is: in, and are the effective refractive indices of the mth and m-1th modes, λ is the wavelength of the optical signal, and L is the geometric length of the multimode fiber; when When the kth order interference is destructive, the wavelength expression is: Where C is the concentration of the nickel ion solution to be measured, T is an arbitrary temperature, and the k-th order interference cancellation peak drift is: △λ k =λ k (C,T)-λ k (0,T0) (6) Among them, λ k (0, T0) is the wavelength of the k-th interference peak in pure water at temperature T0. Due to the thermo-optical effect, the coated sensing film and optical fiber are affected by temperature to a certain extent. The effective refractive indices of the m-th and m-1-th modes are: in, and are the effective refractive indices of the mth and m-1th modes when C = 0 and T = T0, respectively; k c2 and k c1 are the concentration coefficients of the mth and m-1th mode sensing films, respectively, k T1 , k' T1 、k T2 and k' T2 are the first-order and second-order thermo-optical coefficients of the corresponding modes for the optical fiber, respectively.

2. The high-sensitivity nickel ion optical fiber sensor based on reflective intermodal interference according to claim 1, characterized in that: The fiber Bragg grating can monitor the temperature of the environment and transmit the temperature information to the spectrometer. The temperature of the solution environment can be obtained by analyzing the transmission peak of the fiber Bragg grating spectrum.

3. The high-sensitivity nickel ion optical fiber sensor based on reflective intermodal interference according to claim 1, characterized in that: The sensing element is formed by fusing a multimode optical fiber with a single-mode optical fiber. The surface of the multimode optical fiber is corroded to remove a portion of the cladding, and then a sensing film sensitive to nickel ions is coated on the multimode optical fiber.

4. The high-sensitivity nickel ion optical fiber sensor based on reflective intermodal interference according to claim 3, characterized in that: The nickel ion-sensitive sensing membrane is formed by chitosan and polyacrylic acid through a layer-by-layer coating method. The hydroxyl and amino groups of these two polymer materials can effectively absorb nickel ions.

5. The high-sensitivity nickel ion optical fiber sensor based on reflective intermodal interference according to claim 4, characterized in that: The nickel ion sensitive sensing membrane is formed by 2% chitosan and 4% polyacrylic acid through a layer-by-layer coating method.

6. The high-sensitivity nickel ion optical fiber sensor based on reflective intermodal interference according to claim 1, characterized in that: In the sensing element, the optical signal propagates to the end faces of the single-mode fiber and the multimode fiber. Due to the mode mismatch between the single-mode fiber and the multimode fiber, higher-order modes are excited. The optical signal is reflected at the end face of the multimode fiber and re-propagates back to the end faces of the single-mode fiber and the multimode fiber. Some of the higher-order modes and the fundamental mode are transmitted to the single-mode fiber. Since the two modes are of the same origin, their initial phase, electric field vibration direction and frequency are the same, resulting in inter-modal interference. The optical signal is excited to generate higher-order modes. Due to the action of the evanescent wave, the change in the refractive index of the sensing film sensitive to nickel ions will cause changes in the optical path and phase of the optical signal. The sensing film combines with nickel ions, the refractive index changes, and the optical signal received by the spectrometer will also change accordingly.

7. The high-sensitivity nickel ion optical fiber sensor based on reflective intermodal interference according to claim 1, characterized in that: Substituting equations 7 and 8 into equation 5, we obtain: Equations 10 and 11 respectively express the single relationship between the wavelength of the sensor's interference spectrum and the temperature and solution nickel ion concentration. Equation 12 expresses the relationship between the wavelength of the sensor's interference spectrum and the temperature and solution nickel ion concentration, and is expanded into a second-order Taylor series. Substituting Equation 12 and Equation 9 into Equation 6 yields: It can be seen from Equation 13 that the spectral interference cancellation peak drift is related to the nickel ion solution concentration and temperature.

8. The high-sensitivity nickel ion optical fiber sensor based on reflective intermodal interference according to claim 1, characterized in that: The sensor monitors the temperature through cascaded fiber Bragg gratings, which can compensate for the influence of temperature on the sensor. The reflection wavelength of the fiber Bragg grating is: l FBG =2n eff L (14) Among them, n eff The effective refractive index of the grating, Λ is the period of the grating; the fiber Bragg grating has a certain response to temperature. The fiber Bragg grating in the sensor can monitor the temperature changes in the environment and measure the impact of temperature on the sensor at the same time. The impact of temperature on the sensor is compensated by the response of the fiber Bragg grating to temperature.

9. The highly sensitive nickel ion optical fiber sensor based on reflective intermodal interference according to claim 1, characterized in that: In the principle of the sensor, it is analyzed that slight changes in temperature have an impact on spectral interference. When the nickel ion concentration remains unchanged, the expression of the interference cancellation peak and trough and temperature is: Where d is a constant, k = 0, 1, 2, ...; k T1 , k' T1 、k T2 and k' T2 are the first-order thermo-optic coefficient and the second-order thermo-optic coefficient of the multimode optical fiber core and the sensing film in the sensing element, respectively.

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