Ammonia sensor based on small-period long-period fiber grating

Abstract

An ammonia sensor based on a small-period long-period fiber grating, comprising a small-period long-period fiber grating (13) and a gas-sensitive layer (12) coated on the outer surface of a single-mode fiber (11). The small-period long-period fiber grating (13) corresponds to the gas-sensitive layer (12) in position; the small-period long-period fiber grating (13) is of a fiber grating structure inscribed in the single-mode fiber (11); the cladding layer surface of the small-period long-period fiber grating (13) is a refractive index sensitive area; a high molecular polymer is used for the gas-sensitive layer (12); and the refractive index of the high molecular polymer changes along with the change of the concentration of ammonia for ammonia measurement. The ammonia sensor has the advantages of compact structure, high sensitivity, good stability, short response time, low manufacturing difficulty, low cost, and high anti-interference capability, the surrounding environment temperature can be monitored at the same time, and the impact of temperature fluctuation on ammonia measurement is overcome.

Description

An ammonia sensor based on small-period long-period fiber grating Technical Field

[0001] The present invention relates to the field of optical fiber sensing, and in particular to an ammonia sensor based on a small-period long-period optical fiber grating. Background Art

[0002] Ammonia is widely used in the chemical, agricultural, food, and electronics industries. However, because inhaling a certain concentration of ammonia can harm human health, researchers have developed a variety of sensors to monitor its concentration for early warning. Common ammonia sensors are mainly classified into electrochemical, semiconductor, spectral absorption, contact combustion, and fiber optic contact types. Semiconductor ammonia sensors have the disadvantages of high operating temperatures and susceptibility to electromagnetic interference. They also have low detection accuracy and difficulty detecting low gas concentrations at the ppb level. Spectral absorption ammonia sensors can overcome the poor selectivity and high operating temperature problems of electrical gas sensors, but their detection limit and sensitivity are limited by the length of the gas chamber optical path. The long gas chamber optical path restricts the miniaturization and integration of the sensor.

[0003] Fiber-optic contact ammonia sensors have been extensively studied due to their simple structure, light weight, high sensitivity, resistance to electromagnetic interference, and corrosion resistance. Currently reported fiber structures include D-type fibers, micro-nano fibers, tapered fibers, long-period fiber gratings, and tilted fiber Bragg gratings. However, D-type fibers, micro-nano fibers, and tapered fibers are fragile, while long-period fiber gratings and tilted fiber Bragg gratings are susceptible to temperature fluctuations and cannot monitor temperature simultaneously, leading to measurement errors.

[0004] Therefore, those skilled in the art urgently need to provide an ammonia sensor based on a small-period long-period fiber grating that can simultaneously monitor ambient temperature and ammonia concentration.

[0005] Summary of the Invention

[0006] The purpose of the present invention is to provide an ammonia sensor based on a small-period long-period fiber grating to solve the problem that traditional fiber-optic contact ammonia sensors are easily affected by ambient temperature crosstalk and cannot measure ambient temperature and ammonia concentration simultaneously.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] The present invention discloses an ammonia sensor based on a small-period long-period fiber grating, comprising a small-period long-period fiber grating and a gas-sensitive layer coated on the outer surface of a single-mode optical fiber. The small-period long-period fiber grating and the gas-sensitive layer correspond in position to each other. The small-period long-period fiber grating is a fiber grating structure inscribed inside the single-mode optical fiber. The cladding surface of the small-period long-period fiber grating is a refractive index sensitive area. The gas-sensitive layer adopts a high molecular polymer, and the refractive index of the high molecular polymer changes with the ammonia concentration to detect ammonia.

[0009] Preferably, the high molecular weight polymer is polydiallyldimethylammonium chloride and polyacrylic acid.

[0010] Preferably, the small-period long-period fiber grating is obtained by line-by-line writing inside a single-mode optical fiber from which the coating has been removed using a femtosecond laser, the transmission spectrum of the small-period long-period fiber grating includes high-order cladding modes, and the reflection spectrum of the small-period long-period fiber grating includes a series of high-order Bragg reflection peaks.

[0011] The method for preparing the gas-sensitive layer of the ammonia sensor based on a small-period long-period fiber grating as described above comprises the following steps:

[0012] Step 1: soaking the small-period long-period fiber grating in an alkaline solution to ionize the surface of the small-period long-period fiber grating;

[0013] Step 2: Soaking the product obtained in step 1 in a positively charged polydiallyldimethylammonium chloride solution so that the polyacrylic acid can be better bonded to the surface of the single-mode optical fiber;

[0014] Step 3: Soaking the product obtained in step 2 in deionized water and drying it with nitrogen to remove residual molecular groups;

[0015] Step 4: Soaking the product obtained in step 3 in a negatively charged polyacrylic acid solution so that the gas-sensitive layer can combine with ammonia;

[0016] Step 5: Soaking the product obtained in step 4 in deionized water and blowing it dry with nitrogen to remove residual molecular groups;

[0017] Step 6: Repeat steps 2 to 5 until the thickness of the gas-sensitive layer reaches 100 nm to 500 nm.

[0018] The ammonia concentration testing system of the ammonia sensor based on the small-period long-period fiber grating as described above includes a broadband light source, a circulator, an air chamber, a small-period long-period fiber grating ammonia sensor and a spectrometer; the broadband light source, the circulator, the small-period long-period fiber grating ammonia sensor and the spectrometer are arranged and connected in sequence, the small-period long-period fiber grating ammonia sensor is placed in the air chamber, the emission light of the broadband light source is transmitted to the small-period long-period fiber grating ammonia sensor through the circulator, the refractive index of the gas-sensitive layer of the small-period long-period fiber grating ammonia sensor responds to changes in ammonia concentration, and the ammonia concentration information is displayed by the spectrometer.

[0019] The environmental temperature testing system of an ammonia sensor based on a small-period long-period fiber grating as described above includes a broadband light source, a circulator, a small-period long-period fiber grating ammonia sensor, a spectrometer and a tubular furnace; the broadband light source, the small-period long-period fiber grating ammonia sensor, the spectrometer and the circulator are arranged and connected according to position, the small-period long-period fiber grating ammonia sensor is placed in the tubular furnace, the emission light of the broadband light source is transmitted to the small-period long-period fiber grating ammonia sensor through the circulator, the wavelength of the high-order Bragg resonance peak in the reflection spectrum of the small-period long-period fiber grating ammonia sensor changes with temperature to perform temperature detection, and the temperature information is displayed by the spectrometer.

[0020] Compared with the prior art, the present invention has the following beneficial technical effects:

[0021] (1) The device length of the small-period long-period fiber grating of the present invention is shorter than that of the traditional long-period fiber grating, which makes it easier to miniaturize and integrate the sensing system.

[0022] (2) Compared with complex, time-consuming and high-cost thin film deposition technologies such as magnetron sputtering, physical vapor deposition and chemical vapor deposition, the layer-by-layer electrostatic self-assembly film forming method of the present invention is simple and efficient, and the film is uniform and the thickness can be adjusted.

[0023] (3) The femtosecond laser precision processing technology of the present invention can flexibly process fiber Bragg gratings with different parameters, with short processing time and high processing repeatability.

[0024] (4) The present invention can not only detect the ammonia concentration, but also monitor the ambient temperature changes at the same time.

[0025] In summary, the ammonia sensor based on small-period long-period fiber grating of the present invention has the advantages of compact structure, high sensitivity, good stability, short response time, low manufacturing difficulty, low cost, and strong anti-interference ability. It can also simultaneously monitor the ambient temperature and overcome the influence of temperature fluctuations on ammonia detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention will be further described below with reference to the accompanying drawings.

[0027] FIG1 is a schematic diagram of an assembly of an ammonia sensor based on a small-period long-period fiber grating according to the present invention;

[0028] FIG2 is a schematic diagram of an ammonia concentration testing system according to the present invention;

[0029] FIG3 is a schematic diagram of an ambient temperature testing system according to the present invention;

[0030] FIG4 shows the transmission and reflection spectra of the small-period long-period fiber grating of the present invention;

[0031] FIG5 is a transmission spectrum of the ammonia sensor manufactured by the present invention at different ammonia concentrations;

[0032] FIG6 shows the change of the cladding mode resonance wavelength of the small-period long-period fiber grating according to the present invention with the ammonia concentration;

[0033] FIG7 shows the reflectance spectra of the ammonia sensor manufactured by the present invention at different temperatures;

[0034] FIG8 shows the variation of the Bragg resonance peak wavelength of the small-period long-period fiber grating of the present invention with temperature.

[0035] Explanation of the accompanying symbols: 11. Single-mode optical fiber; 12. Gas-sensitive layer; 13. Small-period long-period fiber Bragg grating; 21. Broadband light source; 22. Circulator; 23. Gas chamber; 24. Small-period long-period fiber Bragg grating ammonia sensor; 25. Spectrometer; 31. Tubular furnace. DETAILED DESCRIPTION

[0036] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0037] As shown in Figures 1-8, an ammonia sensor based on a small-period long-period fiber grating includes a small-period long-period fiber grating 13 and a gas-sensitive layer 12 coated on the outer surface of a single-mode optical fiber 11. The small-period long-period fiber grating 13 and the gas-sensitive layer 12 correspond to each other in position. The small-period long-period fiber grating 13 is a fiber grating structure inscribed inside the single-mode optical fiber 11, and the cladding surface of the small-period long-period fiber grating 13 is a refractive index sensitive area. The gas-sensitive layer 12 is made of a polymer, and the refractive index of the polymer changes with the ammonia concentration to detect ammonia.

[0038] Specifically, the working principle of the present invention is as follows: a poly(diallyldimethylammonium chloride) / polyacrylic acid film is grown on the surface of a fiber grating (FBG) via a layer-by-layer electrostatic self-assembly method. The carboxyl functional groups of the polyacrylic acid react with ammonia, causing the refractive index of the coating to change. This change can be detected by the cladding mode of a small-period long-period fiber grating (13). Because the period of the small-period long-period fiber grating (13) inscribed by a femtosecond laser is only 30 μm, far smaller than the period of several hundred microns of a traditional long-period fiber grating (13), the small-period long-period fiber grating (13) not only exhibits a series of high-order transmission peaks in the transmission spectrum, but also a series of Bragg reflection peaks in the reflection spectrum. Ammonia concentration is detected by the wavelength shift of the transmission peak, and ambient temperature is detected by the wavelength shift of the Bragg reflection peak. Because the Bragg reflection peak is insensitive to ammonia, the effect of temperature on the wavelength shift of the transmission peak can be compensated by the relationship between the Bragg reflection peak wavelength and temperature. Therefore, both ammonia concentration and ambient temperature can be measured simultaneously.

[0039] In one specific embodiment, the small-period long-period fiber grating 13 is a fiber grating structure with a period of 30 μm, a duty cycle of 50%, and a length of 3 mm, which is written line by line in a single-mode optical fiber 11 by a femtosecond laser. The gas-sensitive layer 12 is a high molecular polymer polydiallyldimethylammonium chloride and polyacrylic acid, and its thickness is 120 nm.

[0040] The high molecular polymer is polydiallyldimethylammonium chloride (PDDA) and polyacrylic acid (PAA).

[0041] The small-period long-period fiber grating 13 is obtained by line-by-line writing inside the single-mode optical fiber 11 with the coating removed by a femtosecond laser. The transmission spectrum of the small-period long-period fiber grating 13 includes high-order cladding modes, and the reflection spectrum of the small-period long-period fiber grating 13 includes a series of high-order Bragg reflection peaks.

[0042] A method for preparing a gas-sensitive layer of an ammonia sensor based on a small-period long-period fiber grating comprises the following steps:

[0043] Step 1: soaking the small-period long-period fiber grating 13 in an alkaline solution to ionize the surface of the small-period long-period fiber grating 13;

[0044] Step 2: Soaking the product obtained in step 1 in a positively charged polydiallyldimethylammonium chloride solution so that the polyacrylic acid can be better bonded to the surface of the single-mode optical fiber 11;

[0045] Step 3: Soaking the product obtained in step 2 in deionized water and drying it with nitrogen to remove residual molecular groups;

[0046] Step 4: Soaking the product obtained in step 3 in a negatively charged polyacrylic acid solution so that the gas-sensitive layer can combine with ammonia;

[0047] Step 5: Soaking the product obtained in step 4 in deionized water and blowing it dry with nitrogen to remove residual molecular groups;

[0048] Step 6: Repeat steps 2 to 5 until the thickness of the gas-sensitive layer 12 reaches 100 nm to 500 nm.

[0049] In one specific embodiment, the steps for preparing the gas sensitive layer are as follows:

[0050] Step 1: Soak the small-period long-period fiber grating in potassium hydroxide solution for 30 minutes to ionize its surface;

[0051] Step 2: Soaking the product obtained in step 1 in a positively charged polydiallyldimethylammonium chloride solution (0.5 wt.%, pH=3.1) to allow the polyacrylic acid to better bind to the optical fiber surface; specifically, wt.% is the weight percentage;

[0052] Step 3: Soaking the product obtained in step 2 in deionized water and drying it with nitrogen to remove residual molecular groups;

[0053] Step 4: Soak the product obtained in step 3 in a negatively charged polyacrylic acid solution (0.05 wt.%, pH=4.2) so that the gas-sensitive layer can be combined with ammonia.

[0054] Step 5: Soaking the product obtained in step 4 in deionized water and blowing it dry with nitrogen to remove residual molecular groups;

[0055] Step 6: Repeat steps 2 to 5 until the thickness of the gas-sensitive layer reaches 100 nm to 500 nm.

[0056] As shown in Figure 2, an ammonia concentration testing system for an ammonia sensor based on a small-period long-period fiber grating includes a broadband light source 21, a circulator 22, an air chamber 23, a small-period long-period fiber grating ammonia sensor 24 and a spectrometer 25; the broadband light source 21, the circulator 22, the small-period long-period fiber grating ammonia sensor 24 and the spectrometer 25 are arranged and connected in sequence, the small-period long-period fiber grating ammonia sensor 24 is placed in the air chamber 23, the emission light of the broadband light source 21 is transmitted to the small-period long-period fiber grating ammonia sensor 24 through the circulator 22, the refractive index of the gas-sensitive layer 12 of the small-period long-period fiber grating ammonia sensor 24 responds to changes in ammonia concentration, and the ammonia concentration information is displayed through the spectrometer 25.

[0057] The working principle of the ammonia concentration testing system is as follows: the light emitted from the broadband light source 21 passes through the circulator 22 and reaches the small-period long-period fiber Bragg grating ammonia sensor 24. The change in ammonia concentration in the gas chamber 23 changes the refractive index of the gas-sensitive layer 12, and this refractive index change is reflected by the cladding mode in the transmission spectrum of the small-period long-period fiber Bragg grating ammonia sensor 24; the transmitted light containing the ammonia concentration information is received by the spectrometer 25, and the ammonia concentration information is obtained by the change in the resonant wavelength of the cladding mode in the transmission spectrum of the small-period long-period fiber Bragg grating ammonia sensor 24 on the spectrometer 25.

[0058] Figure 5 is the transmission spectrum of the small-period long-period fiber Bragg grating ammonia sensor 24 at different ammonia concentrations. The measurement range is 0.1ppm to 25ppm. When the ammonia concentration is 0.1ppm, the cladding mode resonance peak wavelength responds. Figure 6 is the change of the cladding mode resonance wavelength in the transmission spectrum of the small-period long-period fiber Bragg grating ammonia sensor 24 with the ammonia concentration. It shows a logarithmic change at lower concentrations and an exponential change at higher concentrations. It can be seen from Figure 6 that the curve fit is relatively good, which is consistent with the changing trend of gas sensing based on refractive index changes.

[0059] As shown in Figure 3, an environmental temperature testing system for an ammonia sensor based on a small-period long-period fiber grating includes a broadband light source 21, a circulator 22, a small-period long-period fiber grating ammonia sensor 24, a spectrometer 25 and a tubular furnace 31; the broadband light source 21, the small-period long-period fiber grating ammonia sensor 24, the spectrometer 25 and the circulator 22 are arranged and connected according to position, and the small-period long-period fiber grating ammonia sensor 24 is placed in the tubular furnace 31, and the emitted light of the broadband light source 21 is transmitted to the small-period long-period fiber grating ammonia sensor 24 through the circulator 22, and the wavelength of the high-order Bragg resonance peak in the reflection spectrum of the small-period long-period fiber grating ammonia sensor 24 changes with temperature to perform temperature detection, and the temperature information is displayed by the spectrometer 25.

[0060] The working principle of the ambient temperature testing system is as follows: the light emitted from the broadband light source 21 passes through the circulator 22 to reach the small-period long-period fiber Bragg grating ammonia sensor 24. By adjusting the temperature in the tubular furnace 31, the wavelength of the high-order Bragg resonance peak in the reflection spectrum of the small-period long-period fiber Bragg grating ammonia sensor 24 changes with the temperature; the reflected light containing temperature information is received by the spectrometer 25 through the circulator 22, and the ambient temperature information is obtained by the change in the wavelength of the high-order Bragg resonance peak in the reflection spectrum of the small-period long-period fiber Bragg grating ammonia sensor 24 on the spectrometer 25.

[0061] Figure 7 shows the reflection spectrum of the small-period long-period fiber Bragg grating ammonia sensor 24 at different ambient temperatures. The measurement range is 30°C to 70°C. Figure 7 shows that as the temperature increases, the wavelength of the Bragg reflection peak gradually redshifts (moves to a longer wavelength). Conversely, as the temperature decreases, the wavelength of the Bragg reflection peak gradually blueshifts (moves to a shorter wavelength). Figure 8 shows the change of the wavelength of the high-order Bragg resonance peak in the reflection spectrum of the small-period long-period fiber Bragg grating ammonia sensor 24 with the ambient temperature. The linear fitting R 2 The value is 0.999, indicating that the high-order Bragg resonance peak wavelength has a very good linear relationship with the change of ambient temperature. The slope of the linear fit indicates that the temperature sensitivity of the sensor is 0.0087nm / ℃. Therefore, in practical applications, the actual ambient temperature value can be determined based on the measured Bragg reflection peak wavelength value.

[0062] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0063] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. An ammonia sensor based on a small-period long-period fiber grating, characterized in that: It includes a small-period long-period fiber grating (13) and a gas-sensitive layer (12) coated on the outer surface of a single-mode fiber (11). The small-period long-period fiber grating (13) corresponds to the gas-sensitive layer (12) in position. The small-period long-period fiber grating (13) is a fiber grating structure inscribed inside the single-mode fiber (11), and the cladding surface of the small-period long-period fiber grating (13) is a refractive index sensitive area. The gas-sensitive layer (12) uses a polymer, and the refractive index of the polymer changes with the change of ammonia concentration for ammonia detection.

2. The ammonia gas sensor based on a small-period long-period fiber grating according to claim 1, wherein: The polymer is poly(diallyldimethylammonium chloride) and polyacrylic acid.

3. The ammonia sensor based on a small-period long-period fiber grating according to claim 1, characterized in that: The small-period long-period fiber grating (13) is obtained by inscribing line by line inside the single-mode fiber (11) with the coating layer removed by femtosecond laser. The transmission spectrum of the small-period long-period fiber grating (13) includes high-order cladding modes, and the reflection spectrum of the small-period long-period fiber grating (13) includes a series of high-order Bragg reflection peaks.

4. The preparation method of the gas-sensitive layer of an ammonia gas sensor based on a short-period long-period fiber grating according to any one of claims 1-3, characterized in that: It includes the following steps Step 1: Immerse the small-period long-period fiber grating (13) in an alkaline solution to ionize the surface of the small-period long-period fiber grating (13). Step 2: Immerse the product obtained in Step 1 in a positively charged poly(diallyldimethylammonium chloride) solution so that polyacrylic acid can better bind to the surface of the single-mode fiber (11). Step 3: Immerse the product obtained in Step 2 in deionized water and dry it with nitrogen to remove residual molecular groups. Step 4: Immerse the product obtained in Step 3 in a negatively charged polyacrylic acid solution so that the gas-sensitive layer combines with ammonia. Step 5: Immerse the product obtained in Step 4 in deionized water and dry it with nitrogen to remove residual molecular groups. Step 6: Repeat Steps 2 to 5 until the thickness of the gas-sensitive layer (12) reaches 100 nm to 500 nm.

5. An ammonia concentration testing system for an ammonia sensor based on a small-period long-period fiber grating according to any one of claims 1-3, characterized in that: It includes a broadband light source (21), a circulator (22), a gas chamber (23), a small-period long-period fiber grating ammonia sensor (24), and a spectrometer (25). The broadband light source (21), the circulator (22), the small-period long-period fiber grating ammonia sensor (24), and the spectrometer (25) are arranged and connected in sequence. The small-period long-period fiber grating ammonia sensor (24) is placed in the gas chamber (23). The emitted light of the broadband light source (21) is transmitted to the small-period long-period fiber grating ammonia sensor (24) through the circulator (22). The refractive index of the gas-sensitive layer (12) of the small-period long-period fiber grating ammonia sensor (24) responds to the change of ammonia concentration, and the ammonia concentration information is displayed by the spectrometer (25).

6. An environmental temperature testing system for an ammonia gas sensor based on a small-period long-period fiber grating according to any one of claims 1-3, characterized in that: It includes a broadband light source (21), a circulator (22), a small-period long-period fiber grating ammonia sensor (24), a spectrometer (25) and a tube furnace (31); the broadband light source (21), the small-period long-period fiber grating ammonia sensor (24), the spectrometer (25) and the circulator (22) are arranged and connected according to their positions, the small-period long-period fiber grating ammonia sensor (24) is placed in the tube furnace (31), the emitted light of the broadband light source (21) is transmitted to the small-period long-period fiber grating ammonia sensor (24) through the circulator (22), the wavelength of the high-order Bragg resonance peak in the reflection spectrum of the small-period long-period fiber grating ammonia sensor (24) changes with temperature for temperature detection, and the temperature information is displayed through the spectrometer (25).

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